Tree recruitment of European tree species at their current upper elevational limits in the Swiss Alps

Aim The physical and physiological mechanisms that determine tree-line position are reasonably well understood, but explanations for tree species-specific upper elevational limits below the tree line are still lacking. In addition, once these uppermost positions have been identified, questions arise over whether they reflect past expansion events or active ongoing recruitment or even upslope migration. The aims of this study were: (1) to assess current tree recruitment near the cold-temperature limit of 10 major European tree species in the Swiss Alps, and (2) to rank species by the extent that their seedlings and saplings exceed the elevational limit of adult trees, possibly reflecting effects of the recent climate warming. Location Western and eastern Alps of Switzerland. Methods For each species, occurrences were recorded along six elevational transects according to three size classes from seedlings to adult trees in 25-m-elevation steps above and below their regional upper elevational limit. Two methods were used to compare upper elevational limits between seedlings, saplings and adults within species. First, we focused on the uppermost occurrence observed in each life stage for a given species within each studied region; and second, we predicted their upper distribution range using polynomial models fitted to presence/absence data. Results Species exhibited a clear ranking in their elevational limit. Regional differences in species limits (western versus eastern Swiss Alps) could largely be attributed to regional differences in temperature. Observed and predicted limits of each life stage showed that all species were represented by young individuals in the vicinity of the limit of adult trees. Moreover, tree recruitment of both seedlings and saplings was detected and predicted significantly beyond adult tree limits in most of the species. Across regions, seedlings and saplings were on average found at elevations 73 m higher than adult trees. Main conclusions Under current conditions, neither seed dispersal nor seedling establishment constitutes a serious limitation of recruitment at the upper elevational limits of major European trees. The recruits found beyond the adult limits demonstrate the potential for an upward migration of trees in the Alps in response to ongoing climate warming.