Acclimation of phenology relieves leaf longevity constraints in deciduous forests

Leaf phenology is key for regulating total growing-season mass and energy fluxes. Long-term temporal trends towards earlier leaf unfolding are observed across Northern Hemisphere forests. Phenological dates also vary between years, whereby end-of-season (EOS) dates correlate positively with start-of-season (SOS) dates and negatively with growing-season total net CO2 assimilation (A(net)). These associations have been interpreted as the effect of a constrained leaf longevity or of premature carbon (C) sink saturation-with far-reaching consequences for long-term phenology projections under climate change and rising CO2. Here, we use multidecadal ground and remote-sensing observations to show that the relationships between A(net) and EOS are opposite at the interannual and the decadal time scales. A decadal trend towards later EOS persists in parallel with a trend towards increasing A(net)-in spite of the negative A(net)-EOS relationship at the interannual scale. This finding is robust against the use of diverse observations and models. Results indicate that acclimation of phenology has enabled plants to transcend a constrained leaf longevity or premature C sink saturation over the course of several decades, leading to a more effective use of available light and a sustained extension of the vegetation CO2 uptake season over time. Multidecadal ground and remote-sensing observations of Northern Hemisphere forests show that, at the decadal scale, autumn senescence dates and total net carbon assimilation are positively related, despite a negative relationship at the annual scale. This suggests that acclimation relieves the leaf longevity constraints.

Automated summary

Leaf phenology plays a crucial role in regulating mass and energy fluxes during the growing season. The timing of leaf unfolding has been observed to occur earlier in Northern Hemisphere forests over the long-term. Phenological dates vary between years, with end-of-season dates correlating positively with start-of-season dates and negatively with net CO2 assimilation. This indicates the impact of leaf longevity constraints or premature carbon sink saturation on long-term phenology projections. However, multidecadal observations show a decadal trend towards later leaf senescence, along with increasing net carbon assimilation, contradicting the interannual scale relationship.