Leaf green-up in a semi-arid African savanna - separating tree and grass responses to environmental cues

Abstract

The savannas of Africa are dominated by deciduous trees and perennial but seasonal grasses. Virtually nothing is known about the factors that trigger the onset of greening, or the time-course of leaf-drop in either of these main plant functional types in this biome. The Modis Terra LAI and NDVI dataset (MOD15A2 and MOD13A2) were used to provide a five year record of greenness for a 7 Km X 7 km square centred on the Skukuza flux site (25 Degrees S, 31 Degrees E) between 2000 and 2005. The seasonal and inter-annual patterns of leaf display of trees and grasses were described, using ecological data to un-mix the NDVI of each life form from the total landscape NDVI. The data shows that at a landscape scale savanna trees have a less viable phenological cycle (within and between years) than grasses. Realistic biophysical models of these systems need to take this into account, and using climatic data to predict these dynamics seems to be a feasible approach.