Windstorms are among the most important natural disturbances in Northern Europe, where they cause extensive tree mortality, timber losses and long-lasting changes in forest structure. Large-scale windstorm damage has increased across Central and Northern European forests in recent decades, while in boreal regions such as Finland, warmer winters and shorter periods of frozen soil can further reduce tree stability and increase vulnerability to wind.
The consequences of wind damage, however, continue long after the storm has passed. A large-scale study by researchers at the University of Eastern Finland analysing more than 70,000 wind-damaged areas found that those forests remain vulnerable and may be affected by subsequent bark beetle or snow damage in the years to follow, depending on forest structure, climate, topography and species composition.
“Forest disturbances are often considered separately, but in fact, one disturbance can change the conditions under which the next one occurs,” says Doctoral Researcher Qianqian Tian, lead author of the study at the University of Eastern Finland. “We wanted to understand what happens after wind damage and whether forests subsequently affected by bark beetle or snow damage share the same characteristics.”
Forest structure played an important role in shaping what happened after wind damage. Warmer conditions and forest composition helped distinguish stands affected by bark beetle damage from those affected by snow damage, while mixed stands showed more bark beetle damage than snow damage, compared with pine-dominated stands. These differences suggest that post-storm forest vulnerability depends not only on the initial damage, but also on the structural and environmental conditions of the affected stand.
“This has important practical implications, since it can help us adapt monitoring and forest management after a storm to reduce future losses,” says Professor Blas Mola-Yudego, coordinator of the study at the University of Eastern Finland. “After a storm, a dense and structurally heterogeneous forest in southern Finland may require different attention than a pine dominated stand in North Karelia. Forest risk management should consider what may happen next, rather than focusing only on the damage that has already occurred.”
The timing was also notable. Around 94% of wind-snow damage sequences and 76% of wind-bark beetle sequences occurred within five years of the initial wind disturbance, and more than 90% occurred within eight years.
“The years immediately following a major wind disturbance can therefore be particularly important for monitoring and preventing further damage, but our results also show that we should not look for the same warning signs everywhere,” says Marina Peris-Llopis, a co-author of the study.”
Mari Selkimäki, lecturer in forest planning at the University of Eastern Finland, highlights the importance of the large-scale forest information used in the study: “Combining national forest data with long-term damage records allowed us to look beyond individual storms and identify broader patterns in how disturbances interact across boreal forests.”
The results support more targeted monitoring after storms. Structurally heterogeneous forests containing spruce, particularly in warmer areas, may deserve greater attention for bark beetle damage, whereas forests on steeper terrain may require closer monitoring for subsequent snow damage. Maintaining stand stability and avoiding excessive density may also help reduce vulnerability to interacting disturbances.
These findings are part of a large pan-European research effort aimed at developing more resilient forests and supporting climate-change adaptation across Europe, funded by the EU’s Horizon Europe Programme (project Eco2Adapt) and the Strategic Research Council of Finland (project ForTran).