ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

RED RIVER AREA/INTRODUCTION 129
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) make the vegetational picture of the area more complex. Some of the
oldest bedrock found on the surface in East Texas occurs in Grayson County (Lower
Cretaceous Trinity Group sands). Many microhabitats, and thus increased biological diversity,
result from the outcropping 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 often found nowhere else in Grayson County and
only rarely anywhere in East Texas. These include 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).
In the counties adjacent to the Red River, the presence of sandy and clayey soils in close
proximity to one another, as well as some intermediate type soils, 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 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 entire
western portion of East Texas (Post Oak Savannah, Blackland Prairie) and the Red River
Area are 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
(MacRoberts & MacRoberts 2003b). 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 strictly
local level seems confusing. The geographically intermixed pattern of the Blackland
Prairie and Post Oak Savannah, the irregular boundaries of all the East Texas vegetation
zones, and the unusual mixture of species seen in the Red River Area are all excellent
examples of these phenomena. B