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The increasing role of drought as an inciting factor of bark beetle outbreaks can cause large-scale transformation of Central European forests

Agnish Kumar Das,Marco Baldo,L. Dobor,R. Seidl,W. Rammer,R. Modlinger,Prosper Washaya,K. Merganičová,T. Hlásny

Published 2025 in Landscape Ecology

ABSTRACT

Historically, large-scale outbreaks of the European spruce bark beetle were initiated mainly by windthrows. However, after 2018, a severe drought triggered the hitherto largest bark beetle outbreak observed in Europe, signalling a major shift in the disturbance regime. Develop and test an approach that allows simulating this novel disturbance dynamics and evaluate landscape-scale compound impacts of wind- and drought-initiated outbreaks throughout the twenty-first century. We incorporated drought-initiated outbreaks into the forest landscape simulation model iLand, using critical values of vapour pressure deficit as the outbreak trigger. Forest management records and remote sensing-based disturbance maps were used to derive model parameters and evaluate simulated dynamics in a Central European forest landscape (41,000 hectares). The period 1961–2021 was used for model evaluation, and the years until 2100 for scenario analysis. Incorporating drought as outbreak trigger led to a notable decoupling of wind and bark beetle disturbances, which have historically formed a typical disturbance cascade in European forests. While forest growing stock and species composition were resilient to a wind-dominated disturbance regime, this resilience diminished under the compounded impact of wind- and drought-triggered disturbances. The new disturbance regime caused a persistent decline in Norway spruce and resulted in an overall decrease in landscape-level growing stock. Our findings underscore the urgent need for new approaches to evaluate increasingly complex disturbance dynamics and suggest that the future impacts of bark beetles on forest landscapes may be greater than previously anticipated.

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