Forecasting Bark Beetle Disturbance Risk for Forest Regeneration Planning

Large-scale bark beetle outbreaks can compromise multiple forest ecosystem services, including timber yield, carbon sequestration capacity, and the protective and cultural values of forests. In Central Europe, recent Ips typographus outbreaks have highlighted the increasing vulnerability of spruce-dominated forests under changing climatic conditions.

Historically, major bark beetle outbreaks were usually initiated by windthrow or snow damage, with fallen trees providing suitable host material and enabling rapid beetle population growth. Recently, drought is increasingly recognised as an additional amplifying or even triggering factor. Heat and water limitation impair spruce defence mechanisms, while warmer temperatures benefit beetles by extending their flight periods and accelerating development, potentially allowing additional generations per year. Together, these processes increase the likelihood that both beetle population growth and host susceptibility coincide over multiple consecutive years.

The aim of this work is to develop a predictive model of bark beetle disturbance vulnerability at the European scale. The modelling framework covers the period 1981–2021 and integrates a range of spatially explicit covariates, including climatic variables (temperature, precipitation, drought metrics), stand properties, and topographic characteristics. Model calibration relied on forest management records and remote sensing–based disturbance maps identifying historical bark beetle outbreaks. These disturbance layers provided spatially explicit binary response data and formed the core reference for model training and validation. The performance of a suite of statistical and machine-learning models was evaluated using both spatial and temporal cross-validation.

Such trained models were subsequently applied to projected future climate conditions under a high-emission scenario (SSP5-8.5), with inter-annual climatic variability explicitly incorporated. Ensemble predictions across different models and iterated climate simulations were aggregated to derive spatially explicit estimates of future bark beetle disturbance risk.

The results emphasize the importance of considering disturbance risk at spatial scales relevant for regeneration planning, highlighting species composition, spatial dispersion, and bet-hedging strategies under increasing ecological uncertainty. The outcomes will contribute to a decision-support system currently developed within the RE-ENFORCE project.