A repeat photograph analysis of long-term vegetation change in semi-arid South Africa in response to land use and climate

QuestionsHow has the vegetation of the major biomes (Grassland, Nama-karoo, Albany Thicket, Azonal) of southeastern South Africa changed over the course of the 20th century? How do changes in climate and land-use drivers relate to long-term changes in vegetation? What are the implications of these findings for land degradation hypotheses and future climate change projections for the region? LocationThe biogeographically complex semi-arid, Karoo Midlands region of the southeastern part of South Africa. MethodsWe re-photographed 65 historical landscape photographs, the majority of which dated from 1950 to 1970, to measure long-term changes in the cover of grasses, dwarf shrubs, tall shrubs and alien plants. The cover of each growth form as well as total vegetation cover was estimated from matched photograph pairs with the aid of detailed cover estimates recorded in the field. The change in cover was relativized between sites by dividing the difference in cover between the two time steps by the number of years between photographs, expressed as the percentage change in cover per decade. Significant changes in mean annual rainfall and the standardized precipitation index (SPI) from 27 climate stations were assessed using a Mann-Kendall test for trend. This non-parametric test was also used to assess the significance of long-term trends in the number of cattle, sheep and goats in each of the biomes over the period 1911-1996. ResultsGrass cover and total vegetation cover had increased by between 1.0% to 4.5% per decade and 2.0% to 4.5% per decade, respectively, in all biomes investigated. In contrast, the cover of dwarf shrubs had decreased significantly by between 0.25% and 3.0% per decade, although not significantly so in the Nama-karoo biome. The change in tall shrub cover varied between different biomes but had generally increased in the study area. Alien plants were absent in the historical photographs and had increased significantly but only in Azonal habitats, where increases of 1.5% per decade were recorded. For the majority of climate stations no significant trend in mean annual rainfall and SPI values was recorded, while stocking rate had declined significantly in all biomes by between 36% and 48% from 1911 to 1996. ConclusionsThe findings support the hypothesis that vegetation cover and condition has improved in the semi-arid regions of South Africa. These findings are discussed in light of future projections for the region.