Spatio-temporal change in riparian woodlands of the Kruger National Park: drivers and implications

Verbal accounts, supported by limited ground-based and satellite images, reveal decreasing riparian woodland and a loss of large trees along the rivers of the Kruger National Park (KNP) over the last century. These habitats occupy a tiny fraction of the park’s surface area but fulfil critical ecological functions and provide several ecosystem services. Little research has, however, focused on riparian woodland dynamics across the park. Here a multi-decadal time-series analysis of riparian woodland extent was conducted to identify trends in extent and possible drivers of riparian woody vegetation change. Aerial and satellite imagery (1936 to 2018) was used to measure changes in the extent of riparian woodland tree cover for 18 approx. 10 km long sites along five perennial and nine non-perennial rivers in KNP. This change was compared in a multivariate time-series with river flow and rainfall data from nearby gauging and weather stations, respectively. Particular attention was paid to cumulative flow effects, as well as the frequency and magnitude of large infrequent disturbances (LIDs) such as droughts and floods, which regulate the depth of the water table and may manifest as a physical disturbance. Tree cover fluctuated over the time period and the trajectory of change varied between sites. Most sites (n=11) experienced a decline in overall tree cover over the period while 14 showed a downward trend, six significantly. Overall tree cover increased at six sites, three of which showed an increasing trend (one significantly). There tended to be proportionately higher tree cover loss per year at sites with higher median tree cover. It appears that tree cover decreased substantially at a number of sites following the mega-flood event of 2000 and subsequent large floods over the last decade. It was not possible to generalise responses for the different sites, no doubt because of varying geology, flow regimes and vegetation characteristics at each site, resulting in differing responses to aspects of river flow and rainfall. Peak flow and maximum rainfall events, however, were the strongest significant association with decreases in riparian tree cover, indicating that floods are potentially the biggest drivers of tree loss. Flow variability and cumulative rainfall appear to significantly influence woodland expansion. The initial findings from this study should prompt increased attention to riparian habitats through fine-scale, detailed work aimed at further understanding the dynamics of these systems and determining thresholds for conservation concern in an attempt to ensure persistence of these important ecosystems