Effect of climate warming on the timing of autumn leaf senescence reverses after the summer solstice

Abstract

Climate change is shifting the growing seasons of plants, affecting species performance and biogeochemical cycles. Yet how the timing of autumn leaf senescence in Northern Hemisphere forests will change remains uncertain. Using satellite, ground, carbon flux, and experimental data, we show that early-season and late-season warming have opposite effects on leaf senescence, with a reversal occurring after the year’s longest day (the summer solstice). Across 84% of the northern forest area, increased temperature and vegetation activity before the solstice led to an earlier senescence onset of, on average, 1.9 ± 0.1 days per °C, whereas warmer post-solstice temperatures extended senescence duration by 2.6 ± 0.1 days per °C. The current trajectories toward an earlier onset and slowed progression of senescence affect Northern Hemisphere–wide trends in growing-season length and forest productivity.

Process‐oriented models of leaf senescence are biased towards the mean: Impacts on model performance and future projections

“…The best models for strongly shifting leaf senescence are arguably least biased towards the mean. These models simulated the senescence process based on precipitation and/or temperature during the growing season, or the timing of spring leaf phenology, which constitute legacy effects (e.g., Bigler & Vitasse, 2021; Dox et al., 2022; Xie et al., 2015; Zohner et al., 2023). Thus, our study shows the importance of legacy effects for the leaf senescence process, and further studies are needed for their quantification (Chuine & Régnière, 2017).…”

Decoding autumn phenology: Unraveling the link between observation methods and detected environmental cues

“…Following Zohner et al. (2023), who identify pre‐solstice vegetation activity/temperature and autumn temperature as the main drivers of autumn phenology, two additional analyses were performed. First, for each derivation methodology, a multiple linear regression was calculated with the respective EOS as the dependent variable and growing season first half‐NEP (N 1 ) and autumn temperature ( T AU ) as predictors.…”

“…Following Zohner et al (2023), who identify pre-solstice vegetation activity/temperature and autumn temperature as the main drivers of autumn phenology, two additional analyses were performed.…”

“…This could be because heat stress was shown to lead to earlier rather than later autumn phenology (Xie et al, 2015(Xie et al, , 2018. Furthermore, Zohner et al (2023) recently noted a solstice effect in temperature, with increased temperatures before solstice leading to earlier senescence and after solstice leading to a lengthening of the autumn growing season. Finally, temperature-related effects on senescence seem to be also speciesdependent (Grossiord et al, 2022).…”

“…univariate analysis, differences between direct (DWD/CC) and indirect (MODIS/GPP/NEP) derivation methodologies and between early (DWD:LC/MODIS:SE) and late autumn phenological (DWD:LF/ MODIS:DO) metrics could not be detected.The data source played a crucial role in investigating the interplay between EOS, N 1 , and T AU , as proposed byZohner et al (2023). When keeping autumn temperature constant, the various EOS responses showed a nuanced pattern relative to N 1 (Figure8a): While higher photosynthetic activity in the first half of the growing season lead to a delay in the autumn phenology of GPP:DT, MODIS:DO, and especially CC:Fagus, some of the EOS variables were hardly affected, or even occurred earlier (CC:Fraxinus, GPP:NT, and DWD:LC).…”