Precipitation and temperature drive woody dynamics in the grasslands of sub-Saharan Africa

Abstract

Abstract Understanding the drivers of ecosystem dynamics, and how responses vary spatially and temporally, is a critical challenge in the face of global change. Here we used structural equation models and remote sensing datasets to understand the direct and indirect effects of climatic, environmental, and anthropogenic variables on woody vegetation dynamics across four grasslands regions (i.e., Sahel grasslands, Greater Karoo and Kalahari drylands, Southeast African subtropical grasslands, and Madagascar) of sub-Saharan Africa. We focus on African grasslands given their importance for biodiversity and ecosystem services, the lack of clarity on how they are likely to respond to changes in disturbances, and how such responses vary geographically. This is particularly the case of grass-dominated ecosystems – the focus of our study – rather than more mixed grass-tree regions (e.g., savannas). Rainfall (β = 0.148 [-0.111, 0.398]) and temperature (β = -0.109 [-0.387, 0.133]) showed consistently opposing effects on woody vegetation (average standardised regression coefficients and 95% confidence interval range during 1997–2016) across the four bioregions. Other variables showed overall negligible effects including, for instance, dry season rainfall, soil moisture and, notably, fire. Other relationships were more context-dependent. Only Greater Karoo and Kalahari drylands showed a negative relationship between woody vegetation and fire (β = -0.031 [-0.069, 0.021]). Similarly, in Madagascar we observed strong negative effects of temperature (β = -0.429 [-1.215, -0.259]) and population density (β = -0.354 [-0.651, -0.015]) on burned area, yet these did not result in any significant indirect effects on woody vegetation. Our results clarify the contribution of environmental and anthropogenic variables in controlling woody dynamics at broad spatiotemporal scales and reveal that the widely documented negative feedback between fire and woody vegetation does not necessarily apply across all African grasslands.