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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170 INTRODUCTION/NATURAL HISTORY <strong>OF</strong> BIG THICKET<br />
and shrub species in mesic Texas forests are evergreen, and some forest types and strata of<br />
the Big Thicket can justifiably be called “semi-evergreen” (Harcombe & Marks 1977).<br />
RARE AND ENDANGERED SPECIES <strong>OF</strong> THE BIG THICKET—Five plant species of the Big Thicket, as well as<br />
numerous animals, are considered to be of national conservation concern by the Big<br />
Thicket National Preserve (2002). These include bog coneflower (Rudbeckia scabrifolia,<br />
Species of Concern), Navasota ladies’-tresses (Spiranthes parksii, Federal Endangered), slender<br />
gay feather (Liatris tenuis, Species of Concern), Texas trailing phlox (Phlox nivalis var. texensis,<br />
Federal Endangered), and white firewheel (Gaillardia aestivalis var. winkleri, Species of<br />
Concern). Numerous other plants occurring in the Big Thicket are also quite rare in the<br />
state and are of conservation concern in Texas (Appendix 12). Many of these species, much<br />
more common further east, are known in the state only from one or two Big Thicket counties.<br />
While there is significant variation in the community affiliation of rare species, many of the<br />
rare plants of the Big Thicket (and East Texas in general) are pyrogenic (fire dependent)<br />
species associated with longleaf pine woodlands, a community that has been greatly altered<br />
by human activities including logging and fire suppression.<br />
THE BIG THICKET AS A BIOLOGICAL CROSSROADS—As noted earlier, the Big Thicket has often been<br />
described as a biological crossroads where species typical of areas to the east, west, north,<br />
and south meet and intermingle. Indeed, dry area plants such as Opuntia (prickly-pear,<br />
Cactaceae) and Yucca (Agavaceae) do occur in close proximity to Taxodium (bald-cypress,<br />
Cupressaceae) and other plants typical of swamps. While the Big Thicket is near the ecotone<br />
or transition zone between the eastern deciduous forests and the central North American<br />
grasslands, MacRoberts and MacRoberts (2004a), concluded that there is no evidence to<br />
support the crossroads idea. For example, they note that over 99% of the species in the Big<br />
Thicket are endemic or eastern in affinity. Further, they note that the main source of the<br />
crossroads concept,<br />
…appears to be the presence of xeric sandylands and prairies in southeast Texas. Superficially, xeric<br />
sandylands resemble deserts and some of the genera, but not the species, found in this habitat originated<br />
under desert conditions; cactus, agave, and yucca are obvious examples. But xeric sandylands<br />
are by no means confined to southeast Texas but extend from East Texas northward to Oklahoma<br />
and eastward to North Carolina. The same is true for prairies, which are not confined to the central<br />
and western states but extend across the southern and eastern United States. There is no evidence<br />
that the Big Thicket is a floristic crossroads. Its flora is eastern, notably southeastern.<br />
Indeed, at the species level, there is relatively little evidence of the Big Thicket as a biological<br />
crossroads. However, at the generic and family levels, the Big Thicket and much of the southeastern<br />
U.S. is an area of floristic intermingling. Plants with tropical origins (e.g., Mayacaceae,<br />
Melastomataceae), and those with affinities to the deserts of southwestern North America<br />
(Opuntia, Yucca), mix with plants more typical of the central North American grasslands and<br />
with plants pushed south during the climatic shift associated with glacial times (see page 208).<br />
While the Big Thicket is not unique in this regard, it does clearly share this interesting<br />
biogeographic history with other areas of the southeast.<br />
SIMILARITIES TO TROPICAL RAIN FORESTS—A striking aspect of the Big Thicket is its resemblance<br />
in a number of respects to tropical rain forests. Similarities include:<br />
■ the very tall trees, in some cases over 150 feet (46 m) in height (Fritz 1993),<br />
■ the swollen buttressed bases on trees such as bald-cypress and water tupelo,<br />
■ numerous evergreen species (e.g., southern magnolia, sweet-leaf, wax myrtle, red bay, cherry laurel,<br />
American and other holly species, dwarf palmetto, Smilax species, etc.—in some areas about<br />
“50% of the forest composition is that of evergreens” (McLeod 1971),