THE INFLUENCE OF SOIL DEPTH ON PLANT-SPECIES RESPONSE TO GRAZING WITHIN A SEMIARID SAVANNA

Grassland patches within a semi-arid savanna were evaluated over 45-ye ars for (1) local temporal dynamics of basal area for five dominant gr ass species within long-term heavily grazed and ungrazed treatments, ( 2) the influence of soil depth (resource availability) on vegetation d ynamics, and (3) the applicability of community-level grazing response groups over fine-scale patterns of soil heterogeneity. Temporal patte rns in species composition and basal area were dependent upon soil dep th. In the heavy grazed treatment, Hilaria belangeri dominated deep so ils while Erioneuron pilosum and Bouteloua trifida were restricted to shallow soils. In the ungrazed treatment, removal of grazing resulted in successional changes that were significantly different across soil depths. After 45 years without grazing, Eriochloa sericea was most abu ndant on deep soils while Bouteloua curtipendula was more abundant on intermediate and shallow soils. Community-level functional groups that are based on grazing were not appropriate when multiple pattern-drivi ng variables were considered across multiple scales indicating that fu nctional groups should only be applied to certain processes at specifi c scales. Within the ungrazed treatments, variable soil depths have re sulted in a shifting mosaic in time and space where early- and late-su ccessional species co-exist continuously but spatially separated withi n the community. In the heavily grazed treatment, species are somewhat spatially arranged by soil depths, but much of the inherent heterogen eity is eliminated and species composition is dominated by the three g razing-resistant short-grasses. Broad scale successional changes may a ppear linear and predictable while at finer scales, the same changes m ay be described as non-linear and dependent upon soil depth resulting in thresholds that are partially explained by weather patterns, seed b ank limitations and competitive inhibitions.