Declining global warming effects on the phenology of spring leaf unfolding

Earlier spring leaf unfolding is a frequently observed response of plants to climate warming(1-4). Many deciduous tree species require chilling for dormancy release, and warming-related reductions in chillingmay counteract the advance of leaf unfolding in response to warming(5,6). Empirical evidence for this, however, is limited to saplings or twigs in climate-controlled chambers(7,8). Using long-termin situ observations of leaf unfolding for seven dominant European tree species at 1,245 sites, here we show that the apparent response of leaf unfolding to climate warming (S-T, expressed in days advance of leaf unfolding per degrees C warming) has significantly decreased from 1980 to 2013 in all monitored tree species. Averaged across all species and sites, S-T decreased by 40% from 4.0 +/- 1.8 days degrees C-1 during 1980-1994 to 2.3 +/- 1.6 days degrees C-1 during 1999-2013. The declining S-T was also simulated by chilling-based phenology models, albeit with a weaker decline (24-30%) than observedin situ. The reduction in S-T is likely to be partly attributable to reduced chilling. Nonetheless, other mechanisms may also have a role, such as 'photo-period limitation' mechanisms that may become ultimately limiting when leaf unfolding dates occur too early in the season. Our results provide empirical evidence for a declining S-T, but also suggest that the predicted strong winter warming in the future may further reduce S-T and therefore result in a slow down in the advance of tree spring phenology.