Simulated sensitivity of African terrestrial ecosystem photosynthesis to rainfall frequency, intensity, and rainy season length

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Guan, R., et al. (2017) “Simulated sensitivity of African terrestrial ecosystem photosynthesis to rainfall frequency, intensity, and rainy season length”, Environmental Research Letters, doi:10.1088/1748-9326/aa9f30.

Abstract:Evidence is growing that there are ongoing changes in the statistics of intra-seasonal rainfall variability over large parts of the world. Changes in annual total rainfall may arise from shifts, either singly or in a combination of distinctive intra-seasonal characteristics: rainfall frequency, rainfall intensity and rainfall seasonality. Understanding how various ecosystems respond to the changes in intra-seasonal rainfall characteristics is critical to predict future biome shifts and ecosystem service under climate change, especially for dry and semi-dryland ecosystems. Here, we use an advanced dynamic vegetation model (SEIB-DGVM) and a stochastic rainfall/weather simulator to answer the following question: given the same percentage change in the total seasonal rainfall but realized in only varying one of the three rainfall characteristics (i.e. rainfall frequency, intensity, and rainy season length), to what extent do various ecosystems respond to these rainfall changes in ecosystem productivity? We conducted ensemble simulations for the continental Africa for the realistic range of changes (-20%~+20%) in total rainfall amount but varying in different ways (i.e. three intra-seasonal rainfall characteristics). We find that the simulated ecosystem productivity (measured by gross primary production, GPP) shows distinctive responses to the intra-seasonal rainfall characteristics. Specifically, increase in rainfall frequency can lead to 28% more GPP increase than the same percentage increase in rainfall intensity; in tropical woodlands, GPP sensitivity to changes in rainy season length is ~4 times larger than to the same percentage changes in rainfall frequency or intensity; and finally, the simulated biome distribution shifts is much less sensitive to intra-seasonal rainfall characteristics but primarily to total rainfall amount. Our results reveal three major distinctive productivity responses to seasonal rainfall variability – “chronic water stress”, “acute water stress” and “minimum water stress” - which are respectively associated with three broad spatial patterns of African ecosystem physiognomy, i.e. savannas, woodlands, and tropical forests.

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