Shinners & Mahler's Illustrated Flora of North Central Texas

From the Introduction of Shinners & Mahler's Illustrated Flora of North Central Texas;
copyright 1999 BRIT and Austin College

References cited below are available on-line.

More information on the flora of North Central Texas is available on-line.

Introduction

Overview

North Central Texas is an area of roughly 40,000 square miles, bound on the north by the Red River and extending south nearly to Austin. While small by Texas standards (260,000 square miles), it is about the size of Kentucky. It includes the Blackland Prairie, the Grand Prairie, the East and West cross timbers, and the Red River Area (Fig. 1). The flora includes 2,223 species, about 46 percent of the species known for Texas (Hatch et al. 1990), and a total of 2,376 taxa (species, subspecies, and varieties). This biological diversity is the result of numerous factors, including the region's geologic and climatic variation and its location in the ecotone or transition zone between the eastern deciduous forests and the central North American grasslands. North Central Texas is a mixing ground for plants from the east and west, with different microhabitats even within the same county having radically different plant communities. For the past two centuries, humans have had, and are continuing to have, a tremendous impact on the plants and animals of the region. Presettlement and early settlement conditions were radically different from those found today; the current generation may be the last with the opportunity to preserve even small remnants of the once extensive natural ecosystems.

General Geology of North Central Texas

The geology of North Central Texas is much more interesting and complex than indicated by the gently rolling topography. If one begins about 300 million years ago, during the Pennsylvanian Period (320--286 million years ago), what is now North Central Texas was on the southern edge of a North American continent shaped very differently than it is today. As a result of plate tectonic movements, North America collided with Africa and South America to become part of the supercontinent Pangaea. The outcome of this collision was the uplift and formation of an extensive mountain system including the ancient Appalachians, Wichitas, and Ouachitas. The Ouachita Mountains formed in a line roughly following the western edge of the current Blackland Prairie and farther south, the Balcones Escarpment. They extended across much of Texas, in a line from near Sherman to Dallas to Austin and beyond (Fig. 5). The ancient Ouachita mountain belt also continued to the northeast; the eroded Ouachitas seen today in southeastern Oklahoma and southwestern Arkansas are remnants of this once much more extensive range. In western Texas, to the west of the ancient Ouachita Mountains, crustal areas sagged and low basins formed. Shallow inland seas invaded these low areas, and during the Pennsylvanian, and later the Permian, western Texas served as a collection basin for the sediments that eroded from the Ouachita Mountains to the east. These deep sediments are world-famous for their oil-bearing layers. They are also the source of the bright red, iron-rich (hematite) Permian layers that easily erode and give the modern Red River its name. Over tens of millions of years the Ouachita Mountains gradually eroded, until today all that is left over most of Texas are their roots, deeply buried under thousands of feet of sediments (Spearing 1991).

During the Triassic (245--208 million years ago) and Jurassic periods (208--145 million years ago) and continuing into the Cretaceous Period (145--65 million years ago), Pangaea eventually split apart into separate continents. The North Central Texas region once again became very active geologically with the zone of weakness where the Ouachitas had originally formed serving as the site of continental rifting or breakup between North and South America. It was here, where the continents pulled apart, that huge shallow seas, eventually retreating to become the present-day Gulf of Mexico, began to form (Fig. 6). Into these seas, thick layers of sediment were deposited during much of the Cretaceous Period, the material coming in part from erosion of the Rocky Mountains rising to the west. The shallow seas repeatedly advanced and retreated over much of Texas. In fact, these seas extended to the Big Bend area and at times even connected all the way from the Gulf of Mexico north to the Arctic Ocean. As a result of the varying water depths and other conditions, a number of different layers of Cretaceous sediments were laid down across the state. Fossil-bearing limestones, so common in North Central Texas, were deposited in the shallow seas. Near the ancient coasts, where muds and sands were laid down, dinosaur tracks were sometimes preserved. Excellent examples of these can be seen in Dinosaur Valley State Park near Glen Rose in Somervell County, and others have been found in the Glen Rose Limestone in Comanche and Hamilton counties (Shuler 1935; 1937; Albritton 1942; Spearing 1991).

As a result of the depositional processes described above and subsequent erosion of overlying material, nearly all surface rocks of modern North Central Texas are Cretaceous in origin (Renfro et al. 1973; McGowen et al. 1991) (Fig. 7). An exception is the more recent, stream-deposited sediment along some of the drainages. As indicated above, all these Cretaceous layers were laid down near the margin of an ocean that can be thought of as a greatly expanded Gulf of Mexico. After the Cretaceous Period, as the Gulf retreated farther to the southeast, sediments continued to be deposited on eastern Texas during much of the Tertiary Period (65--2 million years ago). The youngest of these sediments are therefore found near the present-day Gulf Coast (Bullard 1931). During the Tertiary, the major geologic factor shaping North Central Texas was the removal of material by erosion (Sellards et al. 1932; Baker 1960), revealing layers that were once buried. This process can be clearly seen as one travels west across the region. The eastern Blackland Prairie at the eastern margin of the area is developed on relatively young Upper (= Late) Cretaceous layers. Farther west, at higher elevations subject to greater erosion, more and more Cretaceous material was stripped away and progressively older rocks exposed. The only significant non-Cretaceous rocks found in North Central Texas are the older Pennsylvanian-age rocks uncovered by extensive erosion in the Palo Pinto Country in the extreme western portion of the region (Hill 1901; Sellards et al. 1932) (Figs. 7, 8). Here, all Cretaceous layers have been removed, and the much older, Pennsylvanian-age rocks are exposed at the surface.

As indicated above, North Central Texas was near the edge of the ocean during much of the Cretaceous, and as sea levels rose and fell, shallow seas repeatedly covered and then retreated from much of Texas (Sellards et al. 1932; Spearing 1991). Numerous layers of limestone, marl (chalky or limey clays), shale, and sand (Bullard 1931; Baker 1960) were deposited over the area, the type of layer depending on water depth, distance from shore, and other factors. According to Hill (1901),

“In general the sands are near-shore deposits, such as are seen to-day on most ocean beaches. The finer sands were carried a little further seaward than the coarse material. The clays are the lighter débris of the land, which were laid down a little farther from the land border; and so on through the various gradations to the chalky limestones, which largely represent oceanic sediments deposited in relatively purer waters farthest away from the land. The limestones are not all chalky. Some are agglomerates of shells of animals which inhabited the sandy or muddy bottoms; others are old beach wash. The vast numbers of sea shells occurring upon the mountains and prairies of Texas have not been transported, as some people believe. Save that they have been subjected to general regional uplift whereby the sea bottom was converted into land, they are now in the exact locality where they lived and flourished, and the clays and limestones in which they were buried were once the muds of the old ocean bottom.”

Because all these layers are ocean sediments, many have excellent fossils of marine organisms. Some of the most obvious include oyster-like bivalve mollusks, sharks teeth, a type of echinoderm known as heart urchins, and ammonites, the large, extinct, coiled-shell relatives of the octopus and squid. In fact, some North Central Texas rock layers are composed almost completely of fossilized animal remains and the area is well known among fossil hunters.

To the southwest of North Central Texas occurs a rugged area which includes granite and other Precambrian outcrops, variously known as the Burnet Country, Central Mineral Region, or Llano Basin; it has been exposed by the extensive erosion of overlying sediments. To the west of the West Cross Timbers, and thus like the Central Mineral Region outside of North Central Texas, lies the vast area known as the Rolling Plains, underlain by the famous Permian Red Beds. This region at least in part is sometimes referred to as the Red Plains. The strikingly colored, iron oxide-rich, erosional products of these Permian layers give the Red River (originating far to the west) its name. The salinity of the Red River (and thus of Lake Texoma) is also the result of erosion from salt-rich Permian-age evaporation flats through which the river passes on its course east from the Texas Panhandle (Spearing 1991).

Soils of North Central Texas

Soils in North Central Texas vary dramatically, ranging from the characteristic black soil of the Blackland Prairie to the easily erodible sands underlying the West Cross Timbers.

The "black waxy" soils of the Blackland Prairie are derived from Upper Cretaceous rocks, which are sometimes strikingly white in color (e.g., Austin Chalk); through the process of weathering there is a dramatic change in color (Fig. 9). In the words of Hill (1901),

“The Black Prairie owes its name to the deep regolith of black calcareous clay soils which cover it. When wet these assume an excessively plastic and tenacious character, which is locally called "black waxy." These soils are the residue of the underlying marls and chalks, or local surficial deposits derived from them, and hence are rich in lime. Complicated chemical changes, probably due to humic acid acting upon vegetable roots, are believed to cause the black color. The region is exceedingly productive, and nearly every foot of its area is susceptible to high cultivation. In fact, the prairies are the richest and largest body of agricultural land in Texas, constituting a practically continuous area of soil extending from Red River to the Comal. . . .”

More specifically, the Blackland Prairie (also referred to as the Blacklands) has three dominant soil orders: Vertisols, Mollisols, and Alfisols (Fig. 10). The Vertisols develop mainly on the Eagle Ford shale and rocks of the Taylor Group and are characterized by abundant smectitic (= shrink-swell) clays (Hallmark 1993). Upon wetting and drying, these soils often undergo dramatic changes in volume, which can result in significant soil movements. Swelling and shrinking causes cracks up to 50 centimeters or more deep and as much as 10 centimeters wide at the surface (Hallmark 1993). Stories of golf balls or even baseballs or other objects disappearing in deep cracks are not uncommon from longtime residents of Blackland soil areas. The associated soil movements can have dramatic effects on human activities, resulting in uneven or cracked roadways, shifted buildings, and cracked foundations (Hallmark 1993). Only the most elaborately protected houses on many Vertisols are free from at least some cracks or other soil stability problems. These smectitic clay soils are also quite sticky and difficult to manage agriculturally, being easily compacted by farm machinery when wet and forming large clods when plowed dry. Because they can be effectively tilled only within a narrow moisture range, they gained the nickname "nooner soils"---too wet to plow before noon and too dry after noon (Hallmark 1993). The smectitic clays also result in both slickensides and gilgai, two phenomena often seen in Vertisols. Slickensides are planes of weakness in the soil caused by movements associated with shrinkage and swelling. These can result in rather large-scale slippage or failure of soil blocks, which can be problematic in construction (Hallmark 1993). According to Hallmark (1993), slickenside slippage, causing the collapse of the walls of construction trenches, results in Texas workers being crushed to death in trenches almost every year. Gilgai are the microhigh, microlow topography or relief features found on essentially all Vertisols (Diamond & Smeins 1993). On flat areas in the prairie landscape, gilgai typically form circular, almost tub-like depressions, called "hog wallows" by early settlers. These range from about three to six meters across and up to about one-half meter deep (Hayward & Yelderman 1991). On slopes, gilgai take the form of microridges and microvalleys up to about 20 centimeters deep (Miller & Smeins 1988; Diamond & Smeins 1993) (Fig. 11). Both gilgai and the great soil depth of this region are the result of the constant churning and overturn of the shrink-swell, clay-based soils. When these soils shrink during dry weather and large cracks form, loose pieces of soil fall deep into the cracks. Upon wetting, these pieces swell and exert lateral pressure. Material is pushed outward and eventually upward, resulting in depressions rimmed by slightly raised areas (Hayward & Yelderman 1991) (Fig. 12). Gilgai are thus formed and the soil is slowly but constantly churned; the name Vertisol (Latin: verto, turn upward, sol, soil) is derived from this continuous cycle of overturning of the soil (Steila 1993). On the native Blackland Prairie, soil erosion was low because of the dense tall grass community, and also because of the water-trapping capacity of gilgai. Temporary water storage in gilgai depressions of one-half acre foot of water per acre of flat prairie have been estimated. As much as six inches of rain could be temporarily trapped in these structures before runoff began (Hayward & Yelderman 1991). This would have greatly reduced runoff and allowed significant infiltration, particularly important considering that clay soils are often rather impermeable. In fact, early accounts refer to clear runoff and clear streams on the Blacklands (Hayward & Yelderman 1991), in stark contrast to the current situation. However, because thousands of gilgai covered the prairies and created pools of standing water during wet weather, the prairies were at times virtually impassable (Hayward & Yelderman 1991). Under present agricultural conditions, with no plant cover during much of the year and with the suppression of gilgai formation by plowing, erosion rates in the Blacklands are high. Thompson (1993) notes that the Blacklands have one of the highest rates of soil loss on cropland of any major area in Texas. Estimates run from tens to hundreds of times higher than under the original native prairie vegetation (Hayward & Yelderman 1991). Richardson (1993) cites annual erosion figures of 15 tons per acre (t/a) for a cultivated Blackland area compared with only 0.2 t/a for a native grass meadow, a 70-fold increase. Even though gilgai were one of the most evident surface features on the original Blackland Prairie, because they are destroyed by plowing they are rarely observed today. Excellent examples of these "hog wallows," however, can still be seen at the Nature Conservancy's Clymer Meadow preserve in Hunt County as well as on scattered prairie remnants (Fig. 13).

Mollisols are found on the Fort Worth Prairie and the Lampasas Cut Plain on various limestone layers and on the Blackland Prairie on rocks of the Austin Group. All of these areas have high calcium carbonate levels and consolidated parent rocks. Because bedrock is usually just below the surface, rooting and soil water storage are restricted. Typically Mollisols are less useful for agriculture than are Vertisols and at present they tend to be used as pastures or homesites. Shrink-swell phenomena, while still occurring on Mollisols, are less problematic than on Vertisols (Diamond & Smeins 1993; Hallmark 1993). Laws (1962) and Brawand (1984) have studied the characteristics of soils formed from the Austin Chalk in the Dallas area.

Alfisols, which develop principally on bedrocks which are higher in sand and lower in calcium carbonate, are found mainly on the eastern and northern margins of the Blacklands (Hallmark 1993) (Fig. 10). These soils are less fertile than either of the other two types (Hallmark 1993). Another microtopographical feature, mima mounds (also called pimple mounds or prairie mounds) (Fig. 11), were found on essentially all Alfisols within the Blackland Prairie region and can still be observed on certain unplowed prairie remnants (e.g., northern Grayson County (Fig.14) and Tridens Prairie in Lamar County). These are circular, saucer-shaped hills roughly 1 to 14 meters in diameter and up to more than a meter tall. While numerous hypotheses have been proposed, the structures are of unknown and possibly multiple origins (Collins et al. 1975; Diamond & Smeins 1993). Both gilgai and mima mounds increase microhabitat diversity and thus cause vegetational differences over small distances. The overall biological diversity of the prairie is therefore increased (Miller & Smeins 1988; Diamond & Smeins 1993). Due to the different vegetation associated with the different microhabitats of both gilgai and mima mounds, these features are easily discernible in the field at certain seasons of the year (Figs. 13, 14).

The soils of the East and West cross timbers are mainly developed on sandy Cretaceous Woodbine and Trinity strata. An exception are the soils in the western portion of the West Cross Timbers which are developed on gravelly and rocky Pennsylvanian strata. While somewhat variable (due to areas of clay in the Woodbine or Trinity Group sands and the complex nature of the Pennsylvanian strata), in general the soils of the cross timbers are rather loose, often deep sands. In fact, the sandy soils in some areas of the West Cross Timbers were so deep that early wagon travel was difficult because wagon wheels would sink in (Hill 1901). The loose nature of these soils also makes them extremely susceptible to erosion. Dyksterhuis (1948) discussed tremendous gullies formed due to inappropriate cultivation and wind and water erosion resulting in the subsequent abandonment of fields and even whole farms. However, it is because of these sandy soils, which are much more conducive to the growth of woody vegetation than the heavier clay soils typical of the Blackland and Grand prairies, that the cross timbers vegetation is found in this region.

Climate of North Central Texas

North Central Texas is located in a zone of dramatic transition between regional climates. The striking vegetational change from the East Texas deciduous forests on the eastern margin of North Central Texas to the grasslands of the Great Plains just to the west is a vivid reflection of this climatic transition (Stahle & Cleaveland 1995). The climate of North Central Texas is often considered subtropical (Yelderman 1993;; Norwine et al. 1995; Peterson 1995), but a wide range of extremes can be found. Like the rest of the state, the area can be referred to climatically as a "land of contrasts" (Bomar 1983). There is ample reason why locals say, "If you don't like the weather, just wait a few minutes." "Blue northers," cold fronts swinging down from the north and accompanied by rapid drops in temperature of dozens of degrees, are not uncommon (Bomar 1983). Mean annual temperature varies from about 68û F in the south (Williamson County) to about 64û F in the north and west (Griffiths & Orton 1968) (Fig. 15), but temperatures of 0û F and 110û F are not unknown for winter and summer respectively (Tharp 1926), with even more extreme readings observed on occasion. In the heat wave of 1980, temperatures climbed to 113û F (45û C) in Dallas-Fort Worth, and there were 69 days with a temperature of 100û F or above (Bomar 1983). The highest reading, 119û F, was recorded for Weatherford in Parker County in 1980 (Bomar 1983). In the extreme cold spell of December 1983, temperatures were below freezing in Dallas-Fort Worth for 12 straight days (Bomar 1983). The coldest temperatures recorded in North Central Texas include minus 8û F (Dallas-Fort Worth) and minus 13û F (Paris), both in the unusually cold winter of 1899 (Bomar 1983). Native vegetation has evolved with, and is adapted to, such recurrent extremes. A good example of the different effects of the climate on native versus non-native plants was the extensive damage to introduced landscape plants during the winter of 1983, while most native plants were not adversely affected. The mean length of the frost-free period in the area is given in Figure 16.

While native plants are in general adapted to local weather conditions, they can be damaged under exceptional circumstances. An example was the unseasonably late freeze on the night of 11--12 April 1997. Following a period of relatively warm weather, temperatures dropped to substantially below freezing over a large part of North Central Texas. For example, a low of 22û F was recorded for a native habitat (Garnett Preserve) in Montague County (H. Garnett, pers. comm.). The result was substantial damage to the young foliage of many native species and in some cases nearly complete defoliation. Some of the species significantly damaged in Grayson County include Berchemia scandens (supple-jack), Cercis canadensis (redbud), Diospyros virginiana (common persimmon), Fraxinus americana (white ash), Gleditsia triacanthos (common honey-locust), Morus rubra (red mulberry), Platanus occidentalis (sycamore), Quercus macrocarpa (bur oak), Quercus marilandica (blackjack oak), Quercus muhlenbergii (chestnut oak), Quercus shumardii (Shumard's red oak), Quercus stellata (post oak), and Rhus glabra (smooth sumac). Effects on post and blackjack oaks in areas of Cross Timbers vegetation at Hagerman National Wildlife Area (Grayson County) were serious enough that leaf damage was still obvious at a glance in late May. This event is a good example of a false spring weather anomaly. As discussed by Stahle (1990) and Stahle and Cleaveland (1995), a false spring episode includes late winter warmth followed by the movement of polar or arctic air into southern regions. The resulting intense subfreezing temperatures following a warm spell cause widespread damage to cultivated crops as well as native plant species which were advanced in their development by the unusually mild winter temperatures. Forty-four such false spring episodes have been documented in Texas between A.D. 1650 and 1980 (Stahle & Cleaveland 1995). This detailed information can be obtained because frost-damaged cambial tissues leave a permanent record in the annual growth rings of trees and these can be dated dendrochronologically (tree-ring dating) to the exact year of their formation. While there has been a notable decline in the frequency and intensity of false spring episodes in Texas in the last 100 years, the cause of this decrease is not clear (Stahle & Cleaveland 1995).

In terms of precipitation, there is a steep east-west gradient across North Central Texas. Mean annual precipitation ranges from about 46 inches in the northeastern corner of the area in Lamar County to about 24 inches in the westernmost portion of the West Cross Timbers (Dyksterhuis 1948; Griffiths & Orton 1968 ) (Fig. 17). In general, mean annual precipitation decreases about one inch for each 15 miles across Texas from east to west (Bomar 1983), in part as a result of the decreasing influence of moist air from the Gulf of Mexico. Thus there is a rainfall difference of more than 20 inches between the eastern and western boundaries of North Central Texas. This region, like much of the state, is prone to drought, while at other times it receives too much rain in a short time (Sharpless & Yelderman 1993). Severe storms and some of the largest rainfalls in the United States have occurred in this area. According to Hayward et al. (1992), all the point rainfall records for North America are held within a belt 50 miles east and west of a line from Dallas through Waco, Austin, and San Antonio. The town of Thrall, in Williamson County on the eastern edge of the Blacklands, had one of the largest recorded United States rainfalls on 9--10 September 1921, receiving 38.2 inches in 24 hours (Yelderman 1993). In 1978, Albany, in Shackelford County on the western edge of North Central Texas, received 29.05 inches in one day as a result of Hurricane Amelia. The yearly precipitation record for Texas, 109.38 inches, is also from this region. It occurred in 1873 at Clarksville in Red River County near the extreme northeastern tip of the Blackland Prairie (Bomar 1983).

In years past, the incredibly sticky "black waxy" soil of the Blackland Prairie was particularly problematic during wet weather. Personal accounts (e.g., Mosely in Yelderman 1993) describe that under wet conditions the dirt roads were virtually impassable and families actually went hungry until the ground dried enough for people to get to town to obtain food. Drought, however, has been more of a problem, with the lack of water probably always being a limiting factor for humans in the area. The impermeable clay soils, the lack of dependable shallow water-bearing layers, and the scarcity and transitory nature of surface streams made the early Blacklands a particularly inhospitable environment for humans. This is exemplified by early accounts such as the one by D.P. Smythe (1852) who described a trip across the Blacklands:

“The soil improves now at every step becoming more level, and uniformly of a dark rich color, but the water is very bad and scarce, drying up entirely during the heat of the summer. . . . During the forenoon of today we must have traveled some twenty miles without passing over a spot of thin soil; being chiefly the black stiff 'hog wallow' prairie, rolling just enough to drain itself, but entirely destitute of water during the summer. . . . "

Josiah Gregg, another early explorer, indicated that in addition to droughts, the lack of springs or dependable water was "one of the greatest defects of this country" (Fulton 1941).

The concentration of rainfall in spring and fall coupled with hot dry summers makes the water problem even more acute (Yelderman 1993). The severe drought of the mid-1950s exemplifies water difficulties in the area. City water supplies were alarmingly low (e.g., Lake Dallas at 11% of capacity), cattle and sheep ranchers were desperate, wells declined to record levels, and streams barely flowed or dried up (Bomar 1983). At the time of settlement, while shallow wells were used in some areas, cisterns were the only source of water in others (Hayward & Yelderman 1991; Yelderman 1993). Currently, access to deep aquifers, such as the Trinity, and surface storage in large reservoirs (e.g., Lake Texoma, Lake Lewisville, Lake Ray Hubbard), provide water in this water-poor environment (Hayward & Yelderman 1991). However, Simpson (1993) has emphasized that, "Texas has been a water-deficit state since the dawn of recorded history" and that, "The problem will only be exaggerated as population growth expands." Currently, many cities and water-supply corporations in Texas are actively seeking access to more water, and some cities, such as San Antonio, have a serious water supply problem (Simpson 1993). The recurrent water difficulties seen locally are a reminder of the overall scarcity of water in the southwestern United States.

Major storms have also long been a problem in the region, as can be seen by Parker's (1856) account of the effects of a tornado in 1854 near Gainesville in Cooke County. He spoke vividly of trees broken off ten feet above the ground, an ox wagon carried a quarter of a mile, and a sheep blown into the top of a high tree. More recent destructive tornadoes (e.g., Waco 1953, Paris 1982---Bomar 1983, Jarrell in Williamson Co. 1997) and hail storms (e.g., grapefruit size hail in Fort Worth in May 1995) are present-day reminders of the ongoing power of extreme weather events.

From a longer term perspective, pollen, plant macrofossils, and other types of evidence demonstrate that the climate of Texas has changed substantially over the past 15,000 years. At 15,000 years BP, there was a more widespread forest mosaic over most of Texas with boreal species such as Picea glauca (white spruce) in specialized microhabitats (Stahle & Cleaveland 1995). Certain present-day plant distributions, such as the rare western occurrence of plants normally found predominantly in eastern Texas, may thus be relicts of these past climatic conditions. While long-term climate change is well documented, recently attention has focused on the possibility of future climate change in Texas due to human-induced modifications of the atmosphere (e.g., increased C02 concentrations) and the resulting increased greenhouse effect and global warming (e.g., Norwine et al. 1995). While considerable controversy exists over details, there is solid evidence that global atmospheric CO2 concentrations have increased by about 30% since preindustrial times and that this trend can be attributed primarily to human activities (e.g., fossil fuel use, land-use changes, and agriculture) (Houghton et al. 1995). Further, consensus now exists that there is ". . . a discernible human influence on climate" (Houghton et al. 1995).

Plants can contribute to an understanding of climate change in several ways. First, dendrochronology, the study of tree rings, can provide information on past climate and thus a reference point for present and future studies (Stahle & Cleaveland 1992). Extensive tree-ring chronologies based on remnant old-growth Taxodium disticum (bald cypress) stands have provided accurate and well-verified climatic reconstructions for the past 1,000 years for some areas of the southeastern United States, including northwestern Louisiana (Stahle et al. 1988; Stahle & Cleaveland 1992). In North Central Texas, well-documented chronologies based on remnant populations of Quercus stellata (post oak) have yielded detailed information on climate for the past 300 years (Stahle & Hehr 1984; Stahle et al. 1985; Stahle & Cleaveland 1988; 1993; 1995).

Changes in phytogeography (plant distributions) can also indicate climate change. An example is the long-term McWilliams (1995) study of the distribution of Tillandsia recurvata (ball-moss). This species has expanded its geographical range in Texas over the last 80 years, with much of the expansion occurring since the 1940s. McWilliams demonstrated that even slight changes in temperature or moisture conditions can have significant implications for the survival of plants at the margins of their ranges. The eastward expansion into North Central Texas of species adapted to the drier western Rolling Plains and the northward shift of southerly species would both be expected based on climate models which predict increased temperature (and thus evapotranspiration) and decreased regional precipitation and soil moisture (Houghton et al. 1990; Packard & Cook 1995; Schmandt 1995).

The Blackland Prairie

Occurrence of the Blackland Prairie

The Blackland Prairie of Texas is a well-defined band stretching roughly three hundred miles from the Red River (Oklahoma border) south to near San Antonio (Chambers 1948; Sharpless & Yelderman 1993). It is widest at the north, extending from Grayson County east to near Clarksville in Red River County. It narrows to the south, tapering to a point near San Antonio (Fig. 18). The main belt of the Blackland Prairie totals about 4.3 million hectares or roughly six percent of the total land area of Texas (Collins et al. 1975), and is a region slightly larger than the state of Maryland. It coincides almost exactly with a belt of outcropping Upper Cretaceous marine chalks, marls, and shales (Hayward & Yelderman 1991) that upon weathering forms the characteristic black, calcareous, alkaline, heavy clay, "black waxy" soil. In this work we are including the main body of the Blackland Prairie from the Red River south to the Travis County-Williamson County line just north of Austin, but not the San Antonio Prairie or the outlying Fayette Prairie to the east. Topographically, the Blackland Prairie is a nearly level to gently rolling dissected plain (Hallmark 1993); elevations range from about 300 to 800 feet (92 to 244 meters) (Thomas 1962). Roughly speaking, the Blacklands are bounded on the north by the Red River, on the east by the Post Oak Savannah (also called the Oak-hickory) vegetational area, and on the west by the East Cross Timbers and the Lampasas Cut Plain. North of Sherman in Grayson County, the trend of the Blacklands undergoes a shift in direction, turning from north-south to east-west, before ending near Clarksville in Red River County. In this work we use the terms Blackland Prairie and Blacklands interchangeably.

Presettlement and Early Settlement Conditions on the Blackland Prairie

Conditions on the presettlement Blackland Prairies were strikingly different from those found today. Probably the most striking difference was the presence of vast expanses of tall grass prairie. In the words of Parker (1856), traveling with the 1854 Marcy expedition, "After leaving Preston [northern Grayson County], we entered upon the vast plains. . . ." Dr. John Brooke, who emigrated from England in 1848, stated on arriving at the edge of the Blackland Prairie, "It was the finest sight I ever saw; immense meadows 2 or 3 feet deep of fine grass & flowers. Such beautiful colours I never saw. . . ." (Brooke 1848). In describing the area where he settled near Dorchester in south central Grayson County, Brooke (1849) said,

“I can sit on the porch before my door and can see miles of the most beautiful Prairie interwoven with groves of timber, surpassing, in my idea, the beauties of the sea. Think of seeing a tract of land on a slight incline covered with flowers and rich meadow grass for 12 to 20 miles. . . . "

Hill (1901), speaking of the Blacklands in general, said,

“The surfaces of the prairies are ordinarily clad with thick mantles of grass, liberally sprinkled with many-colored flowers, broken here and there by low growths of mesquite trees, or in exceptional places by 'mottes' or clumps of live oaks on uplands, pecan, bois d'arc, walnut and oaks in the stream bottoms; juniper and sumac where stony slopes exist, and post oak and black-jack in the sandy belts.”

Smythe (1852) described the eastern edge of the Blackland Prairie as having " . . .a view of almost boundless Prairie stretching to the north, as far as the eye could reach. . . ." He further referred to it as

“. . .a boundless plain scarcely broken by a single slope or valley, and nearly destitute of trees; (the mesquite appearing but seldom.) Several times during the forenoon not a single shrub or tree could be seen in any direction. . . . The grazing has reached its climax, it would be impossible for natural pasturage to excell [excel] this.”

Kendall (1845) described the southern part of the Blackland Prairie as "beautiful rolling prairies, the land rich, and susceptible of cultivation." Roemer's (1849) descriptions of the same region included "open prairie," "extensive prairies" with mesquite trees and scattered oak groves, "undulating prairie extending. . . an immeasurable distance," and "gently rolling, almost treeless plain." Indeed, on the Blackland Prairie, trees were often rare except as riverine forests along streams or as occasional scattered groves or mottes "such as the one near Kentuckytown that gave Pilot Grove [in southeastern Grayson County] its name, the trees being a major landmark in a featureless terrain." (McLeRoy 1993). The riverine forests along Big Mineral Creek [Grayson County] were described by Parker (1856) as "a rich bottom, thickly grown up with large cotton wood, honey locust, overcup [bur oak], and other heavy timber, besides plenty of the bois d'arc." Roemer (1849) described a trading post he visited in Falls County as "on a hill covered with oak trees, two miles distant from the Brazos, above the broad forested bottom of Tohawacony Creek." He further described the wooded bottomland as having "high, dense trees."

Fire was probably an important factor in maintenance of the original prairie vegetation and had a major impact on community structure (Anderson 1990; Collins & Gibson 1990; Strickland & Fox 1993). Tall grass prairie fires, intensely hot, would have been stopped only by the lack of dry fuel or a change in topography. Even streambank vegetation was susceptible during dry years. The end result was that trees were rare even along some stream banks, and prairie margins probably extended somewhat beyond the limits of the soil types usually associated with prairie (Hayward & Yelderman 1991). Roemer (1849) wrote of a prairie fire as follows:

“. . .we, ourselves, were entertained before going to sleep by the spectacle of a prairie fire. Like a sparkling diamond necklace, the strip of flame, a mile long, raced along over hill and dale, now moving slowly, now faster, now flickering brightly, now growing dim. We could the more enjoy this spectacle undisturbed, since the direction of the wind kept it from approaching us. My companion was of the opinion that Indians had without doubt started the fire, since they do this often to drive the game in a certain direction, and also to expedite the growth of the grass by burning off the dry grass.”

While lightning was an important source of naturally started fires, Native Americans were long present in the region and their use of fire is considered by some to be equally important in having maintained North American grasslands (Bragg 1995).

In summary, the original Blackland Prairie seems to have been predominantly tall grass prairie with trees along watercourses, sometimes scattered on the prairie or concentrated in certain areas (e.g., Pilot Grove) possibly as the result of locally favorable soil conditions or topography.

It is interesting to note that early (back to the 1830s) surveyor records of mesquite as the most common tree in presettlement upland prairies in Navarro County suggest ". . .the legendary spread of mesquite into North Texas by longhorn cattle may be an errant concept" (Jurney 1987). Roemer's (1849) mention of "extensive prairies covered with mesquite trees" also points to mesquite as a natural component of the vegetation. However, mesquite has increased in many areas and the observations mentioned above are not so early as to preclude mesquite having already being spread to some extent by land use changes.

While some question the degree to which mesquite was spread by longhorns, animals have had profound impacts on the vegetation since long before settlement. These range from the obvious effects of the bison or beaver to the more subtle but essential roles of pollination and seed dispersal. Present animal life is much different and some species reduced compared with that of presettlement days. In addition to relatively large present-day species such as the white-tailed deer, coyote, fox, and bobcat, a number of other large or interesting species occurred. According to Brooke (1848) writing of Grayson County, black bears were quite common ("I. . .have never tasted any meat I like better.") as were deer; panthers [mountain lions] and wolves were also present. In Brooke's (1848) words, "I have been out a-shooting Deer and Turkeys alone, and when going up the branches of the Rivers I often come across either bear or wolf. . . ." Strecker (1926a) (based on early fur-trader records) indicated that next to the skins of deer, "those of the black bear were of the most value to the Indians of McLennan County." Strecker (1926a) also reported that gray wolves occurred as far east as McLennan County. He indicated that they

“. . . may never have been very common permanent residents of McLennan County, but in late fall and winter, small packs followed the great herds of buffalo and deer from northwestern Texas and remained here for several months. It was probably only a minority that remained throughout the year. Old settlers refer to packs of from five to eight wolves which they considered small family groups.”

Another predator, the ocelot, is thought to have ranged as far north as the Red River (Hall & Kelson 1959). Strecker (1924), for example, reported that ocelot occurred in the bottoms of the Brazos River near Waco in McLennan County. Even jaguar are believed to have ranged north to the Red River; the last jaguar record from North Central Texas was a large male killed in Mills County (Lampasas Cut Plain) in 1903 (Bailey 1905). Mountain lions probably occurred throughout North Central Texas (Schmidly 1983), with Strecker (1926a) indicating they were common in McLennan County in the middle of the 1800s. However, they were rare by the beginning of the 20th century (Bailey 1905) and since that time have been eliminated over most of the region (Schmidly 1983). The collared peccary or javelina, a small wild pig, was also originally present in the southern portion of the area, north to at least the Brazos River valley in McLennan County near Waco (Strecker 1926a; Schmidly 1983; Davis & Schmidly 1994). Other noteworthy large mammals that previously occurred in appropriate habitats of the Blackland Prairie as well as throughout the rest of North Central Texas include river otter, ringtail, and badger (Schmidly 1983; Davis & Schmidly 1994).

The occurrence of bison was documented by Judge John Simpson of Bonham (Fannin County). Simpson, describing a bison hunt in 1833, reported that hunters found "an immense herd" "on the prairie around Whitewright [Grayson County]" (McLeRoy 1993). Parker (1856), in his 1854 journal, states, "But eight years since, herds roamed around the City of Austin, and were frequently seen in the streets; now there are but few to be found south of Red River." Roemer (1849) described bison on the southern Blackland Prairie as follows:

“When on the following morning at daybreak we entered the prairie on which mesquite trees grew scatteringly, the first object that met our view was a buffalo herd, quietly grazing near us. . . . The whole prairie was covered with countless buffalo trails, crossing in all directions, reminding one of a European grazing ground.”

On a different day, Roemer (1849) indicated,

“They covered the grassy prairie separated into small groups and far distant on the horizon they were visible as black specks. The number of those clearly seen must have been not less than a thousand.”

Pronghorn antelope were also native, occurring at least as far east as Fannin County (Hall & Kelson 1959). Smythe (1852) described a small herd on the eastern edge of the Blacklands, Roemer (1849) mentioned sighting pronghorn antelopes near where the Blackland Prairie and Lampasas Cut Plain come together, and Major G.B. Erath, a pioneer of Waco, indicated that antelope were common in what is now McLennan County in the early to middle 1800s (Schmidly 1983). Erath also reported that small herds penetrated as far east as Milam County on the eastern edge of the Blackland Prairie (Strecker 1926a).

While not native, wild horses, descended from those escaped from the Spanish, were by the early 1800s extremely common in Texas and were probably having a significant impact on the vegetation. Ikin (1841), speaking of Texas as a whole, indicated,

“The wild horse which now roams every prairie, sometimes alone, sometimes in herds of more than a thousand, is not native, but the progeny of those which escaped from the early conquerors of Mexico. He is usually a small bony animal about fourteen hands high, with remarkably clean legs, and other signs indicative of good blood. When congregated in bodies of a thousand, these horses form the most imposing spectacle which the prairies present.”

Strecker (1926a) also reported the wild horse as abundant throughout the Brazos Valley of McLennan County at the time of arrival of the first American settlers. He further indicated that early settlers sometimes shot the wild horses to prevent interference with their domesticated stock.

The bird, reptile, and fish faunas were also conspicuously different in significant ways from those today. Brooke (1848), writing about early Grayson County, mentioned both turkeys and prairie chickens, and Smythe (1852) spoke of hunting "Prairie Hens" in what is now Limestone County on the eastern edge of the Blackland Prairie. According to Pulich (1988), both greater and lesser prairie chickens were common in North Central Texas until the 1880s; these two species were locally extinct by the early 1900s. Oberholser (1974) mentioned a specimen record for the greater prairie chicken from Dallas County with a number of other North Central Texas records west of the Blackland Prairie in Clay, Cooke, Denton, and Navarro counties. There is a questionable record for the lesser prairie chicken from Dallas and also records for this species from Cooke and Young counties to the west of the Blacklands (Oberholser 1974). The extinct passenger pigeon is also well documented for the Blackland Prairie. These birds, known as "wild pigeons" by early settlers, were recorded from Collin, Fannin, and Henderson counties, with a number of records even farther west in the Grand Prairie, Lampasas Cut Plain, and West Cross Timbers (Oberholser 1974; Pulich 1988). This once very numerous species rapidly became extinct in North Central Texas, with 1896 being the last record in the area (Oberholser 1974; Pulich 1988). The ivory-billed, one of the world's largest woodpecker species, was also present in bottomland forests in the Blacklands. Oberholser (1974) lists records for Cooke, Dallas, Fannin, and Kaufman counties with sightings as late as the early 1900s (Pulich 1988). Another extinct species, the Carolina parakeet, was known from eastern Texas (Greenway 1958) and was probably also present in the riverine forests of the Blackland Prairie (Goodwin 1983), especially along the Red and Trinity rivers. Even more surprising, alligators were abundant in places, with Kendall (1845) describing them along the San Gabriel in the southern Blackland Prairie as "too plentiful for any useful purposes." This large reptile occurred in appropriate habitats throughout most of the Blackland Prairie, west to Grayson, Dallas, McLennan, and Williamson counties (Brown 1950; Hibbard 1960; Dixon 1987), and are still known to occur in Dallas County. Kendall (1845) further indicated concerning the San Gabriel that "The stream abounds with trout, perch, and catfish, as do nearly all the watercourses in this section of Texas."

During the Pleistocene, an even more extensive megafauna occurred in the area (Smeins 1988), as shown by the excavation of a mammoth from near Flowing Wells (Grayson County) by Dr. Daniel Schores and a student team from Austin College (D. Schores, pers. comm.). Further, fossils of at least three elephant species, including mammoth and mastadon, are known from the Dallas area (Shuler 1934). An even more impressive site containing a large (20+) mammoth herd was found near Waco in a Brazos River terrace dated around 28,000 years BP (Fox et al. 1992; C. Smith, pers. comm.). Several woody plants found in the Blackland Prairie region seem to have adaptations that are difficult to explain based on interactions with the present fauna. Bois d'arc (Maclura pomifera), honey-locust (Gleditsia triacanthos), and mesquite (Prosopis glandulosa) all have fruits that are adapted for dispersal by large animals (megafauna) and seem to fit Janzen and Martin's (1982) hypothesis that large, now extinct animals were involved in the evolution of certain "anachronistic" plant characteristics we see today. Another such possible characteristic is the protective armature displayed by honey-locust. The long, stout, branched thorns, up to a foot or more long, would seem perfectly reasonable in Africa where there are abundant large herbivores, but rather out of place in northern Texas where currently no large native browsers exist.

In general, the animals of the Blacklands have faunal affinities with the eastern woodlands, the Great Plains, and the southwestern United States (Schmidly et al. 1993). A recent, now very abundant, southern addition to the fauna is the nine-banded armadillo. This species is originally native to South America, and as recently as the 1870s to 1880 was found only at the southern tip of Texas (Strecker 1926b; Phelan 1976). Since that time it has spread extensively and is now found hundreds of miles north of Texas (Hall & Kelson 1959). Armadillos were at least sporadic as far north as the Red River by the early 1930s, but did not become common there until the 1950s (H. McCarley, pers. comm.).

The earliest use of the Blackland Prairie by settlers was as grazing areas for herds of cattle or horses. According to Hayward and Yelderman (1991) ". . . the Blackland Prairie supported some of the earliest of large-scale ranching efforts in Texas, complete with pre-Civil War cattle drives to St. Louis and Chicago." Brooke (1848 ) stated that, ". . . the cattle and horses feed on the prairies all winter; no need of laying up winter food." Parker (1856) wrote of a herd of 1,200 wild cattle being driven north across the Red River at Preston (Grayson County).

While limited "sod plowing" occurred quite early (Smythe 1852), it wasn't until the 1870s and 1880s, with the coming of the railroads and the development of special plows and favorable economic conditions, that extensive "breaking of the prairie" and exploitation of its agricultural potential finally occurred (Hayward & Yelderman 1991). Once farming on the Blacklands was possible, widespread cultivation of the rich soils, perhaps as rich as any in the nation (Hayward & Yelderman 1991), was inevitable and farming quickly replaced ranching. Cotton soon became an important crop and thus began the era referred to as the Cotton Kingdom. According to Sharpless and Yelderman (1993), for seventy years more cotton was grown on the Blackland Prairie than any other region of the world. Hill (1901) said, "In fact these calcareous soils . . . of the Black Prairies are the most fertile of the whole trans-Mississippi region." Others (e.g., Sharpless and Yelderman 1993) have said the soil is arguably the most fertile west of the Mississippi River. Within a very short time, most of the accessible and desirable land was put into cultivation, and according to Burleson (1993), by 1915 the human population on the Blacklands was greater than on any other United States area of comparable size west of the Mississippi. The result was the virtually complete destruction of native Blackland Prairie communities. With the exception of small or inaccessible areas and a relatively few hay meadows valued for their native grasses, almost nothing remains of the tall grass prairies that were once so abundant. Estimates of the destruction of this ecosystem range from 98% (Hatch et al. 1990) or 99% (Riskind & Collins 1975) to more than 99.9 % (Burleson 1993).

Geology of the Blackland Prairie

The Blackland Prairie is on an erosional landscape developed from easily erodible Cretaceous shales, marls, and limestones that dip gently to the east (Hayward & Yelderman 1991). It originally consisted of four somewhat different parallel north-south bands of vegetation: the Eagle Ford Prairie, the Whiterock Prairie, the Taylor Black Prairie, and the Eastern Transitional Prairie. These all correspond to underlying geologic layers (Hayward & Yelderman 1991).

The westernmost and geologically oldest portion of the Blackland Prairie, known as the Eagle Ford Prairie, is developed on the Eagle Ford Shale, Upper Cretaceous material deposited about 92 to 90 million years ago (Hayward & Yelderman 1991). This layer crops out just east of the Woodbine Sand, on which the East Cross Timbers are found. While variable, the Eagle Ford Shale is principally a dark bluish-gray to nearly black shaly clay (Bullard 1931) that weathers to form black vertisol soils supporting prairie vegetation.

Cropping out to the east of the Eagle Ford Shale is the slightly younger Austin Chalk, deposited about 90 to 85 million years ago. This layer, which supports the Whiterock Prairie, forms the elevated backbone or "axis" of the Blacklands (Hayward & Yelderman 1991). It is a strikingly white, very fine-grained limestone, called chalk, made primarily of millions of calcium carbonate cell walls of tiny marine algae. Similar deposits make up the famous white cliffs of Dover in southern England and are used commercially as writing chalk. The Austin Chalk is a relatively resistant hard layer (Dallas Petroleum Geologists 1941) compared to the surrounding shales, and because of this hardness, it forms a rather conspicuous escarpment from Sherman to Dallas and south to Austin. This topographic feature is sometimes referred to as the "white rock escarpment," "white rock scarp," (Hill 1901) or "white rock cuesta," and although it never exceeds 200 feet in elevational difference from the surrounding terrain (usually much less), it is the most conspicuous topographic feature in the Texas Blacklands (Hill 1901; Montgomery 1993). It typically crops out as a west-facing bluff or escarpment overlooking a prairie formed on the less resistant Eagle Ford Shale (Bullard 1931). In striking contrast to the Grand Prairie with its numerous resistant layers, the Austin Chalk is the only resistant, escarpment-forming layer underlying the entire Blackland Prairie. As a result, most of the Blackland Prairie is gently rolling, in contrast to the sharper, more angular topography of the Grand Prairie (Hill 1901). Surprisingly, the extremely white Austin Chalk weathers to form a sticky black soil, typically thinner than, but similar to that derived from the Eagle Ford Shale (Bullard 1931) (Fig. 9). Where this soil is eroded away, as on stream banks, a distinctive flora can be found on the exposed chalky limestone (see description under vegetation). Despite their biological diversity, these exposed chalky areas are of little commercial value and are thus often destroyed by contouring or other types of "remediation."

The layers that crop out to the east of the Austin Chalk are the Taylor marls and sandy marls, laid down about 79 to 72 million years ago. The Taylor Blacklands, the largest of the four Blackland Prairie belts, occur on the soils derived from these rocks (Hill 1901; Hayward & Yelderman 1991). In fact, Taylor sediments underlie about two-thirds of the total Blackland Prairie (Hill 1901). The soils developed on Taylor rocks are the classic deep, rich, calcareous, "black waxy" soils that were formerly so valuable for cotton production.

Finally, the easternmost and youngest Cretaceous rocks supporting Blackland Prairie are those of the Navarro group, deposited about 72 to 68 million years ago (Hayward & Yelderman 1991). These deposits crop out from Red River County in the north, through Kaufman and Navarro counties, south to Williamson County on the southeastern margin of North Central Texas. They break down into a soil with a somewhat higher sand content than the Blackland soils farther west, and support the easternmost of the Blackland Prairies, the Eastern Transitional or Marginal Prairie (Hill 1901; Hayward & Yelderman 1991). While easier to till, these soils are poorer in nutrients and thus not as valuable for farming (Hayward & Yelderman 1991). Immediately to the east of the Navarro Group and east of North Central Texas, on younger sandy deposits of Tertiary-age, the Post Oak Savannah begins, marking the western edge of the eastern deciduous forest.

Vegetation of the Blackland Prairie

According to Gould and Shaw (1983), the Blackland Prairie (and in fact all of North Central Texas) is part of the True Prairie grassland association, extending from Texas to southern Manitoba. This is one of the seven grassland associations of North America recognized in the classification system of Gould (1968) and Gould and Shaw (1983) (Fig. 19). Based on location, climate, and vegetational characteristics, the tall grass prairies of the Texas Blacklands can be considered part of either the True Prairie or Coastal Prairie associations (Collins et al. 1975). They lie at the very southern end of the True Prairie association, but are also connected to the Texas Coastal Prairie. Rainfall values are intermediate and the Blackland Prairies have most of the vegetational dominants of both these areas. According to Collins et al. (1975), adequate data are not currently available for a clear determination. Many authorities, however, recognize the tall grass prairies of the Blacklands as an extension of the True Prairie with little bluestem (Schizachyrium scoparium) as a climax dominant (Allred & Mitchell 1955; Thomas 1962; Correll 1972; Gould & Shaw 1983; Simpson & Pease 1995).

Seven different specific grassland communities occurring on three main soil associations are recognized by Collins et al. (1975) as occurring in the Blackland Prairie. Diamond and Smeins (1993), however, recognize five major tall grass communities in the main body of the Blacklands (Fig. 18). Three of these types, the Schizachyrium-Andropogon-Sorghastrum (little bluestem-big bluestem-Indian grass), Schizachyrium-Sorghastrum-Andropogon (little bluestem-Indian grass-big bluestem) and Schizachyrium-Sorghastrum (little bluestem-Indian grass), are relatively similar, have little bluestem as the prevailing dominant, and occur over the majority of the Blacklands (Diamond & Smeins 1993). Associated species include Bouteloua curtipendula (side-oats grama), Carex microdonta (small-toothed caric sedge), Sporobolus compositus (tall dropseed), Nassella leucotricha (Texas winter grass), Acacia hirta (prairie acacia), Bifora americana (prairie-bishop), Hedyotis nigricans (prairie bluets), and Hymenopappus scabiosaeus (old-plainsman) (Diamond & Smeins 1985). The microtopographical features known as "hog wallows" or gilgai are often found on prairies of these types and provide important microhabitat variation based on differences in water, nutrient relations, and frequency of disturbance (Diamond & Smeins 1993). Vegetational differences associated with the microhighs and microlows are easily observed.

The other two Blackland communities are quite different vegetationally and are relatively limited in occurrence. The Tripsacum-Panicum-Sorghastrum (eastern gamma grass-switch grass-Indian grass) community is ". . . found over poorly drained Vertisols in uplands of the northern Blackland Prairie and in lowlands throughout the Texas tallgrass prairie region" (Diamond & Smeins 1993). Examples can be found in Grayson and Fannin counties. Additional common species include Bouteloua curtipendula (side-oats grama), Carex microdonta (small-toothed caric sedge), Paspalum floridanum (Florida paspalum), Sporobolus compositus (tall dropseed), Acacia hirta (prairie acacia), Aster ericoides (heath aster), Bifora americana (prairie-bishop), Hedyotis nigricans (prairie bluet), Rudbeckia hirta (black-eyed susan), and Ruellia humilis (prairie-petunia) (Diamond & Smeins 1985). The Sporobolus-Carex (silveanus dropseed-mead sedge) community, dominated by Sporobolus silveanus (Silveus' dropseed) and Carex meadii (mead's caric sedge), is found in the northern Blackland Prairie on low pH Alfisols in areas of relatively high precipitation (Diamond & Smeins 1993). An example can be seen on the Nature Conservancy's Tridens Prairie in Lamar County. Other common species found in this community type include Panicum oligosanthes (Scribner's rosette grass), Fimbristylis puberula, Coelorachis cylindrica (Carolina joint-tail), Panicum virgatum (switch grass), Paspalum floridanum (Florida paspalum), Sporobolus compositus (tall dropseed), Aster pratensis (silky aster), Linum medium (Texas flax), and Neptunia lutea (yellow-puff) (Diamond & Smeins 1985). The microtopographical feature known as mima mounds is commonly associated with this community. Like gilgai, mima mounds provide microhabitat variation, increasing the overall biological diversity of the prairie ecosystem (Diamond & Smeins 1985).

Also worth mention is the special assemblage of herbaceous plants often seen on areas of very thin soil and especially on exposed outcrops of the Austin Chalk (Stanford 1995). Species seen in this type of setting in the northern Blackland Prairie (Grayson County) include Baptisia australis (wild blue-indigo), Callirhoe pedata (finger poppy-mallow), Eriogonum longifolium (long-leaf wild buckwheat), Grindelia lanceolata (gulf gumweed), Ipomopsis rubra (standing-cypress), Linum pratense (meadow flax), Marshallia caespitosa (Barbara's-buttons), Oenothera macrocarpa (Missouri primrose), Paronychia jamesii (James' nailwort, and Thelesperma filifolium (greenthread). At some seasons, these outcrops have the aspect of barren eroded rock; in the spring, however, they are covered with spectacular displays of color.

As can be seen above, there is considerable variation in the tall grass prairie communities of the Blacklands (Diamond & Smeins 1993), and disagreement about specific community types (Simpson & Pease 1995). However, common dominant grasses of this tall grass prairie ecosystem include Schizachyrium scoparium (little bluestem), Andropogon gerardii (big bluestem), Sorgastrum nutans (Indian grass), Panicum virgatum (switch grass), Tripsacum dactyloides (eastern gamma grass), Sporobolus compositus (tall dropseed), Eriochloa sericea (Texas cup grass), Paspalum floridanum (Florida paspalum), and Tridens strictus (long-spike tridens) (Collins et al. 1975). Despite similarities in general aspect and even the occurrence of certain species over broad areas, the particular community present and the dominants observed can vary considerably even over short distances, primarily on the basis of differences in soil. Localized patches of a community type well beyond its main zone of occurrence are common, based on soil or other factors. Therefore most of the Blackland Prairie is a complex mosaic of tall grass communities; an example of this can be seen in northern Grayson County where four of the community types discussed above can be seen within a few miles.

Although prairie predominated, some wooded areas were also natural components of the Blackland Prairie region at the time of settlement. Examples include bottomland forests and wooded ravines along the larger rivers and streams, mottes or clumps in protected areas or on certain soils, scarp woodlands on slopes at the contact zones with the Edwards Plateau and Lampasas Cut Plain, and scattered upland oak woodlands similar to the Cross Timbers (Gehlbach 1988; Nixon et al. 1990; Diamond & Smeins 1993). In areas such as Dallas, where the Austin Chalk forms a conspicuous escarpment or bluff, a characteristic woody vegetation is also found in the varied microhabitats associated with this topographic feature. Kennemer (1987) indicated that Fraxinus texensis (Texas ash), Quercus sinuata var. breviloba (shin oak), and Ulmus crassifolia (cedar elm) are dominant. Other noteworthy woody plants of the escarpment include Cercis canadensis var. texensis (Texas redbud), Juniperus ashei (Ash juniper), Morus microphylla (Texas mulberry), and Ungnadia speciosa (Texas buckeye). Farther south, in Bell, Hill, and McLennan counties, the Austin Chalk scarp vegetation is similar. Depending on slope and moisture conditions, characteristic species include Celtis laevigata (sugarberry), Diospyros texana (Texas persimmon), Forestiera pubescens (elbow-bush), Fraxinus texensis, Ilex deciedua (deciduous holly), Juniperus ashei, Juniperus virginiana (eastern red-cedar), Ptelea trifoliata (hoptree), Quercus buckleyi (Texas red oak),Quercus fusiformis (Plateau live oak), Quercus sinuata var. breviloba, and Ulmus crassifolia (Gehlbach 1988).

As indicated earlier, with the exception of preserves, small remnants, or native hay meadows, almost nothing remains of the original Blackland Prairie communities. According to Diamond et al. (1987), all of the tall grass community types of the Blackland Prairie are ". . .endangered or threatened, primarily due to conversion of these types to row crops." Three specific Blackland communities are considered "threatened natural communities" by the Texas Organization for Endangered Species (TOES 1992). Conversion of the Blackland Prairie for agriculture was the most important cause of the destruction of this ecosystem, with only marginal, often steeply sloped land not rapidly brought under cultivation. High prices for cotton and grains eventually resulted in the cultivation of even these marginal areas, ". . .with disastrous effects. Blackland soils on steep slopes, stripped of their protective grass, eroded rapidly. Gullying was everywhere, and in a few years, over much of the marginal slope-lands, as much as three feet of soil had been eroded, exposing barren rock where once was prairie soil" (Hayward & Yelderman 1991). Today, extensive eroded areas and large sections that have been contoured to remedy erosion can be seen in many places throughout the Blacklands.

Existing prairie is still being lost due to a variety of causes. An example is the destruction of Stults Meadow in Dallas, studied in detail by Laws (1962) and Correll (1972). In addition to direct destruction of prairie through cultivation or other uses (e.g., urbanization), existing isolated small prairie remnants are currently being lost through invasion by woody vegetation and introduced species. Given the relatively high rainfall over most of the Blacklands, with the suppression of fire by humans, native trees and shrubs (e.g., Juniperus virginiana---eastern red-cedar, Ulmus crassifolia---cedar elm) as well as introduced species are able to invade and eventually take over areas that were formerly prairie.

Recurrent fire and grazing by bison were natural processes that maintained the Blackland ecosystem; the removal of these processes is a disturbance that causes changes in the vegetation (Smeins 1984; Smeins & Diamond 1986; Diamond & Smeins 1993). In this region, periodic disturbance is essential for the maintenance of prairie. However, even native hay meadows, which are routinely disturbed, are often markedly different from the original vegetation because of the substitution of mowing and particularly past herbicide use in place of fire and grazing. The results include a reduction in broad-leaved plants and an increased abundance of grasses (Diamond & Smeins 1993). While grazing was a natural component of the Blacklands and many other Texas ecosystems, overstocking and thus overgrazing by domesticated animals has also caused a dramatic decline and even near elimination of numerous plants from many areas (Cory 1949). The cumulative effect of all these human-induced changes is that the Blackland Prairie communities have been largely destroyed. Large areas that were once tall grass prairie are now covered by crops or other introduced and now naturalized species such as Bothriochloa ischaemum (King Ranch bluestem), Cynodon dactylon (Bermuda grass), and Sorghum halepense (Johnson grass). Roadsides and pastures are particularly obvious examples; in many cases hardly any native grasses can be found. In these areas there has also been an accompanying dramatic reduction in native forb diversity.

In striking contrast to the terrestrial communities of the Blackland Prairie is the tremendous increase in aquatic habitats. While most native wetlands, including prairie "pothole-like" wetlands, have been lost, with the construction of numerous reservoirs, lakes, ponds, and tanks, there is vastly more habitat for aquatic vegetation than in presettlement days. With the exception of oxbow lakes along some of the larger streams, the only permanent surface water prior to human intervention was in rivers, streams, swampy or marshy areas, beaver ponds, and springs. Introduced as well as native aquatic plants are now widespread and in some cases so abundant as to be problematic weeds. Many aquatic plants probably have populations several orders of magnitude greater than in the relatively recent past. This same pattern holds not just for the Blackland Prairie, but for all vegetational areas within the North Central Texas area.

Cross Timbers And Prairies

Occurrence of the Cross Timbers and Prairies

The Cross Timbers and Prairies (vegetational area 5 of Hatch et al. 1990), an area of about 26,000 square miles (about the size of West Virginia), occupies the region south of the Red River between the Blackland Prairie to the east, the Rolling Plains to the west, and the Llano Basin (Central Mineral Region) and Edwards Plateau to the southwest and south. Vegetationally it is quite diverse and includes the East and West cross timbers, the Fort Worth Prairie, and the Lampasas Cut Plain (Fig. 1). Notable physiographic features included are the Comanche Plateau, the Palo Pinto Country (sometimes referred to as the Palo Pinto Mountains), the mesa and butte country of the Lampasas Cut Plain, and the eastern portion of the Callahan Divide (Fig. 8).

The Cross Timbers, which stretch from Texas north through Oklahoma to Kansas (Marriott 1943; Dyksterhuis 1948; Kuchler 1974), are found in Texas from the Red River south for about 150 miles. They are actually two discrete belts of forest divided by the enclosed Grand Prairie (Dyksterhuis 1948). Surrounded by prairie on both sides (Blackland Prairie to the east, Rolling Plains to the west), they represent a final disjunct western extension of woody components of the eastern deciduous forest before the vegetation changes into the vast expanse of central U. S. grasslands known as the Great Plains. The two separate belts are the East Cross Timbers and the West Cross Timbers, sometimes referred to as the Lower Cross Timbers and Upper Cross Timbers respectively. According to Hill (1887), these names developed because the West or Upper Cross Timbers is at a greater altitude and in a more upstream position relative to the flow of rivers in the area. The East Cross Timbers is a narrow strip (roughly along the 97th meridian) extending from the Red River, in eastern Cooke and western Grayson counties, south to near Waco where it merges with the riverine forests of the Brazos River (Hayward & Yelderman 1991). This southernmost portion of the East Cross Timbers, developed on the sandy terraces of the Brazos River, is sometimes referred to as the "false" East Cross Timbers to distinguish it from the upland or "true" East Cross Timbers farther north. The two areas are continuous but can be distinguished by topography---flat on the river terraces and gently rolling in the uplands (Hayward et al. 1992). The somewhat wider West Cross Timbers stretches west from the Grand Prairie to the beginning of the Rolling Plains and includes the rather rugged Palo Pinto Country (in Eastland, Jack, Palo Pinto, Stephens, and Young counties).

The Fort Worth Prairie portion of the Grand Prairie extends as a continuous body of open grasslands, roughly 10 to 30 miles wide, from near the Red River in the north, south about 110 miles to where it ends in the wooded area along the Brazos River near the Johnson County-Hill County line (Dyksterhuis 1946) (Fig. 1).

The Lampasas Cut Plain, the largest portion of the Grand Prairie, is highly dissected butte and mesa country with extensive lowlands, and can in some ways be considered a northern extension of the Texas Hill Country and Edwards Plateau. It has strong geologic and floristic links with the Edwards Plateau as discussed in the sections below on the geology of the Grand Prairie and the vegetation of the Lampasas Cut Plain. It is, in fact, considered a part of the Edwards Plateau by some authorities (e.g., Riskind & Diamond 1988). The Lampasas Cut Plain extends from the Fort Worth Prairie south and west to the Llano Basin (also called the Central Mineral Region or Burnet Country) and the Colorado River (Figs. 1, 8).

Presettlement and Early Settlement Conditions in the Cross Timbers

As has been noted by historical geographer Richard Francaviglia, the natural and cultural histories of the Cross Timbers are inseparable, for human populations have had an impact on the region for thousands of years---first through the Native Americans' use of fire, then in the 19th century through the European Americans' agricultural and grazing practices, and more recently in the form of urbanization, suburbanization, and conservation activities (Francaviglia, forthcoming). Francaviglia notes that the Texas Cross Timbers has become what geographers call a "vernacular region" in the late 20th century, but that the term was used as early as the 1820s to characterize its distinctive vegetation. The exact origin of the term Cross Timbers is not known, but Dyksterhuis (1948) stated that the name

“ . . presumably alludes either to the fact that this forest extends north and south across, rather than along, the major streams all of which flow eastward; or to the fact that early westward travelers who had left the main body of the great eastern forest and entered upon open prairie found it necessary to cross yet another body of forest before entering upon the grasslands that extended to the Rocky Mountains.”

The abrupt appearance of the East Cross Timbers was quite striking for westbound travelers who had just crossed the extensive open Blackland Prairie. This conspicuous change in the vegetation served as a landmark recognized by almost all early travelers, was a principal marker on early maps (e.g., Holley 1836; Ikin 1941; Gregg 1844; Kendall 1845), and was discussed in many immigrant guides and early explorer accounts (e.g., Marryat 1843; Roemer 1849; Parker 1956). Kennedy (1841), based on accounts of local residents, wrote of the Cross Timbers [not distinguishing East and West],

“This belt of timber varies in width from five to fifty miles. Between the Trinity and Red Rivers it is generally from five to nine miles wide, and is so remarkably straight and regular, that it appears to be a work of art. When viewed from the adjoining prairies on the east or west, it appears in the distance as an immense wall of woods stretching from south to north in a straight line, the extremities of which are lost in the horizon.”

Regarding their use as a landmark, he further stated,

“As might naturally be supposed, the Cross Timber forms the great landmark of the western prairies; and the Indians and hunters, when describing their routes across the country, in their various expeditions, refer to the Cross Timber, as the navigators of Europe refer to the meridian of Greenwich. If they wish to furnish a sketch of the route taken in any expedition, they first draw a line representing the Cross Timber, and another representing the route taken, intersecting the former.”

Together with the west-east rivers such as the Red and farther south, the Trinity, they formed a kind of navigational grid in an otherwise rather featureless landscape (Phelan 1976). Parker (1856) noted that "the long stretches of prairie, although undulating, present no object so prominent as the belt of timber which bounds them." He further described the East Cross Timbers as

“ . . a very singular growth. The one we had now entered is called the Lower Cross Timbers, and is about six miles wide. . . . The timber is a short, stunted oak, not growing in a continuous forest, but interspersed with open glades, plateaus, and vistas of prairie scenery, which give a very picturesque and pleasing variety.”

Marcy (1853, 1866), based on extensive travel in the area, said,

“At six different points where I have passed through it [Cross Timbers], I have found it characterized by the same peculiarities; the trees, consisting principally of post-oak and black-jack, standing at such intervals that wagons can without difficulty pass between them in any direction. The soil is thin, sandy, and poorly watered.”

This statement agrees with numerous early settler accounts (Dyksterhuis 1948). Other early references, however, such as that by Kendall (1845) with the Texan Santa Fe Expedition in 1841, referred to the Cross Timbers in places as "almost impenetrable" and "full of deep and almost impassable gullies." Kendall (1845) further stated,

“The ground was covered with a heavy undergrowth of briers and thorn-bushes, impenetrable even by mules, and these, with the black jacks and post oaks which thickly studded the broken surface, had to be cut away, their removal only showing, in bolder relief, the rough and jagged surface of the soil which had given them existence and nourishment.”

Some other early travelers also considered the Cross Timbers difficult to cross and an obstacle to travel because of the vegetation and topography (Dyksterhuis 1948). Gregg (1844), for example, wrote of them as follows:

“Most of the timber appears to be kept small by the continual inroads of the 'burning prairies;' for, being killed almost annually, it is constantly replaced by scions of undergrowth; so that it becomes more and more dense every reproduction. In some places, however, the oaks are of considerable size, and able to withstand the conflagrations. The underwood is so matted in many places with grape-vines, green-briars, etc., as to form almost impenetrable 'roughs'. . . .”

Another example is Smythe (1852), who referred to the "Lower Cross Timbers" as

“ . . chiefly low scrubby Post Oak groves, extremely tangled and thick, with millions of green briers, . . . making it truly a difficult task to make your way without serious damage to your skin, and clothes.”

Marrayat (1843) described the Cross Timbers in similar fashion:

“During two or three days we followed the edge of the wood, every attempt to penetrate into the interior proving quite useless, so thick were the bushes and thorny briers.”

The Cross Timbers vegetation at the time of contact by Europeans thus probably exhibited considerable variation. The boundaries in particular were probably variable and at least in places were not completely distinct. Parker (1856) described the area just west of the East Cross Timbers but east of what he referred to as the Grand Prairie (east of Gainesville in Cooke County) as follows:

“ . . soon leaving the timber, we entered upon a broken country, consisting of ridges of sand and limestone, interspersed with small prairies and small strips of timber, principally black jack, until we emerged upon and crossed Elm Fork of the Trinity, where, on account of the intense heat, Captain Marcy determined to halt and encamp, thereafter, intending to march by moonlight, until we reached the Grand Prairie.”

The variable nature of the Cross Timbers is also reflected in the following description from Kendall (1845):

“The growth of timber is principally small, gnarled, post oaks and black jacks, and in many places the traveller will find an almost impenetrable undergrowth of brier and other thorny bushes. Here and there he will also find a small valley where timber is large and the land rich and fertile, and occasionally a small prairie intervenes; but the general face of the country is broken and hilly, and the soil thin.”

In at least some areas the timber was extensive. An example is Jordan's (1973) assertion that Forestburg, a small community in southeastern Montague County, boasted six sawmills by 1895.

The animal life of the Cross Timbers was probably similar to that described earlier for the Blackland Prairie. Because of the lower human population size and more native vegetation, slightly more animal life probably survives in the Cross Timbers. A recent (spring 1996) mountain lion sighting in Lake Mineral Wells State Park is an example.

Geology of the East Cross Timbers

The narrow band of woody vegetation between the Blackland Prairie and the Grand Prairie, known as the East Cross Timbers, occurs largely on sandy soil derived from rocks of the Woodbine formation (Hill 1901), which were deposited about 96 to 92 million years ago (Hayward & Yelderman 1991). According to Hill (1901), the formation is "largely made up of ferruginous [iron containing], argillaceous sands, characterized by intense brownish discoloration in places, which are accompanied by bituminous laminated clays." Like the Trinity Group sands underlying the West Cross Timbers, the Woodbine sands are often unconsolidated and rather loose, but differ in having more iron and other minerals. The post oak-blackjack oak vegetation typical of the East Cross Timbers does well on the deep loose soils developed from these unconsolidated layers. In some instances, however, the iron minerals consolidate the sand layers into dark-brown siliceous iron ore (Hill 1901). These iron deposits are so abundant in places that they cap low, wooded, erosion-resistant hills and isolated knobs known as "iron-ore knobs." These are found from Hill County north through Johnson, Tarrant, Denton, and Cooke counties and east into Grayson County (Hill 1901) where the Woodbine formation swings east along the Preston Anticline (a buckle in the strata exposing deeper older layers). The bed of the Red River follows the outcrop of the Woodbine from Grayson County east along the northern boundary of North Central Texas all the way to Red River County in far northeastern Texas (Hill 1901). In general, the Woodbine formation forms a layer of loose brownish sand cropping out just west of the Eagle Ford Shale. Economically the Woodbine is important because its sands contain significant amounts of water and it serves as one of the main aquifers for some areas (Baker 1960).

Geology of the West Cross Timbers

In general, west of the Fort Worth Prairie and north of the Lampasas Cut Plain lies an area of easily erodible sandy soils developed from the Trinity Group (Paluxy, Antlers, and Twin Mountain-Travis Peak sands), the oldest of the Cretaceous layers in North Central Texas (Hill 1901; Renfro et al. 1973). These sands, which represent shallow-water or near-shore sea deposits, are fine grained and so loose that they are ". . .readily cut with pick and spade." They are therefore locally known as "pack-sands" (Hill 1901). The West Cross Timbers have developed in part on the sandy soils derived from such strata. These permeable layers are also important as a source of ground water (Baker 1960), which is still used today by many North Central Texas communities. Because of the rather irregular pattern of outcropping and numerous remnant areas of these sands (Hill 1901), the West Cross Timbers is not an easily delineated region. Further, the sedimentary layers from which the soils of the West Cross Timbers formed are not homogeneous; instead these mainly sandy strata have substantial sections of clay and sandy clay. Therefore, numerous glade-like prairies formed on soils derived from these clayey outcrops can be found scattered through this mostly timbered region (Hayward et al. 1992). The area is further complicated by topographic extensions of the Lampasas Cut Plain that extend north into the West Cross Timbers in areas including Erath and Hood counties. Comanche Peak, a noted landmark in Hood County (Hill 1901), is a good example of such an extension.

North of the easternmost part of the Callahan Divide mesa country, and forming the northwestern portion of the West Cross Timbers, is an area known as the Palo Pinto Country (Fig. 8). This rather rugged region is underlain by the oldest rocks exposed in North Central Texas, deposited during the Pennsylvanian Period (Fig. 7) (the Pennsylvanian extends from 320 to 286 million years ago). According to Hill (1901) the Pennsylvanian (Carboniferous in his terminology) is

“ . . largely made up of soft, impure shales alternating with harder, coarse, brown sandstone and conglomerates, produces ridge-like mountains and a broken belt of country . . . composed of rough-scarped and flat-topped sandstone plains and hills of circumdenudation, surrounded by and overlooking wide clay valleys called 'flats.' ”

The area is deeply dissected and is essentially a cut plain marked by scarps, mesas, and canyons with flat areas of extensive beds of shale outcrop (Hill 1901). The same strata, and consequently landscapes of similar character, extend south of the Callahan Divide and form the Brownwood Country (parts of Brown, Coleman, and Mills counties) (Hill 1901; Renfro et al. 1973).

Vegetation of the Cross Timbers

The East and West cross timbers, with their woody overstory consisting primarily of post oak (Quercus stellata) and blackjack oak (Quercus marilandica), owe their existence to the presence of sandy, slightly acidic soils derived from the Cretaceous Woodbine and Trinity strata (and in the westernmost area to gravelly and rocky Pennsylvanian strata). These soils allow more efficient water infiltration, permit easier penetration of tree roots, and provide more moisture to plants than do heavier clay soils (Allred & Mitchell 1955). The result is that the survival of trees is favored in these areas even though they receive less rainfall than the Blackland Prairie farther east. Hill (1887) first pointed out that the Cross Timbers were developed on sandy soils, and contrasted this vegetation with the adjacent treeless prairies growing on the tight calcareous clay soils developed from limestones.

The original vegetation of both the East and West cross timbers, as based on early accounts (discussed in the section on presettlement conditions), was almost certainly variable, ranging from quite open to dense thickets. However, based on these accounts and on an extensive vegetational study of the West Cross Timbers by Dyksterhuis (1948), presettlement vegetation can probably best be described as a savannah with an oak overstory, but dominated by Schizachyrium scoparius (little bluestem), with two other grasses, Andropogon gerardii (big bluestem), and Sorghastrum nutans (Indian grass), as lesser dominants. Weaver and Clements (1938) also regarded the Cross Timbers as ". . .chiefly oak savanna, in which the grasses are climax dominants." Dyksterhuis (1948) concluded that the current vegetation, even where not cleared for cultivation or pasture, is considerably modified from that present before settlement. He considered the existing understory vegetation a disclimax resulting mainly from overgrazing.

At present, in many Cross Timbers localities, younger trees are often branched to the ground, making movement through the vegetation extremely difficult and denying habitat for the originally dominant grasses; dense cedar breaks are particularly problematic in this regard. Fire suppression, apparently the chief cause of such changes, has thus probably been another major factor responsible for differences from the original vegetation.

Currently, where not completely destroyed, the vegetation ranges from open savannah to dense brush (Correll & Johnston 1970). In addition to the characteristic oaks, other woody species commonly found in the Cross Timbers today include Ulmus crassifolia (cedar elm), Celtis spp. (hackberry), Carya illinoinensis (pecan), Juniperus spp. (juniper), and Prosopis glandulosa (mesquite). Additional common grasses include Bouteloua hirsuta (hairy grama), Bouteloua curtipendula (side-oats grama), Sporobolous compositus (tall dropseed), Panicum virgatum (switch grass), Elymus canadensis (Canada wild-rye), and Nasella leucotrica (Texas winter grass) (Dyksterhuis 1948; Correll & Johnston 1970). Past mismanagement and cultivation have caused many uplands to be covered primarily by scrub oak, mesquite, and juniper with mid- and short-grasses beneath (Hatch et al. 1990).

As the early accounts mentioned above indicate, even in presettlement days the Cross Timbers were probably not continuous unbroken areas of woodland. In the East Cross Timbers some clay is found in the Woodbine formation, and where this clay crops out small prairies are found (Hill 1901). The West Cross Timbers in particular represent a complex pattern of timbered areas interspersed with grasslands or with grassland inclusions (Dyksterhuis 1948). Dyksterhuis (1948) distinguished two areas of woody vegetation within the West Cross Timbers, the "main belt" (developed on sandy Cretaceous strata), and the "fringe" (developed on rocky or gravelly Pennsylvanian strata in the topographically more rugged Palo Pinto Country). The two areas share the same woody species, but differ in other aspects of their vegetation. For instance, Dyksterhuis (1948) found that Buchloe dactyloides (buffalo grass), an important dominant of the drier western plains, was four times as abundant in the fringe as in the main belt. A variety of other plants are more common in one area than the other. Examples include Bouteloua gracilis (blue grama) being more frequent in the fringe, with Bouteloua hirsuta (hairy grama) more common in the main belt (Dyksterhuis 1948). In the fringe in particular, woody vegetation tends to occupy areas of rugged relief with grasses dominating areas of gentler relief (Dyksterhuis 1948).

Tharp (1926) considered the Cross Timbers to be part of the Oak-hickory Association, and as such, part of the eastern deciduous forest. Allred and Mitchell (1955), however, in their broad classification of Texas vegetation, considered the Cross Timbers and even the eastern Texas Post Oak belt to be Post Oak Savannahs that are part of the True Prairie Association. They supported this contention by pointing out that the grasses of the True Prairie are important components in the vegetation of the savannahs. Dyksterhuis (1948), as pointed out above, indicated that little bluestem was the primary dominant. Barbour and Christensen (1993) stated that in the southern part of the tall grass prairie-deciduous forest boundary [including Texas], the ecotone is an oak savannah 50--100 km wide. Clearly, whether classifed as forest or grassland, the Cross Timbers are part of this ecotone. With their rather limited tree diversity and high grass diversity and dominance, they are intermediate vegetationally, as well as geographically, between the eastern deciduous forests and the western grasslands.

One of the most striking features of the Cross Timbers is that this vegetational area contains significant remnants of virgin forests (Stahle & Hehr 1984; Stahle et al. 1985). According to Stahle (1996a), ". . . literally thousands of ancient post oak-blackjack oak forests still enhance the landscapes and biodiversity of. . . the Cross Timbers along the eastern margin of the southern Great Plains. . . ." As as result, this is one of the largest relatively unaltered forest vegetation types in the eastern United States (Stahle & Hehr 1984). The small stature and often poor growth form of post and blackjack oaks made these species commercially unattractive and therefore less subject to systematic logging than other more productive forest types. Extensive dendrochronological (tree-ring) data from post oaks in the Cross Timbers indicate that old-growth remnants of post oak-blackjack forest can be found in numerous localities throughout the region. However, while extensive remnants remain, they are often degraded by various human activities such as heavy grazing or selective cutting and their authenticity is rarely noticed or protected (Stahle & Hehr 1984; Stahle 1996a). In comparison with areas in Oklahoma, the Cross Timbers of Texas are more degraded, in part because of the longer history of settlement (D. Stahle, pers. comm.). Examples of old-growth forests in North Central Texas are found in Comanche County (Leon River), Tarrant County (Fort Worth Nature Center), and Throckmorton County (Nichols Ranch) (Stahle et al. 1985; Stahle 1996a). Tree-ring chronologies extending from about 200 to 300 years have been obtained from these North Central Texas sites, with individual trees dating back to 1681. Such data are readily and harmlessly secured by using a Swedish increment borer to obtain a small diameter (<1 mm) core from the bark to the center of a tree; after careful polishing and under magnification, the annual growth rings can be counted (Stahle et al. 1985; 1996b). Because of the low availability of moisture, rocky or infertile soil, and other factors, the trees of these relict forests, while old, have a slow rate of growth and are of relatively small size, the canopy ranging from only about 6 to 15 meters high (Stahle et al. 1985). Such old-growth forests or ancient individual trees can often be located by environmental factors such as steep, rocky, infertile soils or by the appearance of the individual trees (Stahle & Chaney 1994). Twisted stems, dead tops and branches, canopies restricted to a few heavy limbs, branch stubs, fire and lightning scars, leaning stems, exposed roots or root collars, and hollow voids are all hints of significant age (Stahle 1996a; 1996b) (Fig. 20).

The centuries-long tree-ring chronologies obtained from these relict forests are a valuable source of information about past climate and are particularly important at a time when climate change is a topic of national and global concern. These forests also represent an irreplaceable resource and an unparalleled living record about the North Central Texas area prior to the time of European settlement. Further, because they are relatively unaltered, these remnants may represent areas of significant remaining biodiversity in an otherwise highly altered and reduced diversity environment. Finally, they provide a unique conservation opportunity to preserve some of the last remaining virgin North American forests.

Presettlement and Early Settlement Conditions in the Grand Prairie (Fort Worth Prairie and Lampasas Cut Plain)

Like the Blackland Prairie, the presettlement Grand Prairie was largely a vast grassland, with woody vegetation generally limited to areas along the larger watercourses, as scattered mottes on hilltops or associated with mesas and buttes. Hill (1887) summarized the Grand Prairie as "a prairie region, utterly destitute of timber" and Kendall (1845) described the area as follows:

“To the east, for miles, the prairie gently sloped, hardly presenting a bush to relieve the eye. In the distance, the green skirting of woods, which fringed either border of a large stream, softened down the view. . . .To the west . . . the immediate vicinity was even more desolate, but the fertile bottoms of the Brazos, with their luxuriant growth of timber, were still visible, and the Camanche [Comanche] Peak, rising high above the other hills, gave grandeur and sublimity to a scene which would otherwise have been far from monotonous. "

Referring to another Grand Prairie locality, Kendall (1845) said, "As far as the eye could reach . . ., nothing could be seen but a succession of smooth, gently-undulating prairies."

In reference to the Brazos River valley where it cuts through the Lampasas Cut Plain, Kendall (1845) said,

“The valley of the Brazos at this place abounded with every species of timber known in Texas; grapes, plums, and other fruit were found in profusion; honey could be obtained in almost every hollow tree; trout and other fish were plentiful in the small creeks in the neighborhood, and the woods and prairies about us not only afforded excellent grazing for our cattle and horses, but teemed with every species of game---elk, deer, bears, wild turkeys, and, at the proper season, buffalo and mustang. "

Greer (1935), in describing the Grand Prairie in the period 1850--1890, wrote of an ". . . indescribably beautiful prairie where lush grass swept my mount's sides. . . ." An early settler account (Hattie Richards Sparks in Pool 1964) of Bosque County describes it as follows:

“It was a beautiful prairie country, with sage grass as tall as your head. . . . It was a common occurrence for prairie chickens to fly into the house. I shall never forget when one flew in during our noonday meal and lit in a bowl of soft butter. . . . Wolves were plentiful and I will always remember their howling around our house at night. It was a mournful sound in that open prairie country. ”

Hill (1887) described this region between the two bands of Cross Timbers as "utterly destitute of timber." However, while Smythe (1852) mentioned "immense fields, with the greatest profusion of delicately painted flowers" and "grassy prairies only bounded by the horizon," he also spoke of "prairie, with an occasional strip of woodland," "beautiful groves of Live Oak . . . crowning every hill . . .," and areas with "densely tangled cedar ravines." The Grand Prairie vegetation thus showed considerable variability.

Based on the similarity of vegetation and climate, and on early traveler and settler reports, the original animal life was probably quite similar to that of the Blackland Prairie described earlier. From Kendall's (1845) observations, bison were apparently particularly common:

“ . . I have stood upon a high roll of the prairie, with neither tree nor bush to obstruct the vision in any direction, and seen these animals grazing upon the plain and darkening it at every point. . . . In the distance, as far as the eye could reach, they were seen quietly feeding upon the short prairie grass. . . .”

Kendall (1845) also described seeing many pronghorn antelope on the Upper Brazos. There is even a record of the American alligator as far west as Hamilton County in the central part of the Lampasas Cut Plain (Dixon 1987). Other noteworthy species recorded from the Grand Prairie, but not present there for many decades, include jaguar (Bailey 1905), greater prairie chicken, and passenger pigeon (Oberholser 1974).

In some areas animal populations were apparently reduced early on. Kendall (1845), speaking of another part of the Grand Prairie, said,

“Occasionally a deer would jump suddenly from his noonday rest, and scamper off across the prairie, but other than this no game was seen. The few deer we saw were exceedingly wild, from the fact of there being so many Indians in the vicinity; while the buffalo had evidently all been driven to the south. ”

Native American influence had thus in some areas already modified the fauna, and probably the vegetation as well. Roemer (1849), for example, discussed a Caddo village near the Hill County-Bosque County line as having about one thousand horses and cultivated maize and watermelons.

As with the Blackland Prairie, fire was an important aspect of the presettlement Grand Prairie. Kendall (1845), for example, observed an extensive fire and commented,

“All night the long and bright line of fire, which was sweeping across the prairie to our left, was plainly seen, and the next morning it was climbing the narrow chain of low hills which divided the prairie from the bottoms of the Brazos.”

Settlement, basically the conversion of the prairie to ranching, swept across the Grand Prairie in the relatively short time span of 1850--1860. The importance of ranching and cattle to early Texas is also reflected in the fact that the famous Chisholm and Shawnee cattle trails crossed the Grand Prairie in the 1860s to the 1880s. The general availability of barbed wire in the 1870s and 1880s and subsequent droughts resulted in overstocking and severe overgrazing and thus had a significant detrimental effect on the vegetation (Dyksterhuis 1946). The result of these changes can be seen today in the wide variability of range quality in the area of Grand Prairie still devoted to grazing.

Geology of the Grand Prairie (Fort Worth Prairie and Lampasas Cut Plain)

The following excellent description of the Grand Prairie is from Hill (1901):

“Although often confounded with the Black Prairie, the Grand Prairie differs from it in many minor physical features. In general the surfaces are flat rather than undulating, and the valley slopes are angular (scarped or terraced) rather than rounded. The residual soils and regolith are shallow in comparison with those of the Black Prairie belts, and are of chocolate or brown colors instead of black, although in at least one belt (the Del Rio) the latter color prevails. Owing to the more shallow soil and the decreased rainfall many of the upland areas of the western part of the Grand Prairie are not so well adapted to agriculture, other than grazing, as are those of the Black Prairie, but the valley lands are very fertile and are extensively utilized.

The chief difference between the two regions is that the Grand Prairie is established upon firm, persistent bands of limestones, which are harder than the underlying clay substructure of the Black Prairie region, and which under erosion, result in more extensive stratum plains and more angular cliffs and slopes. These limestone sheets of the Grand Prairie belts also alternate with marls and chalky strata of varying degrees of induration and thickness, and at the base of the whole are unconsolidated sands. The rock sheets of the Grand Prairie are so much harder than those underlying the Black Prairie region and are so conspicuous features in the landscape that, in distinction, the Grand Prairie country has been appropriately called 'the hard lime rock region'. "

In general the surface of the Grand Prairie, especially north of the Brazos [Fort Worth Prairie], is composed of gently sloping, almost level, and usually treeless dip plains, broken only by the valleys of the transecting drainage. These prairies are more continuous and comparatively void of inequalities of erosion along the eastern portion of the area. In the western half, especially south of the Brazos [Lampasas Cut Plain], their surfaces are broken into cut plains, buttes, mesas, and flat-topped divides and are etched by deeply eroded valleys. “

The overall character of the Fort Worth Prairie and Lampasas Cut Plain (Figs. 1, 8) is influenced by such factors as the hardness and slope of the underlying Lower Cretaceous rock layers (Hill 1901). Specific layers cropping out at the surface, from oldest to youngest, include the Twin Mountains-Travis Peak, Antlers, Glen Rose, Paluxy, Walnut, Comanche Peak, Edwards, Kiamichi, Duck Creek, Fort Worth, Denton, Weno, Pawpaw, Mainstreet, and Grayson (Sellards et al. 1932; Renfro et al. 1973). The oldest of these date from the early Cretaceous (which began 145 million years ago), while the youngest, the Grayson, dates from as recently as 98 million years ago (Hayward & Yelderman 1991). Many of these strata include resistant limestone which contributes to the character of what Hill (1901) called the "hard lime rock region." In particular, the Fort Worth Prairie is mostly underlain by layers of firm limestone sloping gently eastward. As one moves from the west to the east, sequentially younger layers are encountered. In general the older western layers disappear under low west-facing escarpments produced by the strata of the next, younger, overlying layer to the east (Hill 1901). It is the exposed ends of the younger layers that form the escarpments and produce the "cuesta" type of topography (Fig. 21) for which the Fort Worth Prairie is known. The Fort Worth Prairie occurs roughly from the Brazos River (northern Hill County) north through Johnson, Tarrant, Denton, and Cooke counties. The area is characterized in large part by gently sloping flat surfaces with thin soil over resistant limestone, extending for miles and ". . . making grass-covered uplands resembling the boundless views of the Great Plains proper" (Hill 1901). Fort Worth itself is built on one of these relatively hard, nearly flat layers, the Fort Worth Limestone.

To the south and southwest these rather flat plains grade into a more rugged, scarped, and dissected area with numerous low buttes and mesas known as the Lampasas Cut Plain (Hill 1901) (Fig. 22). Hill (1901) considered this region to be a modified northern extension of the Edwards Plateau. The term cut plain (or dissected plain) is defined by Hill (1901) as a plain that has been so dissected into remnants by erosion that the level of the original stratum is still recognizable in the summits of the dissected members. The Lampasas Cut Plain is a good example of this type of topography. The eastern portion of the Lampasas Cut Plain, sometimes referred to as the Washita Prairie, is a rolling landscape representing a remnant of the original surface, most of which eventually eroded away to form the highly dissected main portion of the Lampasas Cut Plain to the west (Hayward et al. 1992). This eastern undissected area, while somewhat different topographically, is thus clearly related to the rest of the Lampasas Cut Plain.

The landscape of the main portion of the Lampasas Cut Plain consists of broad grassland valleys that are separated by higher, narrow, often wooded, mesa-like divides (Hayward et al. 1992). These topographic features give the Lampasas Cut Plain a striking and distinctive appearance. The divides have been so eroded that in many places they remain only as flat-topped hills or buttes, often isolated or sometimes in chains. This type of topography can be seen in Bell, Bosque, Comanche, Coryell, Hamilton, Lampasas, Mills, and Williamson counties (Hill 1901); it owes its existence to the hard, white Edwards limestone, the same layer that forms the Edwards Plateau (Hill 1901). In the Lampasas Cut Plain, however, much more of this layer has eroded away, the original surface remaining only as caps on the tops of the relatively few buttes and mesas. Figure 23 is a cross-sectional diagram of such an area. In fact, even though these uplands are the defining feature in the overall appearance of the region, less than ten percent of the area referred to as Lampasas Cut Plain is actually represented by these distinctive flat-topped features (Hayward et al. 1992). The majority of the area is made up of broad sloping valleys between the isolated upland fragments (Hill 1901). Most of the valley floors are formed from the Walnut Clay (Hayward et al. 1992) and the soils derived from this layer supported the original grassland vegetation typical of these valleys. On some of the valley floors, primarily in western portions of the Lampasas Cut Plain, the overlying material has been stripped away to expose the underlying Glen Rose Limestone; in these instances a rockier Glen Rose Prairie developed (Hayward et al. 1992). Often the slopes above the valleys are terraced or benched as the result of differential erosion of the various outcropping layers (Hill 1901).

In some places at the western edge of the Lampasas Cut Plain there is a conspicuous escarpment; this represents the edge of the Edwards limestone. To the south, this layer is less dissected and continues as the surface of that famous physiographic feature of central and western Texas, the Edwards Plateau. Also, some isolated fragments of Edwards limestone extend west through the West Cross Timbers into the Rolling Plains. These fragments cap scattered buttes and mesas in the region known as the Callahan Divide in counties including Brown, Callahan, Coleman, Comanche, and Eastland (Hill 1901) (Fig. 8). The erosion-resistant Edwards limestone, with its tendency to cap and protect more erosion-prone layers, has thus had a major effect on the appearance of large areas of Texas.

At the southern edge of the Lampasas Cut Plain near its boundary with the Blackland Prairie in Bell and Williamson counties, the subsurface geology is particularly important economically. The Edwards Aquifer, the main water source for 1.5 million people in central Texas, extends along the Balcones Fault Zone north to this region. The aquifer, which provides habitat for a number of threatened and endangered species, occurs in the porous Cretaceous layers overlying the Glen Rose Limestone. From the southern part of North Central Texas, the aquifer stretches south to the San Antonio area and west across the Edwards Plateau (Longley 1996). The maintenance of this aquifer is critical not only for the continued existence of a number of plants and animals, but also for the economy of central Texas.

Vegetation of the Fort Worth Prairie

Based on early accounts as described in the section on presettlement conditions, and on a study of relict climax vegetation by Dyksterhuis (1946), a reasonable picture of the original vegetation of the Fort Worth Prairie can be gained. The most striking fact was the absence of trees. In Dyksterhuis' (1946) study of relict areas, Schizachyrium scoparius (little bluestem) was the overwhelming dominant, constituting nearly two-thirds of the total plant cover. Second in importance was Bouteloua curtipendula (side-oats grama), with other significant species including Sorghastrum nutans (Indian grass), Sporobolus asper (tall dropseed), Bouteloua hirsuta (hairy grama), and Andropogon gerardii (big bluestem). Diamond and Smeins (1993) consider the Fort Worth Prairie to be part of the Schizachyrium-Andropogon-Sorghastrum (little bluestem-big bluestem-Indian grass) community, the same tall grass community type found on the shallow soils of the Austin Chalk outcrop in the Blackland Prairie. While the specific underlying strata differ, much of the Fort Worth Prairie, part of Hill's (1901)"hard lime rock region," is also developed on shallow limestone soils. Nassella leucotricha (Texas winter grass) and Bothriochloa laguroides subsp. torreyana (silver bluestem) [referred to by Dyksterhuis as Andropogon saccharoides], species that Dyksterhuis (1946) found to be most abundant in his broad study of the present-day grazing disclimax vegetation, were of almost negligible importance in the relatively undisturbed relicts. The major biotic influence on current vegetation is livestock grazing pressure (Dyksterhuis 1946). In comparison with the Blackland Prairie, much more of the original vegetation of the Fort Worth Prairie has survived, in large part due to the extensive areas of shallow untillable soils that are still used primarily for grazing. Fire suppression and the consequent invasion of woody species has also had an important impact on the vegetation of many areas of the Fort Worth Prairie.

Vegetation of the Lampasas Cut Plain

The vegetation of the Lampasas Cut Plain is more variable than that of the other vegetational areas considered here because of its greater topographic diversity. Depending on the particular conditions present, the vegetation ranges from prairie similar to that found on the Fort Worth Prairie or even the Blackland Prairie, to post oak-blackjack oak woodland similar to the Cross Timbers, to vegetation resembling that of the Edwards Plateau. For example, the easternmost part of the Lampasas Cut Plain, sometimes referred to as the Washita Prairie, is a gently rolling landscape of prairie with scattered oaks on hilltops. This area is characterized in places by rather deep soils with tall grass prairie vegetation similar to the Blackland Prairie, and in other places by shallow soils over hard limestone, with vegetation more closely resembling that of the Fort Worth Prairie (Hayward et al. 1992). To the west, in the more typical butte and mesa country of the main part of the Lampasas Cut Plain, oaks, including Quercus buckleyi (Texas red oak), Quercus fusiformis (live oak), and Quercus sinuata var. breviloba (shin oak), may be found on the rocky Edwards limestone summits of the smaller divides. On the larger divides, areas of deeper soil remain and support westward extensions of the Washita Prairie (Hayward et al. 1992). On the chalky nearly soil-less slopes derived from the underlying Comanche Peak limestone, Quercus sinuata var. breviloba (shin oak), Rhus species (sumac), and Juniperus species (juniper ) may be seen; these dry rocky areas have a distinctly desert-like microclimate (Hayward et al. 1992) and thus support plants with xerophytic adaptations. Below these slopes, on benches in valleys or on the summits of uplands lacking caprock, extensive areas of prairie can be found on the clay soils derived from the Walnut formation where it is exposed. The basal Trinity Group sands (Paluxy, Antlers, Twin Mountains-Travis Peak) underlying the Walnut formation develop Cross Timbers vegetation with Quercus stellata (post oak) and Quercus marilandica (blackjack oak) (Hill 1901). In parts of the western Lampasas Cut Plain, in areas where the overlying strata have been removed to expose the Glen Rose limestone (occurring between layers of the Trinity sands), short-grass prairie, oak-savannah, and woodlands with abundant junipers are found on the thin-soiled, rough, rocky, stair-stepped landscape (Hill 1901; Hayward et al. 1992).

The topographic diversity and deeply cut streams found in various parts of the Lampasas Cut Plain provide important microhabitat variation. In particular, the diverse microhabitats allow the northward extension of many species otherwise found primarily on the Edwards Plateau to the south and southwest. Plants traditionally considered Edwards Plateau endemics (e.g., Amos & Rowell 1988) but found in the Lampasas Cut Plain include Acer grandidentatum Nutt. var. sinuosum (Plateau big-tooth maple), Agalinis edwardsiana (Plateau gerardia), Argythamnia aphoroides (Hill Country wild mercury), Astragalus wrightii (Wright's milk-vetch), Chamaesaracha edwardsiana (Plateau false nightshade), Clematis texensis (scarlet clematis), Garrya ovata var. lindheimeri (Lindheimer's silktassel), Matelea edwardsensis, (Plateau milkvine), Muhlenbergia lindheimeri (Lindheimer's muhly), Nolina lindheimeriana, (devil's-shoestring), Onosmodium helleri (Heller's marbleseed), Perityle lindheimeri (Lindheimer's rock daisy), Prunus serotina var. eximia (escarpment blackcherry), Styrax platanifolius (sycamore-leaf styrax), Pediomelum cyphocalyx (turnip-root scrufpea), Tradescantia edwardsiana (Plateau spiderwort), Triodanis coloradoensis (Colorado Venus'-looking-glass), Verbesina lindheimeri (Lindheimer's crownbeard), and Yucca rupicola (twisted-leaf yucca). When considering vegetation, soils, geologic layers, and general aspects of the landscape, some parts of the Lampasas Cut Plain (e.g., Fort Hood---Bell and Coryell cos.; Meridian State Park---Bosque Co.; bluffs in southern Johnson Co. overlooking the Brazos River) are remarkably similar to the Edwards Plateau; in fact, it could be argued that the Lampasas Cut Plain is simply a northern extension of the Edwards Plateau. A number of plants widely known from the Edwards Plateau also occur not only in the Lampasas Cut Plain, but also in the topographically complex Palo Pinto Country to the north and northwest. Examples include a number of fern species unusual in North Central Texas such as Astrolepis integerrima (star-scaled cloak fern), Cheilanthes eatonii (Eaton's lip fern), Cheilanthes feei (slender lip fern), Cheilanthes horridula (rough lip fern), Pellaea ovata (cliff-brake), and Pellaea wrightiana (Wright's cliff-brake).

Currently in many places in the Lampasas Cut Plain, as well as in the cross timbers, junipers (Juniperus spp.) are a conspicuous component of the vegetation, often crowding out other native species. Because of the control of fire, overgrazing, and other human-caused changes, juniper has become much more common during the last century (Hayward et al. 1992). In fact, juniper is currently one of the most problematic species invading and eliminating native grassland. Fonteyn et al. (1988) emphasized fire suppression as causing a similar transformation from relatively open savannah to shrubland or woodland (in large part due to invasion by Juniperus ashei) on some parts of the Edwards plateau. Mesquite (Prosopis glandulosa), historically much less abundant than at present, shows the same pattern. Originally limited by fire, it has increased greatly in abundance as the result of fire suppression, overgrazing, and the plowing and other disturbances associated with agriculture (Hayward et al. 1992). In general, with the suppression of fire, woody vegetation is currently increasing at the expense of grassland throughout North Central Texas.

Red River Area (Area Adjacent to the Red River)

The narrow band of vegetation found on the primarily sandy soils adjacent to the Red River in the northeastern portion of North Central Texas, specificially in the northern parts of Lamar, Fannin, and Grayson counties, is quite different from the vegetation of the rest of North Central Texas. At least part of this band, which we refer to as the Red River Area (Fig. 1), is often classified as part of vegetational area 3 (Post Oak Savannah) (Correll & Johnston 1970; Hatch et al. 1990). Such a classification is justified because a significant component of the vegetation more typically associated with eastern or southeastern Texas extends west along the Red River in microhabitats with special soil or moisture conditions. Even components of vegetational area 1 (Pineywoods), characteristic of extreme eastern Texas, can be found in this area. In northern Lamar County, the aspect of the vegetation is definitely similar to the eastern deciduous forest. Tall stands of Quercus falcata (southern red oak), abundant Liquidambar styraciflua (sweetgum), Pinus taeda (loblolly pine), Acer rubrum (red maple), Betula nigra (river birch), Carpinus caroliniana (American hornbeam), Crataegus marshallii (parsley hawthorn), bottomland brakes of Arundinaria gigantea (giant cane), Calycocarpum lyonii (cupseed), Osmunda cinnamomea (cinnamon fern), Trachelospermum difforme (climbing dogbane), and herbs such as Lysimachia lanceolata (lance-leaf loosestrife), Monotropa hypopithys (american pinesap), Polygala sanguinea (blood milkwort), Porteranthus stipulatus (Indian-physic), Saccharum contortum (bent-awn plume grass), Sacciolepis striata (American cupscale), Saururus cernuus (lizard's-tail), Stachys tenuifolia (slender-leaf betony), and Veronicastrum virginicum (culver's-physic), are just a few examples of eastern plants found in Lamar County. Even farther west in Fannin County, there are still isolated pockets of eastern Texas vegetation (e.g., Talbot property). Species reaching their known western limits there include Quercus falcata (southern red oak), Quercus nigra (water oak), Quercus phellos (willow oak), Nyssa sylvatica (black-gum), Sassafras albidum (sassafras), Chasmanthium laxum var. sessiliflorum (narrow-leaf wood-oats), Cirsium horridulum (bull thistle), Erechtites hieraciifolia (American burnweed), Impatiens capensis (spotted touch-me-not), Luzula bulbosa (bulb woodrush), Monotropa uniflora (Indian-pipe), Pedicularis canadensis (common lousewort), Pycnanthemum albescens (white-leaf mountain-mint), Sorghastrum elliottii (slender Indian grass), and Woodwardia areolata (narrow-leaved chain fern). Grayson County, the next county to the west, does not have areas dominated by eastern Texas plants as do Lamar and Fannin counties, but there is a significant eastern Texas component to the vegetation. Numerous plant species reach their western limits in Grayson County including Agrimonia rostellata (woodland groovebur), Asimina triloba (pawpaw), Desmodium glutinosum (tick-clover), Liatris aspera (tall gayfeather), Monarda lindheimeri (Lindheimer's beebalm), Podophyllum peltatum (may-apple), Polygonatum biflorum (Solomon's-seal), Quercus velutina (black oak), Thalictrum arkansanum (meadowrue), Triosteum angustifolium (yellow-flowered horse-gentian), and Vaccinium arboreum (farkleberry). A few typically eastern plants extend even farther west into Cooke and Montague counties and beyond.

The area adjacent to the Red River in Grayson County is further complicated by the presence of the Preston Anticline, a post-Cretaceous (Bradfield 1957) fold in the sedimentary strata that brought deeper layers to the surface (Bullard 1931). In places the river valley is two hundred feet below the surrounding area and creeks have cut deep canyon-like valleys. The overall topography near the Red River is thus very rugged (Bullard 1931). Parker (1856) in an early account describes the Texas shore of the Red River as "very bold, presenting a stratification of red clay and white sand, giving a striking and very peculiar appearance in the distance, like chalk cliffs." This different topography and the appearance at the surface of deeper strata otherwise only found far to the west in areas such as the Grand Prairie and West Cross Timbers (e.g., Goodland limestone, Duck Creek limestone, Trinity Group sands) makes the vegetational picture of the area more complex. Many microhabitats, and thus increased biological diversity, result from the cropping out of these deeper strata in the county. For example, in a number of places along the Red River (e.g., Eisenhower State Park, Preston Peninsula, Delaware Bend), the Goodland Limestone forms flat limestone outcrops at the top of rugged cliffs. These areas of very thin soil over flat rock and adjacent slopes and ravines have numerous interesting plant species found nowhere else in Grayson County including Coryphantha missouriensis (plains nipple cactus), Minuartia michauxii var. texana (rock sandwort), Talinum calycinum (rock-pink), Dodecatheon meadia (common shooting-star), and Melica nitens (tall melic).

The proximity of sandy and clayey soils, as well as some intermediate type soils, in the counties adjacent to the Red River, also allows species normally separated ecologically to occur together; this sometimes results in hybridization. An excellent example can be seen in Fannin and Grayson counties where three species of Baptisia (wild indigo) and all three possible hybrids are found in close proximity (Kosnik et al. 1996). These occur either in what early settlers locally called "mixed soil" or in the area of the Preston Anticline where radically different soil types are found over quite small distances.

The basic pattern of the Red River Area is thus one of the eastern Texas forests grading gradually into the much less diverse and more xeric woodlands usually referred to as the Cross Timbers. From an even broader perspective, as discussed in the overview, the whole North Central Texas region is in an ecotone or ecological transition zone between two extensive ecosystems, the eastern North American deciduous forest and the central North American grassland or prairie. In virtually any ecotone, significant areas of vegetational interdigitation are seen; rarely is there a clearcut boundary. One type of vegetation extends deep into another along streams, in-pockets are found in protected areas, and special soil conditions often result in a patchwork pattern of vegetation that at the st