Dryland vegetation functional response to altered rainfall amounts and variability derived from satellite time series data

Abstract. Vegetation net productivity is a key variable in ecosystem functioning. Understanding how its functional response to rainfall in drylands is affected by altered rainfall amounts and variability is therefore vitally important to understand consequences of climatic change for those water-limited ecosystems. Here, we show how this functional response is affected by below and above 30-year-average rainfall conditions in two arid to semi arid subtropical regions in West and South West Africa differing markedly in interannual rainfall variability (higher in South West Africa, lower in West Africa). Shifting linear regression models (SLRs) were used with annual precipitation (satellite derived African Rainfall Climatology 2, ARC2) as explanatory variable and annual satellite-derived vegetation productivity proxies (normalized difference vegetation index, NDVI) as response variable to estimate the gridded vegetation functional response to rainfall. From the SLRs, time series of responses were derived and analyzed along gradients of mean annual precipitation. Vegetation responses to rainfall show a unimodal response along rainfall gradients. While responses for South West Africa are higher during dry periods for mean annual precipitation < 500 mm and spatially more variable, the responses to climate for West Africa are generally low and spatially less dynamic. Those patterns follow differences in interannual rainfall amount variability (higher in South West Africa). Regional peaks of vegetation response to rainfall along mean annual precipitation are found at precipitation values with similar interannual variability in growing season length. Vegetation type (MODIS MCD12C) specific response to rainfall mostly follows observed responses along rainfall gradients leading to region specific responses for each vegetation type. We conclude that higher rainfall amount variability enhances regional-scale vegetation response to rainfall plasticity and thus dryland ecosystem resilience to dry periods. Those results apply irrespective of vegetation type and thus evidence the fundamental role of rainfall variability in ecosystem functioning. Presented results moreover imply that the Sahel region (West Africa) although currently recovering from drought might be highly susceptible to future dry periods.