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

CLIMATE OF EAST TEXAS/INTRODUCTION 65
are less useful for agriculture than are Vertisols, and at present they tend to be used as pastures
or home sites. Shrink-swell phenomena, while still occurring on Mollisols, are less problematic
than on Vertisols (Diamond & Smeins 1993; Hallmark 1993). Mollisols are more common to
the west of East Texas on the Grand Prairie (made up of the Fort Worth Prairie and Lampasas
Cut Plain). The soils of that region are often developed on layers of relatively hard limestone,
and the area has sometimes been called the “hard lime rock region” (Hill 1901). 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 in the Blacklands mainly on the eastern and northern margins
of the main belt of the Blackland Prairie (Hallmark 1993) (Fig. 30), on the western margin of
the Fayette Prairie, and in places on the San Antonio Prairie. These soils, which develop under
forests as well as prairies, are less fertile than either Vertisols or Mollisols (Hallmark 1993).
Mima mounds (also called pimple mounds or prairie mounds), another microtopographical
feature (Fig. 31), were once found on many Alfisols within the Blackland Prairie region and
can still be observed on certain unplowed prairie remnants (e.g., northern Grayson County
(Fig. 34) and the Nature Conservancy’s Tridens Prairie in Lamar County). Mima mounds are
circular, saucer-shaped hills ranging from about 1 to 15 meters in diameter and up to approximately
1.5 meters in height. While numerous hypotheses have been proposed, the specific
cause of these structures is not known with certainty and they are possibly of multiple origins
(Collins et al. 1975; Diamond & Smeins 1993). Though wind (erosion/deposition) appears
to be a widely accepted explanation, other possibilities are that the mounds formed by wave
action, are the products of past drainage patterns (e.g., runoff erosion combined with vegetation
anchoring soil in place), are formed by fossorial (= burrowing) rodents (such as pocket
gophers), or are the result of seismic activity (Diamond & Smeins 1985; Cox & Hunt 1990;
Sletten et al. 1994; Ganse 2003). What is known is that some process operating in the past
(but not presently in the area) is responsible for mima mound formation—this seems clear
since, once destroyed, mima mounds do not reform under current conditions (unlike gilgai
which will reestablish). Both gilgai and mima mounds increase microhabitat diversity and
thus contribute to vegetational differences over small distances. The overall biological diversity
of the prairie is therefore greatly increased by this variation in microtopography (Miller &
Smeins 1988; Diamond & Smeins 1993). Due to the variation in vegetation associated with
the different microhabitats produced by both gilgai and mima mounds, these features are
often easily discernible in the field at certain seasons of the year (Figs. 33, 34). Sometimes a
particularly conspicuous species will be quite obviously associated with a microtopographical
high or low (e.g., Helianthus mollis, ashy sunflower, on mima mounds) and allow these areas
to be spotted from long distances.
CLIMATE AND WEATHER OF EAST TEXAS
East Texas, like the state as a whole, is a region of dramatic climate transition. The striking
vegetational change from the swampy tropical-like forests of Caddo Lake and the Big Thicket
on the eastern margin of East Texas to the grasslands of the Blackland Prairie just to the west
is a vivid reflection of this climatic transition (Stahle & Cleaveland 1995). The climate of East
Texas is considered subtropical (Jordan et al. 1984; Yelderman 1993; Norwine et al. 1995;
Peterson 1995), but a wide range of extremes can be found. Detailed climatic information
about one East Texas site, Nacogdoches, can be found in Chang et al. (1996). Like the rest of
the state, East Texas can be referred to climatically as a “land of contrasts” (Bomar 1995). This
results in part from two major competing factors which dominate the climate of Texas—
frontal systems from the north and west, and moist air moving inland from the Gulf of
Mexico. As these influences interact, dramatic weather often results (North et al. 1995b).
There is ample reason why locals say, “If you don’t like the weather, just wait a few minutes.”