Compound risk in protective forest and natural hazard management

Forests in mountainous areas can lower the frequency, magnitude, and intensity of gravity-driven natural hazards, such as snow avalanches, rockfall and landslides. These so-called protective forests thus constitute a primary natural protection mechanism, which can be complemented by technical protective measures against natural hazards. Their protective effect depends on several factors, including forest structure and management, as well as site characteristics and hazard types.With ongoing climate change, forests are increasingly exposed to stressors and natural disturbances. Environmental stressors create unfavourable conditions that can impair the physiology of trees. In contrast, natural disturbances are discrete events that cause tree mortality leading to a sudden change in the forest structure. Stressors and disturbances can be biotic, such as fungi or insects, or abiotic, such as drought or storms. For example, drought can act as a stressor affecting tree health, or as a disturbance causing tree death. Strong winds can put stress on trees, but they can also cause windthrow, where trees are uprooted or broken. Both phenomena lower forests’ resistance to future stressors and disturbances, as well as their capacity to recover from them.Originating from climate research, compound events are commonly defined as situations where several climatic drivers or hazards co-occur, creating an increased risk to society or the environment. The impacts of compound events across spatial and temporal scales can be significantly greater than the sum of individual drivers or hazards alone. In this study, we transferred this concept to protective forests. Compound events in protective forests are defined as multiple, spatially and/or temporally, interacting climate-induced stressors and disturbances. These events lead to changes in forest structure and composition, which negatively impact the protective effect of forests against natural hazards and create compound risk for people and infrastructure. For example, windthrow and bark beetle infestations can cause large forest openings that create potential release areas for snow avalanches.Compound risks pose novel challenges for pre- and post-disturbance protective forest and natural hazard management. Due to the high level of uncertainty and complexity involved, it is necessary to develop a shared understanding of compound risk. There is also a need to quantitatively assess compound risks to enable the implementation of effective strategies to address and mitigate them.Based on a systematic literature review, we synthesized existing knowledge to develop a definition of compound risk resulting from compound events in protective forests. To assess compound risk for protective forest and natural hazard management, we proposed a methodological framework based on adaptive pathways. Adaptive pathways are a decision-focused approach in climate adaptation research and planning, allowing performance-threshold oriented decision-making under uncertainties. We applied this approach in two case studies and developed scenarios that included a variety of uncertainties regarding compounding stressors and disturbances in forests as well as regarding natural hazards. The method allows the consideration of different forest and natural hazard management strategies for risk-based interventions.