Impacts of global warming on phenology of spring leaf unfolding remain stable in the long run

The impact of spring temperature forcing on the timing of leaf unfolding of plants (temperature sensitivity, S_T) is one important indicator of how and to what degree plant species track climate change. Fu et al. (Nature 526:104-107, 2015) found that S_T has significantly decreased from the 1980-1994 to the 1999-2013 period for seven mid-latitude tree species in Europe. However, long-term changes in S_T over the past 60 years are still not clear. Here, using in situ observations of leaf unfolding for seven dominant European tree species, we analyze the temporal change in S_T over decadal time scales extending the data series back to 1951. Our results demonstrate that S_T shows no statistically significant change within shifting 30-year windows from 1951 to 2013 and remains stable between 1951-1980 and 1984-2013 (3.6 versus 3.7 days °C^-1). This result suggests that the significant decrease in S_T over the past 33 years could not be sustained when examining the trends of phenological responses in the long run. Therefore, we could not conclude that tree spring phenology advances will slow down in the future, and the S_T changes in warming scenarios are still uncertain.