The structure and function of Kalahari Transect vegetation

by Caylor, Kelly Krispin.

The Kalahari Transect provides a unique opportunity to investigate the ways in which small-scale vegetation pattern affects the simulation of plant processes in semi-arid savanna ecosystems. Spatial pattern is aggregated for all trees in the vegetation communities at most research sites, with no occurrence of aggregation in trees at the most southern arid site. Lower than expected understory density is observed at the northern and southernmost sites in the transect. The understory density is significantly higher than expected at intermediate sites. The correspondence between maximum vegetation cluster size and the observed transition in scaling behavior of the NDVI mosaic reinforces the hypothesis that observed multi-scaling is related to the structural characteristics of vegetation canopies across the rainfall gradient. Variability in vegetation structure leads to a range of simulated annual productivity within one site (600 mm) that accounts for 110% of the total range in mean simulated productivity across all sites. Simulated productivity at the various scales of aggregation results in similar overall patterns of average NPP for both trees and grasses, but drastically reduced distributions of productivity due to reduced structural heterogeneity. A coupled energy and water balance model is used to simulate the effects of large tree canopies on soil moisture and water stress across a series of sites spanning a regional moisture gradient in southern Africa. Tree canopies serve to reduce soil moisture stress of under canopy vegetation in the middle of the rainfall gradient. At the dry end of the rainfall gradient, the effect of tree canopies on soil moisture is dependent on the amount of yearly rainfall received. 4