Modellierung des Einflusses von Wildverbiss auf die Schutzwaldentwicklung an der Rigi-Nordlehne

Abstract

Modelling the impact of ungulate browsing on the development of the protective forests of the Rigi-Nordlehne Forests on the northern slopes of the Rigi Mountain in central Switzerland (“Rigi-Nordlehne”) have a key protective function against rockfall, erosion, landslides, flooding and debris flows. Forest management endeavours to guarantee that these forests can maintain their role in the long term. Thus multi-story canopies are favored, being resistant and resilient to natural disturbances such as windthrow. In addition, the mixture of tree species should conform to the guidelines of the national project “Sustainability and success monitoring in protection forests” (NaiS). However, at the present time an imbalance in tree species is evident in the regeneration layer, due to the considerable impact of browsing by wild ungulate populations in these beech and fir forests. To predict future mixtures of tree species, the spatially explicit model of forest development Rigfor was developed and applied to simulate and compare two scenarios of forest dynamics. The scenario “current browsing” suggests that silver fir cannot regenerate and that regeneration of maple and beech is greatly hindered. In the scenario “reduced browsing”, however, a species mixture conforming to the national guidelines can be achieved in the regeneration layer. The simulations show that the amount of beech timber decreases whichever the scenario, while for the larger fir trees in diameter classes above 35 cm no changes are to be expected in the coming decades. However, from 2082 onwards a continuing high browsing rate would lead to the elimination of nearly all silver fir trees at the pole stage and in the long run silver fir may be completely eradicated from these stands. We conclude that under a dense canopy of old trees, silver fir and beech can be regenerated if ungulate browsing is considerably reduced. Spruce regeneration however requires more light and this is not often found on the northern slopes of the Rigi Mountain. Opening up the canopy to foster regeneration of the desired mixture of species is a key measure in order to maintain the long-term protective function of these forests. This will only be possible, however, if the current high browsing intensity can be reduced.