Tree mortality is a crucial process in dynamic vegetation models, yet the representation of major mortality agents has often been limited. A ForestPaths research team at Lund University aims to address this gap through a new study published in Geoscientific Model Development. The paper introduces and evaluates a model for simulating forest damage from European spruce bark beetle (SBB, Ips typographus) outbreaks within the LPJ-GUESS dynamic vegetation model.

In recent years, SBB has become the most significant disturbance agent in European forests, surpassing wind damage as the primary cause of tree loss. Outbreaks have traditionally been triggered by windstorms, which create an abundance of breeding material with no defenses to overcome for the bark beetles. However, over the last decade, drought has also become a main contributor to beetle-induced mortality.

The study incorporates a new SBB damage module into LPJ-GUESS that includes representation of wind damage and forest management practices. The model uses storm and bark beetle damage statistics from Sweden, Switzerland, Austria and France, and combines simple empirical relationships with mechanistic knowledge. The researchers designed the module to reproduce outbreak dynamics - capturing how storm damage, drought stress and temperature-dependent changes in beetle phenology interact. The concept is flexible enough to be adapted to different types of bark beetles by modifying functions related to climate and insect-host interactions.

The model detects the onset and duration of storm-triggered outbreaks reasonably well, though there were some discrepancies in outbreak levels that appear linked to management interventions, such as salvage logging of storm-felled trees and sanitary cutting of infested ones. It also detected increased bark beetle damage in recent warm and dry years, particularly in Sweden and Austria. However, the simulated damage still fell short of reported levels, suggesting that the drought response in LPJ-GUESS may be underestimated.

This new model represents an important new step in assessing the vulnerability of European forests to spruce bark beetle infestations.

Read the full study here.