For the study of long-term processes in forests, gap models generally sacrifice accuracy (i.e., simulating system behavior in a quantitatively accurate manner) for generality (i.e., representing a broad range of systems’ behaviors with the same model). We selected the gap model ForClim to evaluate whether the local accuracy of forest succession models can be increased based on a parsimonious modeling approach that avoids the additional complexity of a 3D crown model, thus keeping parameter requirements low. We improved the representation of tree crowns by introducing feedbacks between (i) light availability and leaf area per tree and (ii) leaf area per tree and diameter growth rate to account for the self-pruning in real stands. The local accuracy of the new model, ForClim v2.9.5, was considerably improved in simulations at three long-term forest research sites in the Swiss Alps, while its generality was maintained as shown in simulations of potential natural vegetation along a broad environmental gradient in Central Europe. We conclude that the predictive ability of a model does not depend on its complexity, but on the reproduction of patterns. Most importantly, model complexity should be consistent with the objectives of the study and the level of system understanding.

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Recent temperature observations suggest a general warming trend that may be causing the range of tree species to shift to higher latitudes and altitudes. Since biotic interactions such as herbivory can change tree species composition, it is important to understand their contribution to vegetation changes triggered by climate change. To investigate the response of forests to climate change and herbivory by wild ungulates, we used the forest gap model ForClim v2.9.6 and simulated forest development in three climatically different valleys in the Swiss Alps. We used altitudinal transects on contrasting slopes covering a wide range of forest types from the cold (upper) to the dry (lower) treeline. This allowed us to investigate (1) altitudinal range shifts in response to climate change, (2) the consequences for tree species composition, and (3) the combined effect of climate change and ungulate herbivory. We found that ungulate herbivory changed species composition and that both basal area and stem numbers decreased with increasing herbivory intensity. Tree species responded differently to the change in climate, and their ranges did not change concurrently, thus causing a succession to new stand types. While climate change partially compensated for the reductions in basal area caused by ungulate herbivory, the combined effect of these two agents on the mix of the dominant species and forest type was non-compensatory, as browsing selectively excluded species from establishing or reaching dominance and altered competition patterns, particularly for light. We conclude that there is an urgent need for adaptive forest management strategies that address the joint effects of climate change and ungulate herbivory.

Ungulate browsing is a major factor influencing tree regeneration. However, it is unclear if the observed increase in ungulate abundance in Central Europe implies increased browsing, and which other factors influence the incidence of browsing. We investigated the impact of forty variables (site, climate, forest and ungulates) on the probability of leader shoot browsing of six tree species which are frequent in Switzerland. The analysis was based on a large dataset including 49 monitoring areas, each containing 25–64 circular plots, in which 10 to 130 cm tall seedlings were repeatedly assessed. Browsing probability was estimated for each plot and year by mixed effects logistic regression and used as a response in random forests to disentangle the influence of the explanatory variables. Browsing probability was positively correlated with ungulate density measures (number culled by hunting or found dead) for all six tree species. Where beyond roe deer, some red deer and/or chamois were present, the browsing probability was higher. Small timber tree stands had less browsing than young growth and thicket stands. Seedlings tended to be more frequently browsed in stands with >80% canopy shading. Browsing increased with increasing understory cover, independent of vegetation category. In conclusion, browsing is a multifactorial phenomenon and ungulate density estimates alone do not explain the whole browsing probability.