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

Summary Within the same forest stand, temperate deciduous trees generally exhibit a distinct pattern in leaf‐out timing, with some species flushing earlier than other species. This study aimed to explain the timing of leaf‐out of various temperate tree species in relation to the risk of freezing damage to leaves. We combined long‐term series of leaf‐out date (14–32 years) of five temperate tree species located in both low and high elevations in Switzerland, daily minimum temperatures recorded at the same sites and species‐specific freezing resistance (LT50) of emerging leaves. We calculated temperature safety margins (the temperature difference between absolute minimum temperature during leaf‐out and species‐specific LT50 values), and date safety margins (time lag between the last day when temperature falls below species‐specific LT50 values and the date of leaf‐out). Leaf‐out occurred when the probability to encounter freezing damage approaches zero, irrespective of 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 safe period. Interestingly, the temperature safety margins did not differ significantly between low and high elevation. Yet, the date safety margin was smaller at high elevation, presumably due to a faster increase in temperature during the leaf‐out period at high elevation. When species‐specific freezing resistance is taken into account, the time of leaf‐out converges among species towards a marginal risk of freezing damage. Thus, leaf‐out time has likely evolved in a way that the risk of freezing damage is minimized over a large spectrum of climatic conditions. Species with a small safety margin against freezing temperature, like Fagus sylvatica, appear to employ photoperiod co‐control of spring phenology, whereas species with a large safety margin depend largely on temperature for the right timing of leaf‐out. Our results offer a new avenue to explain the differences in leaf‐out timing among co‐occurring tree species. They further suggest that in a warming climate, tree species can expand their distribution range to the extent their phenology matches the stochasticity of freezing temperatures in spring. Lay Summary