Woodland expansion in South African grassy biomes based on satellite observations (1990-2013): general patterns and potential drivers

Increases in woody plant cover in savanna grassland environments have been reported on globally for over 50 years and are generally perceived as a threat to rangeland productivity and biodiversity. Despite this, few attempts have been made to estimate the extent of woodland increase at a national scale, principally due to technical constraints such as availability of appropriate remote sensing products. In this study, we aimed to measure the extent to which woodlands have replaced grasslands in South Africa's grassy biomes. We use multiseason Landsat data in conjunction with satellite L-band radar backscatter data to estimate the extent of woodlands and grasslands in 1990 and 2013. The method employed allows for a unique, nationwide measurement of transitions between grassland and woodland classes in recent decades. We estimate that during the 23-year study period, woodlands have replaced grasslands over similar to 57 000 km(2) and conversely that grasslands have replaced woodlands over similar to 30 000 km(2), a net increase in the extent of woodland of similar to 27 000 km(2) and an annual increase of 0.22%. The changes varied markedly across the country; areas receiving over 500 mm mean annual precipitation showed higher rates of woodland expansion than regions receiving <500 mm (0.31% yr(-1) and 0.11% yr(-1), respectively). Protected areas with elephants showed clear loss of woodlands (-0.43% yr(-1)), while commercial rangelands and traditional rangelands showed increases in woodland extent (>0.19% yr(-1)). The woodland change map presented here provides a unique opportunity to test the numerous models of woody plant encroachment at a national/regional scale.