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

Aim Ongoing climate warming has been widely reported to delay autumn phenology, which in turn impacts carbon, water, energy and nutrient balances at regional and global scales. However, the underlying mechanisms of autumn phenology responses to climate change have not been fully elucidated. The aims of this study were to determine whether brightening that was defined as the increase of surface solar radiation and warming during recent decades affect autumn phenology in opposite directions and explore the underlying mechanisms. Location Central Europe. Time period 1950-2016. Major taxa studied Four dominant European tree species in central Europe: Aesculus hippocastanum, Betula pendula, Fagus sylvatica and Quercus robur. Methods We investigated the temporal trends of leaf senescence, preseason temperature and radiation by separating the period of 1950-2016 into two sub-periods (1950-1982 and 1983-2016) and determined the relationship between temperature, radiation and leaf senescence using partial correlation analysis. Results We found a significant warming and brightening trend after the 1980s in central Europe, yet this led to only slight delays in leaf senescence that cannot be explained by the well-known positive correlation between leaf senescence and autumn warming. Interestingly, we found opposite effects between warming (partial correlation coefficient, r = .37) and brightening (r = -.23) on leaf senescence. In addition, the temperature sensitivity of leaf senescence decreased with increasing radiation (-5.08 days/celcius/10(8) J/m(2)). Main conclusions The results suggested that brightening accelerated the leaf senescence dates, counteracting the warming-induced delays in leaf senescence, which may be attributed to photooxidative stress and/or sink limitation. This emphasizes the need to consider radiation to improve the performance of autumn phenology models.