Vegetation shift in Scots pine forests in the Valais accelerated by large disturbances
In the past dozen years, several studies have concluded a vegetation shift from Scots pine to oak (pubescent and sessile) forests in the low elevated zones of the Valais. It is, however, not fully clear in which way such a vegetation shift actually occurs and on which processes such a shift would be based. Two studies, one on the tree demography in the intact Pfynwald and the other on the tree regeneration on the large Leuk forest fire patch, serve to discuss different aspects of the shift from Scots pine to oak. The forest stands of Pfynwald consist of 67% Scots pines and 14% oaks. Regenerating trees are 2–3.5 times more frequent in small gaps than under canopy. In gaps of the Upper Pfynwald, seedlings and saplings of Scots pine are three times more abundant than oaks, while both species regenerate in similar quantities under canopy. In the Lower Pfynwald, young oaks – especially seedlings – are more frequent than Scots pines. A different process is going on at the lower part in the Leuk forest fire patch where Scots pines prevailed before the burn of 2003. While Scots pines regenerate exclusively close to the edge of the intact forest, oaks not only resprout from trunk but also profit from unlimited spreading of their seeds by the Eurasian jay. Regeneration from seeds are hence observed in the whole studied area, independent of the proximity of seed trees. After the large fire disturbance, a mixed forests with a high share of oaks is establishing, which translates to a rapid vegetation shift. The two trajectories are discussed in the light of climate change.
