Declining precipitation frequency drivers earlier leaf senescence by intensifying drought stress and enhancing drought acclimation

Abstract

Precipitation is an important factor influencing the date of leaf senescence (DFS), which in turn affects carbon uptake of terrestrial ecosystems. However, the temporal patterns of precipitation frequency (P_freq) and its impact on DFS remain largely unknown. Using both long-term carbon flux data and satellite observation of DFS across the Northern Hemisphere, here we show that, after excluding impacts from of temperature, radiation and total precipitation, declining P_freq drives earlier DFS from 1982 to 2022. A decrease in P_freq intensified drought stress by reducing root-zone soil moisture and increasing atmospheric dryness, and limit the photosynthesis necessary for sustained growth. The enhanced drought acclimation also explained the positive P_freq-DFS relationship. We found plants experiencing decreased P_freq showed a more rapid response to drought, as represented by a shorter drought response lag, a measure of the time between a drought event and the most severe reduction in vegetation growth. In particular, increased evapotranspiration with shorter drought response lag was observed, further implying an enhanced water acquisition strategy representing drought acclimation as showing in strengthening roots system to deeper water resources. Finally, we found 30 current state-of-art Earth system models largely failed to capture the sensitivity of DFS to changes in P_freq and incorrectly predicted the direction of correlations for approximately half of the northern global lands, in both historical simulations and future predictions under various shared socioeconomic pathways (SSPs). Our results therefore highlight the critical need to include precipitation frequency, rather than just total precipitation, into models to accurately forecast plant phenology under future climate change.