This report summarizes preliminary experiences and results of reforestation studies in 20 windthrow areas in the subalpine zone of the Northern Swiss Alps.
High species diversity was found in tree seeds arriving on the windthrow areas. Seeds of Norway spruce were the most frequent. Seed quantity decreased rapidly with increasing distance from the nearest stand. Seed-fall decreased by 90% at 200 m and by 95% at 400 m from the nearest stand compared to seed-fall within the stand. In a mast year with seed densities of up to 20 million seeds per hectare this may be sufficient for successful colonization of windthrow areas. In years with lower seed production, however, there may not be enough seeds to restock larger open areas.
On most study sites seedlings and saplings mainly consisted of Norway spruce. The bumper seed year of 1992/93 in Norway spruce resulted in high numbers of seedlings in the smaller windthrow areas but not in the larger areas. Up to two thirds of the seedlings and saplings died during the first year after germination. Establishment and growth of the seedlings and saplings was hindered by dense and high vegetation cover, and by browsing ungulates.
Natural regeneration higher than 20 cm that had established itself before the storm was rare on most sites, providing a poor basis for reforestation. The yearly increase in number, however, was considerable. At lower elevations the established regeneration contained more species, but there too it was threatened by browsing ungulates.
To date, plantation contributes much more to reforestation than natural regeneration, even though trees were planted at low densities and patchily. Silver fir, mountain maples, and mountain ash were the species most severely affected by browsing.
Four years after the windthrow event there were still sufficient mycorrhizae in the upper soil layers for successful inoculations of tree seedlings.
Prospects for natural reforestation of the studied windthrow areas generally look well. Medium- to long-term development will lead to species-diverse, multi-structured mountain forests.