Long-term trends towards delayed autumn senescence prevail over short-term effects by high early-season CO2 assimilation

Understanding how leaf autumn phenology varies at different spatio-temporal scales is key to accurately predicting phenological changes under future climate. Recent projections and observations of autumn phenology in deciduous temperate and boreal forests appear conflicting. At the interannual scale, autumn senescence correlates positively with spring leaf-out and negatively with growing season total photosynthesis. These links have been interpreted as the effect of premature carbon sink saturation with potentially far-reaching consequences for carbon cycle projections in a high-CO₂ world. In this study, we use multi-decadal ground and remote-sensing observations to show that these relationships are opposite at the interannual and the decadal time scales. We found a decadal trend towards later autumn senescence in parallel with a trend towards increasing photosynthesis, despite their negative relationship at the interannual scale. Our findings reveal that the leaf longevity constraint has not remained stationary over longer time scales. These shifting relationships suggest a gradual acclimation of phenology, leading to a relief of effects that dominate year-to-year variations. This apparent acclimation implies that in the long run, trees may benefit from increased photosynthesis under rising CO₂ and evade a direct effect by which increased photosynthesis induces an earlier leaf senescence. This apparent plasticity in phenology appears to have driven plants towards optimal functioning in a changing climate.