Effects of precipitation and soil water potential on drought deciduous phenology in the Kalahari


William M. Jolly, tel. +406 243 6230, fax +406 243 4510, e-mail: mattj@ntsg.umt.edu


We utilized an ecosystem process model to investigate the influence of precipitation and soil water potential on vegetation phenology in the semi-arid, drought-deciduous ecosystems in the Kalahari region of South Africa. The timing of leaf flush was assumed to be the first day during which a rainfall event exceeded that day's estimate of potential evapotranspiration after a defined dry season. Leaf senescence was assumed to be a dynamic feedback between soil water potential and net plant carbon gain and was determined by dynamically modeling the effects of concomitant trends in soil water potential and net primary production on leaf area index (LAI). Model predictions of LAI were compared with satellite-derived normalized difference vegetation indices (NDVI) for 3 years at two sites along the Kalahari transect. The mean absolute error for the prediction of modeled leaf flush date compared with leaf flush dates estimated from NDVI were 10.0 days for the Maun site and 39.3 days for the Tshane site. Correlations between model predicted 10-day average LAI and 10-day composite NDVI for both Maun and Tshane were high (ρ=0.67 and 0.74, respectively, P<0.001), suggesting that this method adequately predicts intra-annual leaf area dynamics in these dry tropical ecosystems.