Leaf phenology sensitivity to temperature in European trees: Do within-species populations exhibit similar responses?

Consequences of climate warming on tree phenology are readily observable, but little is known about the differences in phenological sensitivity to temperature between species and between populations within a species. The aim of the present study is to compare phenological sensitivities to temperature of seven woody species between each other and within-species between two geographical areas using both altitudinal and temporal gradients (Abies alba, Acer pseudoplatanus, Carpinus betulus, Fagus sylvatica, Fraxinus excelsior, Ilex aquifolium and Quercus petraea). The timing of leaf unfolding was monitored (i) over 2 years along two altitudinal gradients in the Pyrénées mountains (six species), and (ii) over 22 years in Fontainebleau forest (four species). Three species were present in both areas which allowed us to compare their phenological sensitivity to temperature over altitudinal and temporal gradients. Along altitudinal gradients, we observed for all species an advance in leaf unfolding with decreasing elevation, ranging from 11 to 34 days 1000m⁻¹ for beech and oak, respectively. Across the temporal gradient, we found significant advances in leaf unfolding for oak (-0.42daysyear⁻¹) and ash (-0.78daysyear⁻¹) since 1976, whereas no significant advance was observed for beech and hornbeam. For both gradients and for all species, significant correlations were found between leaf unfolding dates and temperature, except for beech in the temporal study. Moreover, we highlighted that phenological sensitivity to temperature was very similar between the two geographically separated populations (Pyrénées and Fontainebleau forests). Thus, oak had the strongest sensitivity (-7.48 and -7.26days°C⁻¹ in altitudinal and temporal gradient, respectively) and beech had the lowest (-2.09 and -2.03days°C⁻¹). Our results suggest that population sensitivity to global warming might be stable for a given species, in spite of its possible local adaptation.