TREELIM – Population processes and evolution

B2 Population dynamics

There is no indication of recruitment limitation at the tree species limit. At all transects we found a broad spectrum of young recruits near the uppermost mature individuals of each species, and a lot of recruitment going on above the current mature tree species limit (the only exception, Sorbus aucuparia, that grows already at the life form limit, our only treeline species). We conclude that recruitment is not constrained at the tree species limit of these major European tree taxa by the recent past climate (paper by Vitasse et al. in press in Journal of Biogeography).
The present section supplements the previous one, which aimed to assess the reproductive potential of broad-leaved tree species at their upper elevational limits. Indeed, to go further in the exploration of the biological processes which could control the upper elevational limits of broad-leaved tree species, we investigated an in situ population dynamic survey. The aim was to quantify the current tree recruitment pattern around elevational limits of broad-leaved tree species. Hence, we addressed a basic question: does the altitudinal limit of adult trees differ from the young ones? The survey was conducted at regional scale and provided accurate data to position the elevational limits of the study tree species.

Three helper students were engaged for 2 months in summer 2011 to conduct the population dynamic survey supervised by a postdoc of the project. The survey was investigated in the two regions of the Swiss Alps where the project mainly focused, that is, around Chur (eastern Swiss Alps) and around Martigny (western Swiss Alps). Surveys were conducted along forested elevational transects replicated three times in each of the two regions. Along each transect, the presence of seedlings, saplings and adults of all tree species were recorded every 25 m of elevation increase following the fall line of the slope. At every step an area of about 500 m2 was surveyed on both the left and the right side of the virtual line of the transect. Trees were classified into three age/size categories: (1) seedling for height < 0.5 m, (2) sapling for height [0.5–4 m] or for height > 4 m but with a diameter at breast height (d.b.h.) < 0.15 m, and (3) adult for height > 4 m and d.b.h. > 0.15 m. 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 on presence/absence data.

Our survey revealed that all species were represented by young individuals in the vicinity of the limit of adult trees. Furthermore, tree recruitment of both seedlings and saplings was detected and predicted significantly beyond adult tree limits in most of the species (Fig. 5). Across regions, seedlings and saplings were on average found at elevations 73 m higher than adult trees.

In agreement with the results presented in the previous section, our results clearly show that under current conditions, neither reproduction nor seedling establishment constitute a serious limitation of recruitment at the upper elevational limits of major European trees. The recruits found beyond the adult limits constitute a potential for an upward migration of trees in the Alps in response to ongoing climate warming.