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

Abstract

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 ~57 000 km and conversely that grasslands have replaced woodlands over ~30 000 km , a net increase in the extent of woodland of ~27 000 km 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 and 0.11% yr , respectively). Protected areas with elephants showed clear loss of woodlands (-0.43% yr ), while commercial rangelands and traditional rangelands showed increases in woodland extent (>0.19% yr ). The woodland change map presented here provides a unique opportunity to test the numerous models of woody plant encroachment at a national/regional scale.

African dryland ecosystem changes controlled by soil water

“…We attribute these contrasting trends to the fact that VOD and NDVI represent different characteristics of the vegetation and infer an increase in the relative proportion of the woody component of the vegetation in African drylands (Andela, Liu, van Dijk, de Jeu, & McVicar, ; R. Fensholt et al, ). The results add further weight to the idea, widely reported over the last decades (Mitchard & Flintrop, ; Skowno et al, ; Wigley, Bond, & Hoffman, ), that woody encroachment prevailed in African drylands during 1993–2012. Moreover, hot spot areas of increased VOD coincide with areas where woody vegetation increased has been reported (Tian, Brandt, Liu, Rasmussen, & Fensholt, ; Zhang et al, ).…”

Drought Dynamics and Vegetation Productivity in Different Land Management Systems of Eastern Cape, South Africa—A Remote Sensing Perspective

“…According to studies carried out in the region, the problem of bush encroachment is present in some of the areas along the coastal zone [80,81]. In addition, Skowny et al [82] showed that areas receiving more than 500 mm MAP show higher rates of woodland expansion. Calculation of MAP for Eastern Cape highlighted the eastern part of the province receiving >500 mm MAP and thereby being prone to bush encroachment according to the findings of Skowny et al [82].…”

“…In addition, Skowny et al [82] showed that areas receiving more than 500 mm MAP show higher rates of woodland expansion. Calculation of MAP for Eastern Cape highlighted the eastern part of the province receiving >500 mm MAP and thereby being prone to bush encroachment according to the findings of Skowny et al [82]. To mask out these areas, spatial information on invasive species or high resolution data in combination with training samples from the field would be necessary.…”

Human impacts in African savannas are mediated by plant functional traits

“…a,b; Buitenwerf et al ., ; Mitchard & Flintrop, ; Stevens et al ., ,b). Savanna tree cover does fluctuate naturally over decades, but numerous African savannas are now on a trajectory of increasing woody biomass, as noted by meta‐analyses of long‐term change (Stevens et al ., ), analysis of photographic records (Wigley et al ., ; Buitenwerf et al ., ; Ward et al ., ) and regional observations derived from remote sensing (Andela et al ., ; Mitchard & Flintrop, ; Stevens et al ., ; Skowno et al ., ). Critically, observed rates of encroachment across Africa are accelerating (Stevens et al ., ).…”