Understanding range limits of tree species from first principle
Classical approaches at understanding range limits of tree species are using statistical procedures correlating species distribution with environmental variables. In this synthesis, I will present the results of a multidisciplinary project on the physiological causes of the range limits of temperate deciduous tree species. The project combined climatology, biogeography, dendrology, population and reproduction biology, stress physiology, phenology and evolutionary biology. Results from in situ elevational (Swiss Alps) and latitudinal (Alps vs Scandinavia) comparison, from common garden and phytotron studies as well as laboratory experiments for 8 common European tree species will be presented.
Results show that tree top temperature minima can be predicted from weather station data and the ranking of the range limits of species was similar across elevation and latitude. Unlike wide spread assumption, absolute temperature minima in winter had no explanatory power for the geographic distribution of the species. There was no recruitment limitation at the current upper elevational limit of any species. Reciprocal common gardens revealed that the environment had a much stronger influence on growth and phenology of seedlings than provenance. Spring flushing in adults trees is timed in a way that the probability to encounter freezing damage is minimized, with a uniform safety margin across the entire elevational range of all taxa. More resistant species flush earlier than less freezing resistant species. Tree ring formation at the range limit is not related to variation in season length, but to growing season temperature. Young trees, grown at temperatures colder than at their natural range limit, show signs of incomplete lignification, tissue formation. We conclude that the range limits of the examined taxa are set by the interactive influence of phenology, freezing resistance in spring, and the time required to mature tissues. Microevolution of spring phenology solves the compromise between demands set by species-specific freezing resistance of immature tissue and evolutionary life history traits related to tissue maturation.