To examine the speed of tree regeneration in lower subalpine forests (1300-1700 m a.s.l.) of the European Alps after blowdown events, we developed a spatially explicit simulation model. Calibrated with data from 10 years of observation on extensive blowdown areas in Switzerland, the model incorporates the interactions between the changing herb layer and the growth of Norway spruce (Picea abies (L.) Karst.). The model consists of a vegetation model and a tree model: the vegetation model simulates spatial changes in the herb layer using a cellular automaton combined with a Markov transition matrix. Herb layer changes include the dispersal of tall herbs and raspberry (Rubus idaeus) on different microsite types, the growth of pioneer tree species (Sorbus aucuparia), the decay of fallen trunks, and their colonization by plants. The tree model simulates growth and demographic processes of spruce trees (P. abies) with an individual-based approach, including seed dispersal, germination, establishment, growth and mortality as a function of herb layer properties. The model was evaluated through sensitivity analysis along with stepwise multiple rank regression. As a measure of the goodness of fit, we compared the model results with independent field data. The sensitivity analysis showed that seed availability, seedling survival and the presence of advance regeneration are the key processes for successful reforestation. The model reliably simulated the observed tree height distributions after 8 and 10 years (R 2 between 0.81 and 0.93). Long-term simulations yielded initially sparse spruce regeneration, which is retarded by unfavourable microsite conditions. The results indicate that the recovery of the forest to protect against avalanches and landslides may not be achieved for several decades on blowdown sites with large distances to the nearest forest edge, and thus low seed supply. On sites where less than 10% of the surface was covered by logs and with higher seed supply, the protection against avalanches and landslides may be sufficient again about 50 years after a blowdown event. ?? 2005 Elsevier B.V. All rights reserved.
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