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

226 INTRODUCTION/ORIGIN AND DIVERSITY OF EAST TEXAS FLORA
(2,877—Arkansas Vascular Plant Manual Committee 2002), Louisiana (2,952—MacRoberts
1984), and Oklahoma (2,549 species—Taylor & Taylor 1994), is considered, East Texas
again appears particularly diverse. The striking diversity of the area is also apparent when
it is realized that there are 202 vascular plant families (as treated here) in East Texas. This
diversity at the family level exceeds all but a handful of states (e.g., Alabama, Florida, Texas).
When compared (using the same taxonomic approach to families) with its nearest neighbor
to the north, East Texas has 11 more families than all of Oklahoma (Taylor & Taylor 1994),
even though Oklahoma is substantially larger.
A number of interrelated factors contribute to this diversity:
■ GEOLOGIC AND ASSOCIATED HYDROLOGIC VARIATION resulting in considerable edaphic variation—e.g.,
very dry to very wet conditions including bogs and baygalls; acidic to calcareous substrates ranging
from Carrizo sands to Weches outcrops and Catahoula sandstone; deep highly fertile soils to highly
leached areas and rock outcrops.
■ CLIMATIC VARIATION—e.g.,average annual rainfall ranges from nearly 60 inches at the southeastern
portion of the area to less than 28 inches at the southwestern margin.
■ HIGH HABITAT DIVERSITY—Large numbers of different habitats occur in a relatively small area, each
of these supporting a diverse assemblage of species (this is particularly noteworthy for the Big Thicket;
see further discussion on page 164).
FIG. 130/ PERCENTAGE OF EASTERN AND WESTERN SPECIES VERSUS LONGITUDE ACROSS TEXAS.(MODIFIED FROM MACROBERTS & MACROBERTS
2003B), WITH PERMISSION OF THE AUTHORS.