Understanding alpine tree line dynamics: An individual-based model

In addition to the effects of climate change, land use change is expected to trigger dynamic processes that may shift alpine tree lines upwards. To gain a conceptual understanding of the spatio-temporal processes shaping tree line patterns, we developed an individual-based and spatially-explicit model of spontaneous forest regeneration at the alpine tree line. Remote sensing data were integrated with the succession processes for individual trees in an object based image analysis approach. The model presented here predicts patterns of natural forest regeneration on a summer pasture in the Austrian Central Alps between 1954 and 2006. Different model scenarios are compared to the actual forest regeneration during the study period to validate the model's structure and its sensitivity to changing input parameters. The model's main process parameters are tested in four scenarios: A baseline scenario with the most likely input parameters derived from empirical studies in the literature; and three scenarios with differing parameters for seed production, seed dispersal kernel, and seedling competition with the ground vegetation. The tests largely confirm the model's formulation. The baseline scenario correctly predicts a significant upwards shift of the tree line elevation. Moreover it is demonstrated that (1) fecundity is a crucial factor in the expansion of forest into open land, (2) land cover and land use history have a significant impact on the emergence of forest regeneration patterns and (3) the dispersal kernel of Norway spruce has a long-distance component. We thus identified the key processes operating under environmental change from the characteristics of spatial changes in tree line patterns over the last 50 years.