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A functional explanation of low temperature tree species limits

Final Report Summary - TREELIM (A functional explanation of low temperature tree species limits)

Why do trees reach a low temperature range limit?

What stops trees from spreading beyond certain boundaries when it gets cold? All plant species, trees included, reach a low temperature limit, either at high latitude or high elevation. These limits vary with species, but the mechanisms that cause these limits are largely unknown. TREELIM aimed at deciphering the causes of species-specific range limits in a set of most important European deciduous tree species in a multi-disciplinary approach. This included the identification of range limits with geographical information systems (GIS), the quantification of reproductive success of trees at the current range limit (including tree demography), an assessment of the actual temperatures at canopy level in a large-scale micro-meteorological survey, an in depth exploration of bud and leaf freezing resistance, a transplantation experiment with more than 4000 young trees along two different elevational gradients to explore genotypic vs. phenotypic traits related to tree success at the range limit (e.g. phenology), the study of growth trends as one approaches the range limit, and a series of physiological laboratory tests to quantify low temperature effects under controlled conditions. TREELIM was the first ever coordinated attempt at deciphering the critical processes that control tree species range limits. All previous attempts were looking at partial processes in isolation or they used purely statistical (correlative) approaches based on species distribution.

TREELIM revealed that absolute minimum temperatures in winter are neither correlated with species range limits, nor do they exert any risk given the hardiness of native taxa. Species find their northern (Scandinavian) range limit at much lower temperature extremes in winter than at their high elevation limits in the Alps (from 4K to 14K colder). Temperatures measured right at the top of mature trees did not significantly differ from nearest weather station data when scaled to the appropriate altitude. To our surprise seeds collected from individuals growing at the range limit were viable and large populations of seedlings and saplings were found beyond the current range limit of adults, suggesting no current recruitment limitation. The transplant experiment revealed a small genotypic and large environmental control over plant traits including growth and phenology. Trees show substantial annual stem diameter increment until close to their range limit. There is clearly no gradual decline with temperature (elevation, latitude), and the number of available trees gets very small over the last few hundred meters as growth becomes marginal. Trees growing under such marginal conditions still store plenty of carbohydrate reserves, hence these individuals are not carbon-limited at the range edge.

The range limit of temperate deciduous trees emerged to be controlled by a delicate interaction between three factors: (1) the freezing tolerance during bud break and leaf emergence, (2) the associated controls of phenology that ensure a close to zero risk of freezing damage and (3) the resulting constraints on the remaining length of the growing season in relation to species-specific life history traits (such as latewood maturation, winter bud formation, fruit ripening). Phenology is the adaptive tool by which species manage to avoid inherently dangerous low temperature exposure during spring, with the species-specific tradeoff set by insufficient time to complete the seasonal growth cycle (risk of winter damage of immature tissue). With - to date - 24 publications, the results of TREELIM offer a theoretical framework that unifies freezing resistance, phenology and life history traits and provides the empirical foundation for modeling species range limits on a mechanistic basis.