Vegetation, soils, and surface hydrology of playa landforms in the Rio Grande Plains, Texas

Abstract

Playas in the Rio Grande Plains of southern Texas were compared with respect to their: 1) size, shape, soil properties, and microtopography, 2) vegetation composition and structure, 3) surface water accumulation potential, and 4) disturbance history. Transitions from playa to drainage woodlands were also quantified with respect to soils and vegetation. Playas were typically oval shaped basins ranging from 0.14 to 3.81 ha in size. Physiognomy ranged from grassland (treeless) to savanna (woody basal area < 70 m²/ha) to woodland (woody basal area > 70 m²/ha). When trees were present, Prosopis glandulosa or Acacia smallii were typically dominant or co-dominant. Neighboring woodlands on sandy loam drainage landforms had a greater overstory woody density (255 [] 58 stems/ha) than playas (18 [] 3.6 stems/ha); however, overstory woody basal area was not significantly different between the landforms. Three woody species (Acacia smallii, Sesbania drummondii, and Parkinsonia aculeata) were unique to playas. Herbaceous standing crop biomass was statistically different between playas (81 [] 24.4 to 198 [] 29.8 g/m²) and inversely correlated with tree basal area (r² = 0.36), with the contribution of grasses to total biomass ranging from 55% to 92%. Playa soils were Vertisols or vertic Mollisols consisting of clayey surfaces with shrink-swell properties. Mean surface pH values (6.5 [] 0.1), EC (0.231 [] 0.03 and 0.350 [] 0.05 dS/m), clay content (~ 40%), and bulk densities (1.8 [] 0.02 g/m³) for both subgroup classifications were not comparable. Thus, there was no apparent edaphic basis for the observed differences in vegetation. The extent and duration of inundation varied among playas, depending on basin microtopography. Despite anaerobic conditions associated with inundation, woody plant cover has increased in five of the eight playas in the past 44 years. Decreases in woody density occurred in two playas subjected to prescribed fire and herbicide treatments. The third basin has been grass-dominated and treeless since 1950, apparently owing to its retention of standing water for extended periods. With the exception of this deep playa basin, disturbance (e.g. fire, herbiciding, and chaining) appears to have been more important than topographic factors in shaping vegetation structure and composition in playas.