Increase in plant reliance on past precipitation associated with greening and drying

Water is indispensable for life on Earth. Plants use water either from recent precipitation (within a month) or from past precipitation stored in deeper soil (P_P; at least a month ago) to maintain metabolism and growth. It is widely known that plants tend to rely more on P_P to buffer against short-term rainfall deficits. However, how this reliance has changed in response to global change remains unclear. Here we develop a novel framework to estimate temporal changes in plant reliance on P_P during the past four decades. Observational data reveal that 42% of tropical and subtropical natural ecosystems have experienced a significant increase in plant reliance on P_P over the period 1982-2021 (P < 0.05). Such an increase is consistent with greening during the late growing season in drylands and drying during the wet-to-dry transitional period in non-drylands, when short-term precipitation fails to meet plant water demand. Adaptive changes in root depth and species composition may further facilitate this change in P_P reliance, especially in drylands. Our study highlights the importance of P_P in ecosystem functioning and implies an increasing ecosystem resilience to climate variability.