Physiological ecology of the low temperature range limit of European deciduous tree species

Low temperature is one of the most important factors that can exert the overarching pattern in species distribution of major European deciduous tree taxa along latitudinal and elevational gradients. Apart from low temperature, time could be an important constraint for tree species to complete growth and fully harden tissues in late summer/autumn before the occurrence of freezing temperatures. As part of the ERC-project ‘TREELIM’ the present PhD thesis aimed to assess the effect of (1) low temperature extremes, (2) mean temperatures during the growing season and (3) length of the growing season on climatic boundaries of eight major European deciduous tree species. In detail, we investigated the freezing resistance of buds and leaves before, during, and after the period of leaf emergence at the upper elevational limits of these tree species in the Swiss Alps. Further, we assessed the secondary growth of these taxa over the last 30 years using tree cores collected along elevational gradients. We aimed at disentangling the effects of temperature and time on tree growth and survival, by combining freezing resistance and tree growth measurements with spring phenology. In all investigated species, freezing resistance of buds was tightly correlated with the advancement of spring phenology (bud development). Interestingly, species’ spring phenology is perfectly adapted to escape damaging freezing temperatures to leaves, irrespective of elevation. Thus, more freezing resistant tree species can flush earlier and extend their growing season in spring. The elevational time-shift of bud burst allows species to minimise the risk of freezing damage in spring at the cost of a shorter duration of the growing season at higher elevations. Tree-ring data revealed no change in growth along a large part of the studied elevational gradient, with a rather abrupt growth decline towards species’ elevational limits. Interestingly, mean temperatures during the growing season have a strong effect on tree growth independent of the length of the growing season, while the length of the growing season becomes only effective at warmer temperature. This inconsistency might be explained by the over-proportional increase of tissue formation with warmer temperature. An increasing length of time enables many cell divisions at warm temperature, whereas only few additional cell divisions are possible at cold temperature. Interestingly, the physiological low temperature limit for any tissue formation correlates with species’ elevational limits. Nevertheless, tree species are not growth limited at their elevational limit. We rather suggest that freezing resistance, the associated timing of spring phenology, and the resulting length of the growing season jointly reflect species-specific life history requirements that control tree species cold range limits.