A considerable number of studies have investigated the phenology of European beech using models, experimental controlled conditions, or descriptive surveys of patterns in situ. In spite of this interest, there is no consensus about the environmental factors controlling bud burst in beech, especially about the role of photoperiod and chilling temperature (cold temperature effective to release bud dormancy). However, recent experimental and modelling studies provide new insights into the means by which these environmental factors control beech phenology. This present contribution aims to reconcile contradictory hypotheses about the main environmental factors controlling bud burst date of European beech. First, we review the main published results on the environmental control of beech phenology both in controlled and in natural conditions. Second, supported by the findings of recent studies, we propose a new theory for the role of photoperiod during the chilling phase for explaining spatial and temporal variations in bud burst phenology of European beech. Examples using long-term data from the Swiss Alps and Germany are presented to support this theory. The possible impacts of future and ongoing climate warming on beech phenology are discussed. Finally, due to interactions between chilling, forcing temperature, and photoperiod, we assert that beech phenology follows a nonlinear trend across biogeographical gradients such as changes in elevation or latitude and that the bud burst date of beech is expected not to undergo significant changes in response to global warming, especially in warmer climates.
1. Attempts at explaining range limits of temperate tree species still rest on correlations with climatic data that lack a physiological justification. Here, we present a synthesis of a multidisciplinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrology, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from reciprocal common garden and phytotron studies for eight European broadleaf tree species. 2. We show that unlike for low-stature plants, tree canopy temperatures can be predicted from weather station data, and that low-temperature extremes in winter do not explain range limits. At the current low-temperature range limit, all species recruit well. Transplants revealed that the local environment rather than elevation of seed origin dominates growth and phenology. Tree ring width at the range limit is not related to season length, but to growing season temperature, with no evidence of carbon shortage. Bud break and leaf emergence in adults trees are timed in such a way that the probability of freezing damage is almost zero, with a uniform safety margin across elevations and taxa. More freezing-resistant species flush earlier than less resistant species. 3. Synthesis: we conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation.
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