Vegetation resilience to precipitation anomalies in savanna and grassland biomes in arid and semi-arid Africa

Background/Question/Methods

Climate Change is assumed to alter temperature and precipitation patterns throughout (semi-)arid Africa and to even increase the high climatic variability which is characteristic for drylands. Projections indicate an increase in temperature and in intra- and interannual variability of rainfall, leading to erratic switches between high and low rainfall years. Savanna and grassland biomes represent the majority of terrestrial ecosystems in Sub-Saharan Africa. The two biomes are assumed to differ in their buffering capacity for rainfall variability including extreme events such as droughts. Besides biome type, grazing intensity also seems to play a crucial role for vegetation resilience to drought.

Our study aims to assess the resilience of savanna and grassland vegetation to droughts, comparing monitoring data from sites with different grazing intensities. We analyze 11 long-term plant productivity studies from Sub-Sahara Africa (7 savanna and 4 grassland studies). Aboveground net primary production (ANPP) and rain-use efficiency (RUE; ANPP/annual precipitation) are used as proxy for vegetation performance. We evaluate vegetation response to droughts in the past decades with respect to two aspects of resilience, i.e. drought resistance and recovery. We define resistance as the difference in ANPP and RUE between drought years and the average of years with average rainfall. Similarly, recovery is tested as differences in ANPP and RUE between drought and post-drought years.

Results/Conclusions

From preliminary and published studies, we know that the grassland and savanna biome show distinct responses to precipitation and precipitation anomalies. The coupling between productivity and precipitation is stronger in grasslands as compared to savanna systems (Ruppert et al. 2012). At the same time grassland sites are also more resilient to precipitation anomalies, while savanna sites are more prone to degradation via bush encroachment and/or erosion. By improving the understanding of the role of rainfall anomalies, and specifically its interference with grazing intensity, in savanna and grassland systems, this study might help adapting the right management strategies to climate change.