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

162 INTRODUCTION/NATURAL HISTORY OF BIG THICKET
(pers. comm.) points out that beech does occur in the Lance Rosier Unit of the Big Thicket
National Preserve in the “lower thicket.” While McLeod’s ecological definition seems the most
accurate way of generally delineating the Big Thicket, many other strikingly different vegetation
types (e.g., xeric sandylands, baygalls) are interspersed throughout his “loblolly pinewhite
oak-beech-magnolia forest.” These have been variously classified by subsequent
authorities as follows.
Watson’s (1975) early work divided the Big Thicket into seven plant communities:
1) Prairie; 2) Palmetto-hardwood flats; 3) Stream floodplains (subdivided into Lower bottomland
and Upper bottomland); 4) Arid sandyland; 5) Acid bog-baygall; 6) Longleaf
pinelands (subdivided into Longleaf pine uplands and Pine savannah wetlands); and 7)
Beech-magnolia-loblolly pine association. The National Park Service (based in part on
Watson) recognized 10 ecosystems in the Big Thicket National Preserve: Baygall, Beechmagnolia-loblolly,
Cypress slough, Longleaf pine upland, Oak-gum floodplain, Palmettohardwood
flats, Pine savannah wetlands, River edge, Roadside, and Arid sandylands
(Peacock 1994). Ajilvsgi (1979, also based in part on Watson) distinguished nine communities:
Mixed-grass prairies, Palmetto-oak flats, Sweet gum-oak floodplains, Bay-gallberry
holly bogs, Longleaf-black-gum savannahs, Longleaf-bluestem uplands, Beech-magnolialoblolly
slopes, Oak-farkleberry sandylands, and Roadsides. Marks and Harcombe (1981)
recognized four broad types and subdivided them as follows: Uplands (Sandhill pine forest,
Upland pine forest, Wetland pine savannah), Slopes (Upper slope pine oak forest, Mid slope
oak pine forest, Lower slope hardwood pine forest), Floodplains (Floodplain hardwood pine
forest, Floodplain hardwood forest, Wetland baygall shrub thicket, Swamp cypress tupelo
forest), and Flatlands (Flatland hardwood forest). While the terms used in each of these
systems are descriptive and, in general, accurate in portraying the vegetational diversity
present, plant ecologists currently give these communities different names more readily
allowing comparison to communities in other geographic areas (see page 90 for a detailed
classification and discussion of Pineywoods vegetation).
VARIABLES INFLUENCING THE VEGETATION—The plant life of the region is the result of complex interactions
between a number of often highly related factors, including geological ones (e.g., type
of parent material, topography, soils, hydrology), climate, succession, and fire. Though rainfall
is heavy throughout the Big Thicket (Parent 1993) and well-distributed through the
year—the difference between the driest and wettest months being only 5 cm (2 inches)
(Marks & Harcombe 1981)—relatively small differences in topography and different parent
materials and soils produce a striking diversity of vegetation types. The importance of soils
in influencing Big Thicket vegetation is reflected by the following statement by Harcombe et
al. (1993). “Most of this area is covered by closed forests which vary in structure and species
composition along a soil texture gradient” [italics ours].
While a mosaic of vegetation can be found nearly throughout the Big Thicket, certain
plant communities are much more common in some areas than in others. In the far north,
on the Oligocene Catahoula and Miocene Fleming geologic formations, areas of “barrens” and
small isolated prairies are found, as well as hardwood-dominated ravines and areas of upland
longleaf pine. Limited areas of Wet Herbaceous Seeps (“hanging bogs”) often form when the
impenetrable Catahoula Formation is overlain by more porous layers. In contrast, where the
calcareous (and thus basic) Fleming outcrops at the surface, there are unusual occurrences of
calciphilic (= calcium-loving) plants in certain localities in Jasper and Newton counties
(Bridges & Orzell 1989a; J. Liggio, pers. comm.).
A little to the south, the younger (late Tertiary, Pliocene), sandy Willis Formation (e.g.,
near Woodville) has eroded to form rolling topography and permeable, well-drained, but
often relatively moist, rich, sandy soils. The increased soil moisture and relatively dissected
topography apparently allow more fire-susceptible vegetation to survive here. The result is