Management recommendations for Alpine protection forests: the importance of regeneration quality and initial stand composition

Abstract

The protection of infrastructure against gravitational natural hazards is one of the most important ecosystem services (ES) of mountain forests in Alpine countries. For a continuous provision of this ES, forests need to have a high protective effect, e.g., high canopy cover and/or stem numbers, while being resistant to and resilient after disturbances by being well-structured and stable, having a species composition adapted to the local site conditions and sufficient regeneration, all on a relatively small spatial scale. While "natural" forests may fulfill these prerequisites without human intervention, management history and high levels of ungulate browsing have produced unsustainable stand structures in many protection forests that need to be improved by management. The general principles of protection forest management are well established, but there are no quantitative, science-based recommendations for management regimes, i.e., specific sequences of interventions, that ensure a continuous protective quality. Our goal was to derive such recommendations for different stand types across three elevational zones, from mixed forests of the upper montane to spruce forests of the subalpine zone. We used an updated version of the model ProForM to simulate stand development under different levels of ungulate browsing, testing a large number of management regimes that vary in the spatial aggregation of tree removal, the intensity and interval of the interventions. We investigated the influence of browsing pressure and management on the protective quality using Boosted Regression Trees and Beta regression. High levels of ungulate browsing had such a strong negative effect on the protective quality that it could not be improved through forest management. This underlines the need for maintaining ungulate densities in Alpine forests at levels that allow for the successful regeneration of all key tree species. In stands that are influenced less by browsing, the protective quality can be improved through management in many cases, with specific management recommendations differing mostly depending on the initial stand conditions and, to a lesser extent, on the elevational zone. Well-structured stands provide a high protective quality without management interventions during at least a century across all elevational zones. In young and in mature stands, we generally recommend management regimes with relatively long return intervals of 30 to 40 years and low intervention intensities of 10 to 20% basal area removal.