Growth and carbon relations of temperate deciduous tree species at their upper elevation range limit

1.Temperature is one of the most important drivers of range limits. Here we aimed at disentangling the direct effect of low temperature and the indirect temperature effect via the length of the growing season on radial growth and carbon resources of deciduous temperate tree species at their high elevation limit in the Swiss Alps. 2.Trees of 8 species were cored along 3 elevational gradients of ca. 1000 m up to the specific high elevation range limit. We correlated basal area increment (BAI) with mean temperature during the growing season and length of the growing season, derived from a thermal time model. Stem sapwood of cored trees was analysed for non-structural carbohydrate concentrations. 3.The frequency of negative event years (exceptionally narrow tree rings) did not significantly increase with elevation except for Fagus sylvatica. Late season non-structural carbohydrate concentrations remained at a high level across elevations in all species, suggesting that trees are not carbon limited at their high elevation limit. 4.Annual BAI showed no significant change over a wide range of elevations, before it gradually decreased over the last few hundred (300-500) metres below the range limit, with an abrupt decrease in few species at the range limit. Annual BAI correlated with the mean temperature during the growing season in the uppermost 400 m of elevation. The length of the growing season had only a significant effect on BAI at warm mean temperatures during the growing season (i.e. at lower elevation or during warmest summers). 5.Synthesis: While temperature has a strong effect on wood formation, the length of the growing season is negligible for growth at high elevation due to the low rate of growth at low temperature and the over-proportional increase of growth with warmer temperatures. We ruled out a direct growth limitation by low temperature as the limiting factor of the upper distribution limits, and rather suggest that the formation of range limits (necessary the rate of growth) is set by a minimum requirement of warmth and season length to fully mature key tissues such as seeds, shoots or winter hardy buds.