Abstract
Leaf unfolding in temperate forests is driven by spring temperature, but little is known about the spatial variance of that temperature dependency. Here we use in situ leaf unfolding observations for eight deciduous tree species to show that the two factors that control chilling (number of cold days) and heat requirement (growing degree days at leaf unfolding, GDDreq) only explain 30% of the spatial variance of leaf unfolding. Radiation and aridity differences among sites together explain 10% of the spatial variance of leaf unfolding date, and 40% of the variation in GDDreq. Radiation intensity is positively correlated with GDDreq and aridity is negatively correlated with GDDreq spatial variance. These results suggest that leaf unfolding of temperate deciduous trees is adapted to local mean climate, including water and light availability, through altered sensitivity to spring temperature. Such adaptation of heat requirement to background climate would imply that models using constant temperature response are inherently inaccurate at local scale.
Highlights
Leaf unfolding in temperate forests is driven by spring temperature, but little is known about the spatial variance of that temperature dependency
The key hypothesis that we explore in this study is that longterm mean background biogeographical conditions determine the spatial heterogeneity of spring leaf unfolding (LU) and its Growing Degree Days requirement (GDDreq), reflecting evolutionary mechanisms through which plants have adjusted their growth strategies in order to maximize their fitness under those specific biogeographical conditions
We argue that biogeographic constraints on plant phenology can be detected by analyzing the spatial response of stands long-term mean LU and GDDreq, instead of their inter-annual variability
Summary
Leaf unfolding in temperate forests is driven by spring temperature, but little is known about the spatial variance of that temperature dependency. Radiation intensity is positively correlated with GDDreq and aridity is negatively correlated with GDDreq spatial variance These results suggest that leaf unfolding of temperate deciduous trees is adapted to local mean climate, including water and light availability, through altered sensitivity to spring temperature. Such adaptation of heat requirement to background climate would imply that models using constant temperature response are inherently inaccurate at local scale. Biogeographic constraints on LU include all environmental variables that impose long-term adaptation of LU and its GDDreq to optimize fitness under local conditions These include climatic variables, such as site-specific occurrence of late frost events, drought occurrence, low or high light extremes, that may need to be avoided and require shifts in growing season to enable maximum tree fitness. Site-specific interactions with neighboring competitors, pathogens and herbivores may induce spatial differences in LU and its weather dependency, in order to maximize tree fitness
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