Convergence of leaf-out towards minimum risk of freezing damage in temperate trees

Summary1. Within the same forest stand, temperate deciduous trees generally exhibit a distinct patternin leaf-out timing, with some species flushing earlier than other species. This study aimed toexplain the timing of leaf-out of various temperate tree species in relation to the risk offreezing damage to leaves.2. We combined long-term series of leaf-out date (14–32 years) of five temperate tree specieslocated in both low and high elevations in Switzerland, daily minimum temperatures recordedat the same sites and species-specific freezing resistance (LT50) of emerging leaves. Wecalculated temperature safety margins (the temperature difference between absolute minimumtemperature during leaf-out and species-specific LT50 values), and date safety margins (timelag between the last day when temperature falls below species-specific LT50 values and the dateof leaf-out).3. Leaf-out occurred when the probability to encounter freezing damage approaches zero, irrespectiveof climatic conditions (low vs. high elevation) and species (early- and late-flushing species).In other words, trees leaf out precisely at the beginning of the probabilistically safeperiod. Interestingly, the temperature safety margins did not differ significantly between lowand high elevation. Yet, the date safety margin was smaller at high elevation, presumably dueto a faster increase in temperature during the leaf-out period at high elevation.4. When species-specific freezing resistance is taken into account, the time of leaf-out convergesamong species towards a marginal risk of freezing damage. Thus, leaf-out time haslikely evolved in a way that the risk of freezing damage is minimized over a large spectrum ofclimatic conditions. Species with a small safety margin against freezing temperature, likeFagus sylvatica, appear to employ photoperiod co-control of spring phenology, whereas specieswith a large safety margin depend largely on temperature for the right timing of leaf-out.5. Our results offer a new avenue to explain the differences in leaf-out timing among co-occurringtree species. They further suggest that in a warming climate, tree species can expand theirdistribution range to the extent their phenology matches the stochasticity of freezing temperaturesin spring.