How to manage windthrows in Central Europe to prevent bark beetle outbreaks?

Bark beetle (Ips typographus) epidemics in Europe are typically triggered by excessive availability of freshly dead trees and trees with compromised defense, which often occur after windstorms or droughts. Subsequently, enlarged beetle populations migrate to the surrounding forests, which were not affected by the primary disturbance. Removal of windfelled trees (salvage or sanitation logging) is therefore a frequent management response to prevent the build-up of bark beetle populations. Yet, the effectivity of the removal remains poorly understood, particularly when the outbreaks are amplified by faster beetle development cycles and reduced tree defense under climate change conditions.

Moreover, diverse ownership, management objectives and limited resources often restrict salvaging operations, and the final effect on bark beetle populations is thus even less clear. To better understand the interplay between climate, management, bark beetle populations, and host trees, we use the process-based forest landscape and disturbance model iLand. We studied differences between the removal of windfelled trees applied evenly across the landscape, focused on the vicinity of roads (scenario of limited logging resources) and concentrated in a contiguous block (scenario of spatially diversified management objectives) on a 16 050 ha forest landscape in Central Europe. We found that the removal of >80% of all windfelled trees is required to substantially reduce bark beetle disturbances. Focusing on the vicinity of roads created a “fire break effect” on bark beetle spread, and was moderately efficient in reducing landscape-scale bark beetle disturbance. Block treatments substantially reduced outbreaks in treated areas. Leaving parts of the landscape untreated (e.g., conservation areas) had no significant amplifying effect on outbreaks in managed areas. Our research suggests that the management of interacting disturbances from wind and bark beetles requires much more complex considerations than are currently practiced.