PARENT SESSION
Oral Session #80: Plant Ecology .
Presiding: G. Platenkamp
Thursday, August 8. 9:00 AM to 11:30 AM. Grand Ballroom West, Radisson.

Biotic and abiotic constraints on woody plant seedling establishment in semi-arid savannas.

THOMAS, LEIGH^*,1, WELTZIN, JAKE¹, STURDIVANT, LISA², ¹ University of Tennessee, Knoxville, TN² University of Arizona, Tucson, AZ

ABSTRACT- In semi-arid regions, spatial and temporal variation in soil moisture is probably the most important driver of shifts in plant distribution and abundance. High densities of grasses may deplete soil moisture resources and inhibit woody plant seedling establishment. Furthermore, soil moisture availability may also be mediated by soil texture. We investigated biotic and abiotic constraints on woody plant establishment in semi-arid savannas by constructing twenty-four 1.5m x 1.5m plots on soils with either high or low clay content. Three levels of grass density (intact, 50% removal, and 100% removal) and two levels of watering (ambient and watered) were assigned to each plot on both soil types. Mesquite (Prosopis velutina) seeds were planted in August 2001. Soil moisture was measured to a depth of 20cm using time-domain reflectometry probes. Dependent variables included soil volumetric water content (_v) and seedling emergence, recruitment, and survivorship. At the time of seedling emergence, _v was greater (p<0.005) on the high clay soil (9%) than on the low clay soil (7%) and greater (p<0.03) in graminoid removal plots (9%) than in intact plots (8%). All seedling response variables were inversely correlated with graminoid biomass (p<0.0001) and correlated with soil clay content (p<0.0001). Results indicate that soil moisture is mediated by both graminoid biomass and soil texture and may be the ultimate constraint on woody plant seedling establishment in semi-arid savannas.

KEY WORDS: Prosopis velutina, Semi- arid savanna, Woody plant encroachment