Aim We compared the upper limits of 18 deciduous tree species with respect to elevation in Switzerland and latitude in Europe. We hypothesized that species would exhibit the same relative positions along elevation and latitude, which can be expected if species have reached their thermal cold limit along both gradients.
Location Europe and Switzerland.
Methods We developed a method to identify a least biased estimate of the elevational and latitudinal cold temperature limits of species and for comparing the relative rank positions with respect to these two limits. We applied an algorithm to calculate the elevation of the potential tree line for each point in the gridded landscape of Europe and Switzerland. For each occurrence of each species, the elevation was extracted from digital elevation models. The vertical distance between the elevation of the potential regional climatic tree line and the uppermost species occurrences was calculated and used for comparisons between elevation and latitude.
Results We found a strong relationship between the thermal latitudinal and elevational distances of species' cold limits to the potential tree line, with only marginally significantly different rank positions (P = 0.057) detected along elevational and latitudinal gradients. A first group of nine species showed very similar thermal distances to the potential tree lines along elevation and latitude. Among these species, eight showed a significant decrease in their elevational limit towards high latitudes across mountainous regions of Europe. A second group of seven species occupied a climatic niche closer to the tree line at the edge of their latitudinal range, and only two species did not fill their thermal niche.
Main conclusions Our study provides support for the common concept of a species range-environment equilibrium. Notably, we did not detect a stronger deviation for the filling of thermal niches at latitudinal limits compared with elevational limits, although the former involves a species covering a much greater geographic distance.