Reversing effects of seasonal warming on autumn leaf senescence across the Northern Hemisphere

Under climate warming, tree autumn phenology exhibits divergent trends, with both advances and delays observed across species and regions, reflecting its complex climatic responses. Although it has been recognized that both spring and autumn warming affect autumn leaf senescence, the way trees integrate these seasonal climate signals remains unclear. By combining phenological records of 615 tree species from 3,282 ground sites and two satellite products across the Northern Hemisphere, with controlled seasonal warming experiments, we demonstrate that spring and autumn temperatures exert opposing effects on autumn leaf senescence, with spring warming advancing and autumn warming delaying it. However, in warmer regions and periods, these temperature effects undergo a reversal, with intensified spring warming shifting autumn senescence from advance to delay and elevated autumn warming shifting it from delay to advance. Seasonal warming experiments show that spring affects leaf senescence primarily through carbohydrate dynamics, whereas autumn acts through direct warming effects that regulate chlorophyll maintenance. Earth system model projections indicate that these phenological reversals are already occurring in warmer regions and will likely intensify progressively over the period 2020–2100. Analyses show that autumn temperatures exert a stronger influence than spring temperatures, likely by intensifying water stress and accelerating metabolic processes that trigger earlier leaf senescence, suggesting that future warming will predominantly lead to earlier autumn senescence, especially in warmer regions. Our results reveal seasonally dependent and nonlinear climate controls on leaf senescence, highlighting the urgent need to integrate cross-seasonal warming effects into Earth system models to improve predictions of forest carbon cycling and climate feedbacks in a warming world.