THE ROLE OF SAND THICKNESS AND PALEOTOPOGRAPHY ON PHREATOPHYTE AND NON-PHREATOPHYTE SPECIES DISTRIBUTION ALONG A NORTH - SOUTH TRANSECT OF THE SOUTH TEXAS SAND SHEET

The South Texas Sand Sheet (STSS), lies north of the lower Rio Grande Valley and spans ~7,200 km². Despite the lack of running water, limited precipitation (50-67 cm/yr) and high summer temperatures (39 ^oC), discernable patches of honey mesquite (Prosopis glandulosa)occur amid a distinct grassland ecosystem, indicating phreatophytic water uptake behavior. To elucidate the role of sand thickness and paleotopography on tree distribution, a 70km long, N-S stratigraphic cross-section spanning the width of the STSS has been rendered along Texas Interstate 69C.The cross-section indicates localized zones of substantial STSS thickness (>4m) as well as zones where the underlying Goliad Formation is either exposed or just below the surface (<0.5m).Spatial patterns of both phreatophytic and non-phreatophytic vegetation where analyzed with remote sensing. The spectral signature of three classes of vegetation was measured by conducting a supervised classification of well-delineated plant assemblages throughout a transect of equal length to the cross-section and with a 1,400m buffer on the periphery of Texas Highway 69C. Patch and mosaic patterns where comprised of three vegetation assemblages: P glandulosa/Vachellia farnesiana, Q. virginianaand a matrix of herbaceous plants (forbs & grasses). The geographic distribution of vegetation assemblages corresponded to specific lithological parameters and features. Namely, Q. virginianatends to occupy locations with the highest values for two parameters; (1) STSS thickness and (2) elevation. P. glandulosa/V. farnesianaassemblages as well as the herbaceous matrix occupied zones where the STSS unit was relatively thin (<0.5m) and the Goliad formation was either exposed or more proximal to the surface. A notable outlier in the distribution of P. glandulosa/V. farnesianais the locality which coincides with the largest Q. virginianapatch where the thickness of the STSS is highest. This distribution anomaly may be due to the presence of two prominent paleovalleys within the Goliad Formation at these sites.