Ecoregions of Texas, AS-199 - Texas Commission on Environmental ...

frameworks were examined. These include the <strong>Texas</strong> ecoregions <strong>of</strong> the LBJ School <strong>of</strong> Public Affairs (1978),<strong>Texas</strong> ecological areas shown by McMahan et al., (1984), Bezanson (2000), and Telfair (<strong>199</strong>9), the USFSsections and subsections (Keys et al., <strong>199</strong>5), ecoregions <strong>of</strong> the Nature Conservancy (The Nature ConservancyEcoregional Working Group <strong>199</strong>7, The Nature Conservancy 2003), Major Land Resource Areas (USDA SCS1981) and later derivatives developed by NRCS, and the natural land-use regions <strong>of</strong> Barnes and Marschner(1933), among others. Also <strong>of</strong> major importance in our process <strong>of</strong> regionalization were the mental maps thatlocal experts brought to discussion meetings, reviews, or field reconnaissance trips.We used USGS 1:250,000-scale topographic maps as the base for delineating and digitizing the ecoregionboundaries. Although this map series is dated, it does provide quality in terms <strong>of</strong> the relative consistency andcomparability <strong>of</strong> the series, in the accuracy <strong>of</strong> the topographic information portrayed, and in the locationalcontrol. It is also a very convenient scale. Fifty one <strong>of</strong> these maps give complete coverage <strong>of</strong> <strong>Texas</strong>.RESULTS AND REGIONAL DESCRIPTIONSWe have divided <strong>Texas</strong> into twelve level III ecoregions (Figure 1, p. v) and 56 level IV ecoregions(Figure 2, p. vi). Although these level IV ecoregions still contain some heterogeneity in factors that can affectwater quality and biotic characteristics, they provide a more detailed framework and more precise ecoregionboundaries than the earlier national-scale ecoregions (Omernik 1987). The ecoregion framework also providesmore homogeneous units for inventorying, monitoring, and assessing surface waters than the commonly usedhydrologic unit frameworks or political unit frameworks (Omernik and Bailey <strong>199</strong>7, Omernik and Griffith<strong>199</strong>1, Griffith et al., <strong>199</strong>9, Omernik 2003). Major river basins drain strikingly different ecological regions. Amap poster <strong>of</strong> the ecoregions <strong>of</strong> <strong>Texas</strong> has been published by USGS (Griffith et al., 2004).23 Arizona/New Mexico MountainsThe Arizona/New Mexico Mountains are distinguished from neighboring mountainous ecoregions bylower elevations and an associated vegetation indicative <strong>of</strong> drier, warmer environments, due in part to theregion’s more southerly location. Chaparral is common at lower elevations; pinyon-juniper, and oak woodlandsare found at lower and middle elevations; and the higher elevations are mostly covered with open to denseponderosa pine forests. Forests <strong>of</strong> spruce, fir, and Douglas-fir are common in the Southern Rockies (21) andthe Wasatch and Uinta Mountains (19), but they are found only in limited areas at the highest elevations in thisregion. Only a small portion <strong>of</strong> Ecoregion 23 occurs in <strong>Texas</strong>. The Guadalupe Mountains on the <strong>Texas</strong>-NewMexico border comprise the southernmost peaks <strong>of</strong> the Arizona/New Mexico Mountains ecoregion.23a Chihuahuan Desert SlopesThe Chihuahuan Desert Slopes <strong>of</strong> the Guadalupe Mountains in <strong>Texas</strong> form theleading edge <strong>of</strong> a giant uplifted Permian reef created from the accumulated remains <strong>of</strong>algae, sponges, and marine bivalves (Spearing <strong>199</strong>1). The white, 2000 foot high cliffface dominates the landscape <strong>of</strong> the northern Trans-Pecos region <strong>of</strong> <strong>Texas</strong>. The lowerslopes <strong>of</strong> the mountains, composed <strong>of</strong> eroded limestone, shale, and sandstone, representa continuation <strong>of</strong> the Chihuahuan Desert ecosystem; soils and vegetation in much<strong>of</strong> Ecoregion 23a are similar to those in the Low Mountains and Bajadas (24c) <strong>of</strong> the Chihuahuan Desertecoregion (24). Water is scarce; the few streams that originate from springs at higher elevations do not persistbeyond the mouths <strong>of</strong> major canyons.The lower elevation boundary <strong>of</strong> the Chihuahuan Desert Slopes ecoregion (23a) coincides with the base <strong>of</strong>the Guadalupe Mountains at about 4500 to 5000 feet. Climate, vegetation, and soil do not differ significantlyon either side <strong>of</strong> the mapped boundary, but the line marks a physiographic and geological transition to themountainous ecosystem with its cliffs and canyons. The upper boundary <strong>of</strong> Ecoregion 23a is very generalized;it indicates the passage <strong>of</strong> desert shrubs and grassland to woodland and surrounds the core <strong>of</strong> the forested area<strong>of</strong> the <strong>Texas</strong> Guadalupe Mountains. The scale <strong>of</strong> mapping is not detailed enough to represent strips <strong>of</strong> riparian4<strong>Ecoregions</strong> <strong>of</strong> <strong>Texas</strong>