Assessing the effects of human-induced land degradation in the former homelands of northern South Africa with a 1 km AVHRR NDVI time-series

Abstract

There is a pressing need for an objective, repeatable, systematic and spatially explicit measure of land degradation. In northeastern South Africa, there are large areas of the former homelands that are widely regarded as degraded. A time-series of seasonally integrated I km, Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data was used to compare degraded rangelands [mapped by the National Land Cover (NLC) using Landsat Thematic Mapper (TM) imagery] to nondegraded rangelands within the same land capability units (LCUs). Nondegraded and degraded areas in the same LCU (paired areas) were compared by: testing for differences in spatial mean SigmaNDVI values, calculating the relative degradation impact (RDI) as the difference between the spatial mean SigmaNDVI values of paired areas expressed as a percentage of nondegraded mean value, investigating the relationship between RDI and rainfall and comparing the resilience and stability of paired areas in response to natural variations in rainfall. The SigmaNDVI of degraded areas was significantly lower for most of the LCUs. Relative degradation impacts (RDI) across all LCUs ranged from 1% to 20% with an average of 9%. Although SigmaNDVI was related to rainfall, RDI was not. Degraded areas were no less stable or resilient than nondegraded. However, the productivity of degraded areas, that is the forage production per unit rainfall, was consistently lower than nondegraded areas, even within years of above normal rainfall. The results indicate that there has not been a catastrophic reduction in ecosystem function within degraded areas. Instead, degradation impacts were reflected as reductions in productivity that varied along a continuum from slight to severe, depending on the specific LCU