Atmospheric brightening counteracts warming-induced delays in autumn phenology of temperate trees in Europe

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Zhaofei Wu, Shouzhi Chen, Shuxin Wang and Yongshuo H. Fu, Beijing Normal University, Hans J. De Boeck, University of Antwerp, Nils Christian Stenseth, University of Oslo, Nils Christian Stenseth, University of Agder, Jing Tang, University of Copenhagen, Jing Tang, Lund University, Yann Vitasse, Swiss Federal Institute for Forest, Constantin Zohner, ETH Zurich

Background/Question/Methods
Ongoing climate change is affecting autumn phenology of temperate forests, which in turn impacts carbon, water, energy and nutrient balances at regional and global scales. However, the underlying mechanisms of autumn phenology responses to global warming have not been fully elucidated. Warmer autumns under climate change are generally expected to delay leaf senescence of trees. However, the effects of concomitant increases in solar radiation (hereafter referred to as atmospheric brightening) since 1980 in Europe, caused by reductions in cloud cover and airborne pollution, have not been quantified so far. Based on extensive in situ observations of leaf senescence for four dominant European tree species at 1,160 sites over the period 1950-2016 in central Europe, we investigated the temporal trends of leaf senescence in response to warming and brightening.
Results/Conclusions
We found a significant warming and brightening trend after the 1980s in Central Europe, yet this led to only slight delays in leaf senescence which cannot be explained by the well-known positive correlation between leaf senescence and autumn warming. Interestingly, we found contrasting effects between warming (partial correlation coefficient, r = 0.42) and brightening (r = -0.25) on leaf senescence. In addition, the temperature sensitivity of leaf senescence decreased with increasing radiation (-5.2 days·℃-1/108J·m-2), which may be attributed to photooxidative stress and/or sink limitation. Our findings provide empirical evidence for an antagonistic effect of radiation and temperature on leaf senescence, suggesting that brightening accelerates the leaf senescence process, limiting the warming-induced delays in leaf senescence. This emphasizes the need to consider radiation to improve the performance of phenology models.