TREELIM – Biogeography and climatology

A1 Realized niche and macro-climate (GIS, Elevation vs. latitude)

The ranking of major European deciduous tree taxa with respect to their cold climate limit is consistent across the two test regions in the Alps and in S-Scandinavia. We thus can conclude, that the taxa examined have been tracking regional climate and arrived at their thermal niche envelope. This study also provides support for the concept of the species range–environment equilibrium. Notably, we did not find stronger deviation in filling thermal niches at the latitudinal limits as compared to the elevational limits, although the former involve a species to cover a greater geographical distance. Thermal equilibrium seems thus decoupled from potential geographic non-equilibrium. This is a key observation for all forthcoming data analysis and interpretation and provides support for other studies using Ecological Niche Models. Hence, we gave this part priority (paper by Randin et al. in press).

We compared the cold-end range limits of eighteen deciduous tree species along elevational gradients in Switzerland and a latitudinal gradient in Scandinavia. We hypothesized that species exhibit the same relative position along elevation and latitude, which can be expected, if species have reached their thermal cold limit at both high elevation and high latitude.
We developed a method to identify a least-biased estimate of the elevational and latitudinal cold temperature limits and for comparing relative rank positions along these two limits. We applied an algorithm to calculate the elevation of the potential treeline for each point in the gridded landscape of Europe and Switzerland. For each occurrence of each species, elevation was extracted from digital elevation models. The vertical distance between elevation of the potential regional climatic treeline and uppermost species occurrences was calculated and used for comparisons between elevation and latitude.
We found a strong relationship between the thermal latitudinal and the elevational distance of species’ cold limits to the potential treeline with only marginally significant different rank positions (P=0.057) along elevational and latitudinal gradients. A first group of nine species showed very similar thermal distances to both the elevational and latitudinal potential treeline (Fig. 2). Among them, eight showed a negative relationship between elevation and latitude across the different mountain regions of Europe. A second group of seven species occupied climatic niches closer to the treeline at their latitudinal range edge. Only two species did not fill their thermal niche at the latitudinal upper limit, based on weather station derived data.
Our study provides support for the common concept of the species range–environment equilibrium. Notably, we did not find any strong deviation in filling thermal niches at the latitudinal limits as compared to the elevational limits, although the former involve a species to cover much more geographic distance. Thermal equilibrium seems therefore decoupled from potential geographic non-equilibrium.
Recently, Randin et al. (in revision) showed that the poleward thermal limits of nine deciduous tree species in Europe match the upper elevational thermal limits in the Alps well. This suggests that these species share a common isotherm at the two limits. However, a proper identification of the temperature-based parameters that control the two limits still remains difficult when empirically relating large-scale occurrence datasets to geographic climate layers. Indeed, such data may not accurately reflect the temperature extremes or the frequency of such extremes (Randin et al. in prep; Lundquist et al. 2008). In contrast, direct in situ measurements of temperature at different heights above ground of standing trees may better reproduce the temperature experienced by the different life stages of a deciduous tree species.
This study thus aims at (1) developing (from the literature) temperature-based variables that are potential candidates as determinants of cold limits of deciduous species at the thermal equilibrium; (2) verifying whether the patterns of these variables are found at both elevational and latitudinal limits of species at the equilibrium; and (3) ultimately producing a typology of cold limits for species at the thermal equilibrium.
We selected a set of five European deciduous species that (i) have been reported to be in thermal equilibrium at their cold limits at high elevation in the Swiss Alps and at their high latitude limit in northern Europe (Figure 2), and (ii) which limits are found both in the Alps and in the south of Sweden. These species are namely Quercus petraea, Fagus sylavtica, Acer pseudoplatanus, Prunus avium and Fraxinus excelsior.
During summer 2009, we placed temperature loggers at the cold limits of the five species in two regions of Switzerland. Sites were selected by combining occurrences from the Swiss National Forest Inventory (NFI) and a survey. Loggers recorded during two years. In Sweden, sites were selected with the maps produced by the EUFORGEN project (http://www.euforgen.org) and a survey before placing the loggers in September 2010 for one year.