ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...
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NATURAL HISTORY <strong>OF</strong> BIG THICKET/INTRODUCTION 163<br />
larger areas of beech-magnolia-loblolly forests (e.g., in the Beech Creek Unit of Big Thicket<br />
National Preserve) but fewer areas dominated by longleaf pine (Harcombe et al. 1993).<br />
Because the Willis is well-drained, wetland communities are in general restricted to stream<br />
bottoms or to Wet Herbaceous Seeps/hanging bogs on hillsides (Watson 1975).<br />
Still further south, on the Pleistocene Lissie (including the Bently and Montgomery)<br />
Formation, xeric sandylands and longleaf pine uplands can be found on higher, dry sandy<br />
terraces between the streams, or in the case of longleaf pine, on mounds on the nearly flat<br />
savannah wetlands. In general though, the Lissie and the still younger Beaumont (also<br />
Pleistocene) (e.g., near Beaumont and Little Pine Island Bayou) underlie very flat areas close<br />
to sea level—hence they have a high water table and are more subject to flooding and water<br />
retention. In addition, the soils of the Lissie and Beaumont formations have a higher clay<br />
content, particularly in the subsoils. These soils are thus less permeable and often poorly<br />
drained, favoring plants with greater water requirements or tolerance. Wetland communities<br />
such as palmetto-hardwood flats, wetland longleaf pine savannahs, and bald-cypress<br />
sloughs are common in this flat terrain (Smeins et al. 1982; Bridges & Orzell 1989b; Parent<br />
1993). Even here, slight differences in topography and soil moisture—such as mima mounds<br />
(see page 65), old terraces, or slightly higher areas between streams—can have a major<br />
impact on the plant species present. For example, on the Beaumont Formation, longleaf<br />
pines are essentially restricted to mima mounds (Watson 1975). On the very southern edge<br />
of the Big Thicket, on the Beaumont, wetland longleaf pine savannahs and scattered areas<br />
of prairie are transitional to what was in presettlement times a large expanse of Coastal<br />
Prairie, underlain in general by clay soils.<br />
Two other cases further exemplify the influences of soil, topography, and hydrology on<br />
vegetation. Particularly interesting are areas where tight impermeable clay soils and flat<br />
terrain result in very poor drainage and standing water during part of the year. Such areas<br />
are known as Wet Pine Savannahs or savannah wetlands (see page 97). The growth of<br />
woody plants in these areas is apparently retarded by waterlogging and high acidity. In addition,<br />
because of their slow growth, trees in such areas are vulnerable to fire for relatively<br />
longer (MacRoberts & MacRoberts 1993a; 2001). Also, Wet Pine Savannahs have soils that<br />
tend to be acidic and very poor in nutrients, and carnivorous plants do well—in fact, 14<br />
species in four different genera, Sarracenia (pitcher plants), Drosera (sundews), Pinguicula<br />
(butterworts), and Utricularia (bladderworts), are found in East Texas. As with all carnivorous<br />
plants, nutrients (especially nitrogen), rather than calories, are obtained through carnivory—<br />
these plants still obtain the energy they need for growth and development through photosynthesis.<br />
In contrast to the poorly drained Wet Pine Savannahs, the excessively drained, dry, white<br />
quartz sand soils of the Dry Uplands on Deep Coarse Sands (also known as xeric or arid sandylands)<br />
(see page 92) support such dry-adapted plants as Cnidoscolus (bull-nettle), Opuntia<br />
(prickly-pear), Quercus incana (bluejack oak), Vaccinium arboreum (farkleberry), and Yucca.<br />
Factors affecting succession have long been important in influencing the vegetation of<br />
the Big Thicket (and much of the southeastern United States). These include such diverse<br />
phenomena (at widely varying scales) as recurrent hurricanes, special edaphic conditions,<br />
beaver activity, oxbow lake formation, the gradual filling of bogs, and fire. Of all of these, fire<br />
is probably most important. According to MacRoberts and MacRoberts (2000), “about half of<br />
the plant communities of the Longleaf Pine Ecoregion require periodic fire or they gradually<br />
succeed to other communities, losing key species and structural integrity.” For example,<br />
prairies, small seeps/bogs, pine savannah wetlands, and longleaf pine uplands were all pyrogenic<br />
communities maintained during presettlement times at least in part by fire (the fires<br />
due to both natural causes and the activities of Native Americans). Under current fire suppression<br />
regimes, these fire-adapted communities have been (and are still being) replaced by<br />
different suites of plants (Watson 1975; Simberloff 2000b). In fact, fire suppression is one of