In recent years, forests in the canton of Grisons have been increasingly affected by natural disturbances such as windthrow and bark beetles. Researchers from the WSL Institute for Snow and Avalanche Research (SLF) have now identified which forests are particularly vulnerable. The study should help to determine priorities in forest management.
Mountain forests are vital for human beings because of the many functions they perform. As well as protecting settlements and roads from natural hazards such as avalanches and rockfall, they clean the air and store carbon, provide recreational space and a habitat for numerous animal and plant species, and play an important economic role by supplying renewable raw materials.
Until the late 19th century, mountain forests were intensively exploited and grazed, but in recent decades they have tended to become denser and their surface area has expanded. Today, almost a third of the canton of Grisons is forested, of which approximately 60% is protection forest. The extent of natural disturbances such as windthrow, forest fires and bark beetle outbreaks has increased in recent years as a result of forest densification and increasingly also as a result of climate change. Such developments pose a challenge for forest management. It is therefore crucial, from a forest management perspective, to know where, when and how the forest should be maintained, to ensure that the risk from natural disturbances remains as low as possible despite an increasingly challenging environment and that the forest can perform its functions as sustainably as possible.
In a recent study carried out in collaboration with the Technical University of Munich (TUM), the Swiss Federal Institute of Technology Zurich (ETH Zurich) and the Canton of Grisons Office for Forest and Natural Hazards (AWN), SLF researchers combined satellite data with data on wood harvesting and forest management in the canton of Grisons and historical maps of forest cover. While satellite data had already been used for earlier studies, it had not previously been possible to distinguish whether disturbances were natural or the result of human interventions such as thinning. “With this new method, we can see relatively easily and over a large area what has happened where and when in the forests. We can now identify which natural disturbance is involved, whether bark beetle, snow breakage or windthrow, for example,” says Peter Bebi, co-author of the study and head of the SLF's Mountain Ecosystems Research Group.
The models show that natural disturbances, especially windthrow and bark beetles, occur most frequently at lower elevations, on shallow and south-facing slopes. Spruce-dominated and dense stands showed a higher susceptibility to natural disturbances than structured mixed stands. Younger forests, i.e. those established in the 20th century, were significantly more susceptible to natural disturbances than forests that were already present before 1880. “This finding is new and contradicts the widespread belief that management should focus on older forests because they are overaged,” says Bebi. Homogeneous and closed-canopy spruce forests, which are also less valuable for biodiversity and recreation, are particularly prone to disturbances. “So in many respects it makes sense to intervene at an early stage and ensure that spruce forests do not grow too dense. As long as spruces have relatively long crowns, they can respond positively to changes,” explains Bebi. While regeneration interventions may increase susceptibility to natural disturbances in the short term, in the longer term the forest usually becomes more diverse and more resilient.