Resource redistribution mediated by hydrological connectivity modulates vegetation response to aridification in drylands

Water scarcity poses a significant life constraint in global drylands that determines species adaptations and mosaic of exposed bare areas and vegetation patches. Runoff-water redistribution resulting from this spatial configuration has been suggested as a key process controlling water availability for vegetation and ecosystem functioning. However, the potential of this process to ameliorate the negative impacts of aridification in drylands remains unclear, and there is no empirical evidence of its relevance on natural ecosystems under different levels of aridity and disturbance regimes. To address this gap, we analysed temporal series of the normalized vegetation index (NDVI, a proxy of vegetation functioning) along a regional aridity–disturbance gradient under current and future climatic conditions. We found that mean NDVI increases in areas of runoff water accumulation (calculated using a water redistribution index) until a certain threshold, above which vegetation patches are not able to retain extra runoff water. Once thresholds were identified, we analysed the role of water redistribution on vegetation dynamics by analysing temporal series of monthly NDVI in a space–for–time substitution approach. The obtained results provided further evidence of the runoff water redistribution on vegetation, triggering a positive feedback between water accumulation and vegetation growth. Results obtained by the combination of the obtained model with climatic data from the 6th IPCC report suggest that this feedback could ameliorate the expected negative effects of aridification in drylands. However, this effect is partially counterbalanced in scenarios of high human disturbance and in areas where vegetation is not able to trap and retain the extra amount of resources given by runoff. Overall, our results provide empirical evidence of the relevance of runoff redistribution as a key process linking vegetation patterns to climate resistance in drylands that underscores its importance in the analysis and modelling of drylands' responses to aridification.