Impact of changing precipitation variability on carbon budgets of global semi-arid savannas

Global warming leads to increased precipitation variability, impacting vegetation and the terrestrial carbon sink. While the impact of mean annual precipitation on vegetation and the carbon cycle is well-studied, recent research emphasizes the importance of intra-annual precipitation variability in precipitation-productivity relationships. In drylands the effects of changed precipitation variability on ecosystem functioning are still not fully understood, and therefore also not captured well in Earth system models. Available studies cover either multiple sites, but focus on the effects of changing inter-annual precipitation variability on productivity, or focus on single dryland sites to assess the effect of changing seasonal precipitation variability on carbon fluxes. A multi-site analysis of the effect of intra-annual precipitation variability on carbon fluxes across global semi-arid savannas is still missing.

Here, we contribute to closing this gap by incorporating data from multiple eddy covariance measurement stations located in semi-arid savanna ecosystems around the globe. We examined the impacts of precipitation variability on ecosystem carbon fluxes by: i) assessing the sensitivity of ecosystem CO₂ fluxes to precipitation amount, frequency and intensity on the annual scale, ii) evaluating the importance of each metric in explaining seasonal CO₂ fluxes, and iii) understanding direct and indirect effects of precipitation metrics on ecosystem CO₂ fluxes in different seasons.

On the annual scale precipitation variability has a positive effect on both gross primary productivity and ecosystem respiration. However, these effects partly cancel out, with different ecosystem processes dominating in different seasons. On the seasonal scale, both precipitation frequency and intensity explain more variance in net CO₂ fluxes than precipitation amount. Linear mixed effect models show that models containing all three metrics together have the most explanatory power. Structural equation models show that across seasons soil water content is the main mediator of precipitation impacts on CO₂ fluxes. In the next steps we will investigate how other site properties, such as canopy cover and height, mean annual precipitation or soil composition, modulate the effects of precipitation variability on the ecosystem CO₂ fluxes.