Vegetation dynamics in African drylands: an assessment based on the Vegetation Degradation Index in an agro-pastoral region of Botswana

Changes in vegetation conditions are induced both by climatic and human factors. Knowledge about how changes in vegetation conditions relate to its degradation is often lacking, although needed to sustainably manage rangeland resources in drylands. Contributions made to develop land management options require assessing vegetation dynamics and degradation in Palapye, an agro-pastoral region of high economic and biodiversity importance in eastern Botswana. The Vegetation Degradation Index (VDI) was applied to 18-year (1986-2016) time series of annual Normalized Difference Vegetation Index (ANDVI) images to establish vegetation degradation levels, i.e. relatively undegraded, low degradation, medium degradation, and high degradation. Plant species metrics and biophysical variables were then examined at nine sites with the aim of explaining variations in the vegetation degradation levels. Approximately 95% of the study area experienced low degradation and 11% had significant negative trends in ANDVI. Species richness, diversity, and rainfall explained 49% of the variance in degradation levels. Species diversity and richness were highest in Moeng (a relatively undegraded site), whereas they were lowest in Lesenopole (a low degradation site). A probable reason being that Colophospermum mopane contributed 87% of individual trees and shrubs at Lesenopole. Species diversity and richness were also negatively associated with VDI. The non-occurrence of some species such as Sclerocarya birrea and Lonchocarpus capassa on highly degraded sites suggests a link between species composition and the level of degradation. This study combined the use of theremote sensing-based VDI with field-based plant species data for validating vegetation degradation levels in a dryland context.