Drought, climate change and vegetation response in the succulent karoo, South Africa

Introduction More than 90% of South Africa is either arid or semi-arid and drought is a characteristic feature of the climate. Because drought has significant ecological and socio-economic impacts, investigations into its causes, consequences and mitigation have been regularly undertaken in South Africa over the last 100 years. Recent climate change scenarios suggest that there will be an increase in the frequency of extreme events, including drought, particularly in the winter-rainfall region of southern Africa as a result of the predicted pole-ward retreat of rain-bearing frontal systems. Such predictions raise two important questions. Firstly, is there evidence in the climate record that annual rainfall has already declined and the incidence of drought has increased over the last 100 years in the winter-rainfall region of South Africa? Previous analyses of the historical winter rainfall record found little evidence either for a decline in rainfall or an increase in drought over the 20th century. However, a more detailed analysis of Namaqualand’s rainfall record described the pattern as spatially complex, with some areas exhibiting wetter and others drier conditions since 1950. Unequivocal evidence for a recent increase in drier conditions, such as was recorded for the Sahel in the 1970s and 1980s, will help motivate the region to prepare for the inevitable consequences of less rain and help offset some of the worst effects of drought. A detailed analysis of the historical rainfall record will also place any future drought in its proper context in terms of the intensity and duration. The second question which arises from the climate change scenarios is concerned with the response of vegetation to drought. Both the fynbos and succulent karoo biomes, which characterise the winter-rainfall region of South Africa, are internationally recognised for their high levels of biodiversity and endemism. Oscillating wet and dry climatic conditions in the succulent karoo and a moderate climatic history within the succulent karoo have both been suggested to be instrumental in the development of high levels of floral diversity. The moderate climatic history argument has lead to the suggestion that the flora is vulnerable to the effects of climate change. What will be the impact of less rain and a higher incidence of drought on the vegetation of these biomes? While several studies explored the impact of future climate change on fynbos and succulent karoo biome vegetation none has assessed the historical impact of drought on fynbos species and only four studies have been undertaken in the succulent karoo biome. All have focused on the response of leafand stem-succulent shrubs relative to non-succulent evergreen and deciduous shrubs and have returned contradictory results. Some studies have reported that leaf succulents were negatively affected by drought while at other sites leaf succulents were either hardly affected at all or were less affected than non-succulent shrubs. In the first part of this paper we examine the 20th century rainfall record at six stations across the succulent karoo biome to test whether annual rainfall has declined and whether or not the incidence of drought has increased since 1900. In the second part we review the results of previous studies to understand the impact of drought events on both adult plants and seedlings which occur within the region.