Focal Area

Biotic Agents

Biotic agents, ranging from insects, pathogens, and large herbivores are increasingly shaping disturbances across pan-European forests.

Once considered manageable and localized, these threats have escalated in scale and complexity due to climate change, forest homogenization, and global trade. From bark beetle outbreaks to ash dieback, biotic disturbances pose major systematic risks to forest health and biodiversity across Europe.

Challenges in Managing Biotic Disturbances across Pan-Europe

The landscape of biotic threats is multifaced and rapidly evolving. Recent years have seen unprecedented bark beetle outbreaks, particularly in spruce forests and often driven by extreme droughts. These outbreaks have disrupted timber markets, forest management, and ecological stability. However, the challenge extends well beyond spruce forests; mortality in pine, fir, and beech trees is rising due to combined stressors, while invasive organisms like the pine wood nematode, oak lace bug, and emerald ash borer continue to expand their range.

Forest areas recovering from past disturbances are particularly vulnerable, often becoming hotspots for invasive plant species and can suffer from excessive browsing by large ungulate populations. These pressures, together with increasing extreme climate, may critically hinder forest regeneration. Traditional management approaches focused on suppression and control are becoming inadequate in these new high-risk conditions. A shift toward resilience-oriented strategies is urgently needed, emphasizing diversity, adaptive capacity, and cross-sectoral coordination.

  • Biotic Agents
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Tomáš Hlásny

Expert insight

Tomáš Hlásny

Tomáš Hlásny, a forest researcher with over two decades of experience, offers a deeply informed perspective shaped by years of scientific inquiry, policy engagement, and field observation. His work bridges ecology, management, and climate science, emphasizing the need to treat forests as dynamic socio-ecological systems with an emphasis on resilience.

“Future risks will be higher and more complex, and disturbance dynamics will differ from what we know today. In the faces of this uncertainty, our compass is resilience, grounded in diversity across scales. This remains our primary strategy for facing the unknown: distributing risk and enhancing recovery capacity”.

His vision of resilience includes fostering diversity across species and landscapes, avoiding monocultures and adapting forest composition to future climates. Mixed and structurally complex forests not only resist outbreaks better but also recover more effectively. Tomáš stresses that resilient forests may not align with traditional productivity-focused models, requiring a shift in forestry values and practices.

FoRISK’s Role in Enhancing Biotic Resilience

FoRISK is helping forest professionals across pan-Europe navigate the growing complexity of biotic threats. By fostering expert networks, promoting science-based strategies, and supporting adaptive and preventive management, FoRISK contributes to a future where forests are better equipped to withstand and recover from biotic disturbances.

FoRISK’s commitment to collaboration, capacity building, and knowledge sharing ensures that Europe’s forests are not only aware of biotic risks, but resilient enough to face them.

Key actions supported by FoRISK include:

Transboundary monitoring and expert exchange
Transboundary monitoring and expert exchange:

Sharing knowledge across regions helps detect threats early and apply proven strategies in new contexts.

Resilience-oriented training and silviculture
Resilience-oriented training and silviculture:

Equipping practitioners with the tools and education needed to design diverse, adaptive forest landscapes that resist and recover from biotic stress.

Accessible resources via the FoRISK Intelligence Hub
Accessible resources via the FoRISK Intelligence Hub:

Providing decision-makers with up-to-date information and guidance on biotic threats, forest health, along with adaptive and preventive strategies.

Biotic agents such as insect pests and pathogens interact dynamically with wildfire behaviour and forest fire dynamics:

  • Vegetation Mortality and Fuel Continuity:
    Insect pests and pathogens can cause widespread tree mortality and canopy thinning, leaving behind large amounts of dry, dead biomass. This not only increases surface and canopy fuel loads but also alters light and moisture conditions in the understory. The resulting environment promotes the growth of flammable grasses and shrubs, creating a more continuous and connected fuel layer. Together, these changes significantly heighten the risk of intense and fast-spreading wildfires[1].
  • Disturbance Synergy:
    The interaction between biotic disturbances and fire is often synergistic. For example, forests affected by bark beetle epidemics, due to increased fuel loads from dead trees, retained dry needles that elevate crown fire risk, and canopy openings that dry out surface fuels. These conditions make forests more flammable and harder to manage. These conditions make forests more flammable and harder to manage. In turn, severe fires can create stress conditions that favour further pest outbreaks. These feedback loops are becoming more frequent under climate change, which intensifies both pest activity and fire regimes[2].
  • Pathogen-Fire Buffering:
    In some cases, wildfires can reduce the impact of forest diseases. For instance, fire may destroy infected host trees or alter microclimates in ways that suppress pathogen survival and transmission. A study on Phytophthora ramorum (sudden oak death) found that recently burned areas had lower disease reinvasion and host mortality rates.

[1] Fire and Insect Interactions in North American Forests
 
[2] Global review on interactions between insect pests and other forest disturbances | Landscape Ecology

Biotic Agents

Intersectional Biotic Agents

Windstorms and biotic agents (such as insect pests and pathogens) interact in complex ways that can significantly influence forest health and disturbance dynamic:

  • Windthrow as Breeding Habitat for Pests: Windstorms often uproot or snap trees, leaving behind weakened or dead wood. This creates ideal breeding conditions for bark beetles and other wood-boring insects, which prefer freshly downed trees for colonization. These populations can then spread to healthy trees, triggering outbreaks[1].
  • Tree Vulnerability from Biotic Damage: Trees weakened by pathogens or insect infestations (e.g. root rot fungi or bark beetles) are structurally compromised and more likely to suffer windthrow during storms. This increases the extent of storm damage and contributes to cascading forest disturbances[2].
  • Facilitated Pathogen Spread: Windstorms can break branches, strip bark, and expose roots, creating entry points for pathogens. These injuries, combined with stress from mechanical damage, make trees more susceptible to infection and disease spread[3].

[1] Climate change amplifies the interactions between wind and bark beetle disturbances in forest landscapes.

[2] Climate change amplifies the interactions between wind and bark beetle disturbances in forest landscapes

[3] Storms, wind throws and their consequences

Biotic Agents

Intersectional Biotic Agents

Biotic agents such as insect pests and pathogens interact dynamically with wildfire behaviour and forest fire dynamics:

  • Biotic agents such as insect pests and pathogens interact dynamically with wildfire behaviour and forest fire dynamics.
  • Fuel Load Amplification:
    Insect outbreaks such as bark beetles and defoliators can cause widespread tree mortality, leaving behind large quantities of dry, dead biomass. This deadwood increases surface and canopy fuel loads, making forests more prone to intense and fast-spreading wildfires. The timing and severity of outbreaks play a critical role in determining whether they amplify or dampen fire behavior[1].
  • Canopy Structure Alteration:
    Pathogens and pests can thin forest canopies by killing host trees, which alters light and moisture conditions in the understory. This can lead to increased growth of flammable grasses and shrubs, creating a more continuous fuel layer that facilitates fire spread[2].
  • Disturbance Synergy:
    The interaction between biotic disturbances and fire is often synergistic. For example, forests affected by bark beetle epidemics may experience more severe fires, which in turn can create conditions favorable for further pest outbreaks. These feedback loops are becoming more frequent under climate change, which intensifies both pest activity and fire regimes5.
  • Pathogen-Fire Buffering:
    In some cases, wildfires can reduce the impact of forest diseases. For instance, fire may destroy infected host trees or alter microclimates in ways that suppress pathogen survival and transmission. A study on Phytophthora ramorum (sudden oak death) found that recently burned areas had lower disease reinvasion and host mortality rates5.

[1] Fire and Insect Interactions in North American Forests

[2] Global review on interactions between insect pests and other forest disturbances | Landscape Ecology

Windstorms and biotic agents (such as insect pests and pathogens) interact in complex ways that can significantly influence forest health and disturbance dynamic::

  • Windthrow as Breeding Habitat for Pests: Windstorms often uproot or snap trees, leaving behind weakened or dead wood. This creates ideal breeding conditions for bark beetles and other wood-boring insects, which prefer freshly downed trees for colonization. These populations can then spread to healthy trees, triggering outbreaks[1].
  • Fuel Load Amplification:
    Insect outbreaks such as bark beetles and defoliators can cause widespread tree mortality, leaving behind large quantities of dry, dead biomass. This deadwood increases surface and canopy fuel loads, making forests more prone to intense and fast-spreading wildfires. The timing and severity of outbreaks play a critical role in determining whether they amplify or dampen fire behavior[1].
  • Tree Vulnerability from Biotic Damage: Trees weakened by pathogens or insect infestations (e.g. root rot fungi or bark beetles) are structurally compromised and more likely to suffer windthrow during storms. This increases the extent of storm damage and contributes to cascading forest disturbances [2].
  • Facilitated Pathogen Spread: Windstorms can break branches, strip bark, and expose roots, creating entry points for pathogens. These injuries, combined with stress from mechanical damage, make trees more susceptible to infection and disease spread [3].

[1] Climate change amplifies the interactions between wind and bark beetle disturbances in forest landscapes.

[2] Climate change amplifies the interactions between wind and bark beetle disturbances in forest landscapes

[3] Storms, wind throws and their consequences