The Vulnerability of Forests to Natural Disturbances

The Analysis Notes present, in four pages, the main points of discussion on a current topic relevant to the areas of activity of the Department of Agriculture, Agri-Food, and Food Sovereignty. Depending on the topic, they take a forward-looking, strategic, or evaluative approach.

Analysis n°224

The Vulnerability of Forests to Natural Disturbances

Forests are increasingly threatened by natural disturbances, which are being exacerbated by climate change. This note explores the growing risks that will emerge in the future. It details several vulnerability factors and the associated environmental and economic impacts. In an uncertain context, understanding the interplay between the various components of risk makes it possible to prioritise actions that can mitigate it. Proactive management of forest fuel or the recovery of storm-damaged timber, for instance, are measures that can help address this challenge.

Introduction

Forests are affected by so-called “natural” disturbances, whether biotic (e.g. pathogens) or abiotic (e.g. fires). Their occurrence may nevertheless be of anthropogenic origin, as is the case for the majority of fires in France. Climate change intensifies these disturbances and places an increasing threat on the functions of forests: timber production, carbon sequestration, biodiversity reservoir, etc. The recent multiplication of extreme events, such as the fires in France during the summer of 2022 or the wildfire events that devastated forests in the Aude department in August 2025, has heightened the visibility of these disturbances in public debate. Awareness has also grown of their role in the decline of the land carbon sink¹.

The concepts associated with the notion of “risk” are numerous, each carrying a specific meaning. This note clarifies the constituent elements of the risks generated by natural disturbances in forests and illustrates them with concrete cases concerning Europe (including France) and the United States.

The first part defines the components of risk (hazard, asset, vulnerability) and presents trends of the future evolution of hazards for the main disturbance agents. The second part reviews some of the environmental and economic consequences of current and future disturbances. Finally, examples of practices to reduce vulnerability through forest management or organisational solutions are described.

Hazard and Vulnerability: Two Major Components of Climate Change-Related Risks in Forests

Risk, a Composite Notion

Risk is a polysemous term, defined by the Intergovernmental Panel on Climate Change (IPCC) as the potential for adverse consequences for a system to which certain values and objectives are attached². It is a composite notion, resulting from the encounter of three components: a hazard, an element exposed to that hazard (or asset), and its vulnerability. The hazard corresponds to the physical phenomenon capable of causing harm. The notion covers both the characteristics of the event (e.g. wind speed, burnt area) and its probability of occurrence. Hazards that correspond to punctual events (e.g. fire, storm) cause catastrophic mortality, i.e. sudden and massive, in forest stands. Others have more diffuse impacts, such as droughts, which correspond to periods of thermal and/or rainfall anomalies compared to past references.

The exposed element here is the forest ecosystem, which possesses use and non-use values for society and provides ecosystem services (e.g. timber production, landscape amenities). The asset corresponds to the proper ecosystem functioning of the forest. Independently of its degree of exposure, i.e. presence in a given area, a forest is characterised by a certain level of vulnerability, meaning a predisposition to being negatively impacted. It may be vulnerable due to its predisposition to damage (sensitivity) or its limited ability to adjust to and recover from damage (adaptive capacity).

Figure 1 – Breakdown of the concept of risk

^{Reading: risk emerges from the encounter of a hazard that affects an exposed element characterised by a certain level of vulnerability.}

^{Source : IPCC, 2022, Climate Change 2022: Impacts, Adaptation and Vulnerability}

Vulnerability may be intrinsic, linked to the physiological traits of trees, the composition and structure of stands, etc.³. For instance, some species are more resistant to fires due to thicker bark, or better adapted to rapid recovery, by resprouting (e.g. oaks) or via seed banks (e.g. Aleppo pine). Conversely, a forest with a high presence of fuel (e.g. dense understorey) is more prone to high-intensity fires. Forest vulnerability may also be extrinsic, linked to forest management, spatial planning, or land use. For example, the presence of roads makes forests more accessible for prevention and firefighting, but also more exposed to fire outbreaks caused by humans. Similarly, the existence of a monitoring system enables earlier detection of pathogen invasions.

Climate Hazards in Forests Are Intensifying

Climate change increases risk by worsening disturbance regimes (the hazard component), in other words by altering the frequency, intensity and seasonality of disturbance events and the areas they affect. These effects may be direct, for example when water stress due to lack of rainfall and longer, more intense periods of high temperatures weaken trees. They may also be indirect, particularly when new pathogens migrate and become viable (e.g. pine processionary caterpillar) or when endemic species enter epidemic phases (e.g. spruce bark beetle).

Between the periods 1950–2000 and 2001–2019, damage caused by natural disturbances increased by 17% in Europe. Storms accounted for 46% of damage across both periods, followed by fires (24%), bark beetles (17%), other biotic agents (8%, e.g. diseases such as ash dieback and chestnut blight), and abiotic agents (8%, e.g. drought, snow). The increase in damage has been particularly marked in recent years for fires and bark beetles (Figure 2). The latter were responsible for 50% of the timber harvest in Germany in 2021. In France, in Bourgogne-Franche-Comté, 30,000 hectares of coniferous forests containing spruces (18% of the regional total) were destroyed or harvested between 2018 and 2022 due to bark beetle attacks. Since 2023, the crisis has affected all altitudinal levels of the Jura massif, including above 1,000 m⁴. Droughts, meanwhile, affected 0.5 million hectares of forests between 1987 and 2016 in Europe, and four of the five most severe events occurred after 2000⁵.

Figure 2 – Annual damage due to natural forest disturbances in Europe, 1950–2019

^{Source : Patacca M. et al. 2022, “Significant increase in natural disturbance impacts on European forests since 1950”, Global Change Biology}

Southern Europe could, during the 21st century, experience a 40 to 100% increase in annual burnt area. The fire season could lengthen by 20 days, and the probability of “megafires” increase tenfold^{6, 7}. Temperate and boreal regions would also be affected, albeit to a lesser extent. In France, fire risk is expected to rise particularly sharply in the West and along the Mediterranean coastline⁸. For a +4 °C warming by the end of the century, the fire season would last 131 days and fires would affect 67% of the south-eastern quarter of the country, compared with 78 days and 28% at present⁹.

Climate change accelerates the development cycle of pathogens and extends the areas favourable to them. The main epidemic currently underway is that of the spruce bark beetle. In western Europe, the damage it causes could be six times higher in 2021–2030 compared with 1971–2010¹⁰.

Storms are rare and complex events that are difficult to model. Climate change could increase wind speeds and make extreme events more common. In central Europe, a storm that is currently expected once a century could see its return period shortened to around 70 years by 2100¹¹.

The different hazards interact with one another. For example, a forest damaged by a storm is more vulnerable to pathogens, and a drought increases the likelihood of fires. These interactions are expected to intensify under climate change, particularly with regard to biotic risks, making it necessary to adopt a “multi-hazard” perspective¹².

Natural Hazards Affect Forest Functions

Complex Ecological Interactions

The occurrence of a hazard directly alters the structure and composition of the forest, as well as the ecosystem services it provides. In the short term, many trees may be destroyed or damaged, and biogeochemical cycles and habitats disrupted. The reorganisation phase that follows immediately after the hazard is crucial¹³, as it is when the communities that will make up the future forest establish themselves. Several outcomes are possible, ranging from full resilience (same composition and structure) to transition towards a non-forest state (in the case of major and/or repeated hazards), with intermediate states also possible.

Biodiversity is affected through the impacts of disturbances on habitats. An affected area may see its species richness (number of species present) increase, decrease or remain stable¹⁴. This ambiguous relationship varies depending on the species and the dynamics of disturbances. Negative impacts often concern species that favour closed canopies (e.g. lichens), while others depend on disturbances to survive and establish themselves in open environments. The highest biodiversity levels, at landscape scale, are observed for disturbances of medium severity and size. Conversely, extreme events are generally harmful due to their tendency to homogenise habitats over large areas. The disappearance of a particular species may also be caused by certain hazards (e.g. pathogen affecting a specific tree species, such as ash dieback).

A global meta-analysis has shown a negative impact of disturbances on most ecosystem services, particularly carbon sequestration. Soil quality and organic content, timber production and water quality are also affected¹⁵. Fires, for instance, increase soil erosion, reduce the recreational value of forests, and cause carbon emissions and release volatile compounds that can affect human health. According to a recent estimate, the three main disturbance agents in Europe (storms, fires, bark beetles) currently threaten the provision of forest ecosystem services by 10.4 to 12.3%. In total, 20 million hectares of forest (9.7% of the total area) would see their multifunctionality threatened, particularly in central and western Europe (Figure 3).

Figure 3 – Areas affected by high risk of loss of forest ecosystem services in Europe

^{Reading: the coloured areas correspond to those most affected (beyond the 8th decile) by the risk to the ecosystem service considered. Other areas are also affected, but the level of risk is lower.}

^{Source : Lecina Diaz, J., et al., 2024, “Ecosystem services at risk from disturbance in Europe’s forests”, Global Change Biology}

From Timber Losses to Economic and Climate Impacts

Natural hazards pose risks to timber production and the wood industry. A major crisis, such as a storm, causes in the short term an influx of lower-quality timber on the market, reducing prices and disrupting trade¹⁶. Later, the damage causes a reduction in timber supply and pressure on prices (which rise), potentially lasting for several decades. In the Czech Republic, the 2018 bark beetle crisis caused an increase of 5.3 to 18 million m³ (+340%) in salvaged damaged timber compared with 2017, leading to a drop in timber prices from €55/m³ to €15/m³ (–73%)¹⁷.

Disturbances also affect the value of forest land. For example, it decreased by an average of 10% between 1984–2003 and 2001–2020 in western US states, due to the multiplication of fires and droughts. Even unburnt areas are affected, to a lesser extent, through a change in risk perception by buyers¹⁸.

Hazards also pose a risk of non-permanence for carbon stocks mobilised to mitigate climate change. In Europe, in the future, carbon losses linked to natural disturbances could be of the same order of magnitude as the gains expected from forest management aimed at mitigating climate change¹⁹. Due to the inherent uncertainty of meteorological phenomena, the forest carbon sink may vary greatly from year to year²⁰. For example, Storm Vaia alone reduced Italy’s forest carbon sink by 4% in 2018²¹. Recovering damaged timber for industry nevertheless makes it possible to transfer part of the carbon to downstream sectors, partially mitigating these consequences.

The introduction of new pests could cause further degradation of the carbon sink. Seidl et al.²² estimate that 10% of European forest carbon would be exposed to five major new pests in Europe by 2080 under a high-warming scenario (RCP 8.5). An invasion by these pests would increase carbon losses due to natural hazards from +2.3% (for beech canker) to +50% (for the Asian longhorn beetle, which attacks several hardwood species). Beyond the direct mortality due to hazards, the decline of the carbon sink may also be attributed to the loss of photosynthetic activity by dying trees (Figure 4).

Figure 4 – Breakdown of the decline of the French forest carbon sink between 2010 and 2020

^{Source : Secrétariat général à la planification écologique, 2023 1}

Reducing Vulnerability to Mitigate Risk

Direct action on hazards is often difficult, but risk can be reduced by lowering the vulnerability of exposed forests. Interventions are possible at different levels. Two examples are presented here: fuel management and the recovery of damaged timber. Many other measures, which we do not develop here, can also contribute to reducing vulnerability: diversification of forest composition and structure, enhanced monitoring, raising awareness among local stakeholders, etc.

Managing Forest Fuel to Reduce Fire Risk

In forests, the presence of fuel in the understorey (e.g. low vegetation, branches) increases the risk of fire. It raises the probability of fire outbreaks and their ability to intensify and spread. When fuel is dry and continuous, the risk is even greater.

Removing this vegetation preventively reduces forest vulnerability, particularly by lowering sensitivity, while also affecting the hazard component. This is, for example, the aim of the legal obligations for clearing undergrowth. These apply, in 43 French departments, to land located within 200 m of forest areas, within a radius of 50 m around buildings. These “wildland-urban” interface zones are particularly vulnerable, due to the high potential for fire outbreaks and the presence of buildings, people and economic activities. Similar measures exist elsewhere, for instance in Catalonia, Spain. In some regions, grazing is used to reduce fuel, through payments for environmental services²³.

A recent study assessed the impact of forest fuel management in California, where fires caused average annual damage estimated at $117 billion between 2017 and 2021²⁴. By 2050, the increase in fire intensity could be offset by the annual treatment of 240,000 hectares. The current target is to treat 400,000 hectares, which would avoid losses of $10.9 billion for an estimated annual cost of $3 billion, while each year of delay in achieving this target would lead to $4 billion in losses. However, according to the economists behind the study, the optimal area to treat to maximise benefits would be 1.6 million hectares per year, four times higher than the current target.

Fuel can also be deliberately burnt preventively. Prescribed burning is a preventive measure that reduces the fuel load to prevent fire outbreaks. In France, it is planned by the State, local authorities or their representatives, and carried out by professionals. This practice also helps to maintain the good condition of certain ecosystems. Another solution, tactical fires and backfires, are firefighting techniques used during a blaze. They aim to channel the fire and support firefighting personnel.

Recovering Damaged Timber to Reduce Economic Impacts

When storms occur, they generate a sudden influx of damaged timber. Recovering it enables owners to generate income, which is then reinvested in forest management. Being able to commercialise this “crisis timber” improves the adaptive capacity of forests. Conversely, leaving such timber on site increases the likelihood of another crisis (e.g. disease, fire outbreak), raising the sensitivity (and therefore vulnerability) of the stand to potential cascading hazards.

Following a major crisis, wood recovery may be coordinated and supported by the State. It mainly involves two methods: storing timber for later processing locally; or exporting it to other regions²⁵. These interventions help to ease pressure on local markets and smooth out negative economic impacts (e.g. price falls). First, an assessment of the damage must be carried out to prioritise operations. Rapid intervention is particularly necessary in the case of disease, to avoid its spread and timber degradation. During storage, timber health can be stabilised by treating it once removed from the forest (heat treatment, fumigation) and by ensuring proper storage conditions (ventilation, distance from forest, contact with the ground).

A timber mobilisation plan was implemented in France following Storm Klaus (2009), which destroyed 42.5 million m³ of timber, mainly in the Aquitaine region. It included direct support for timber storage (creation and rehabilitation of storage areas) and transport outside the region, for a total of €126 million. Ultimately, 55% of the 29.5 million m³ of timber harvested post-storm benefited from direct aid. An economic model²⁶ showed that this plan encouraged storage (+25%), accelerated and increased timber recovery (+33%), although it did not prevent the fall in prices (–60% for maritime pine). Over ten years, the plan is estimated to have generated an economic gain of €44 million for the forest-wood sector. An even more storage-focused strategy would have been more beneficial but would have slowed the recovery of prices, to the detriment of local producers.

In the future, more damaged timber will be available due to the worsening of disturbance regimes. A recent foresight study²⁷ emphasises the importance of anticipating this dynamic, not only for the economic aspects mentioned above, but also for maintaining the forest carbon sink. Upstream, it will be necessary to increase proactive harvesting of declining timber and prioritise it, in crisis years, over standard harvesting. Downstream, processing and storage capacities will need to be increased. Innovation will also be a key factor, in order to make better use of downgraded timber and, where possible, direct it towards long-life uses (e.g. construction), which are more beneficial from a climate perspective.

Conclusion

The vulnerability of forests to natural disturbances, exacerbated by climate change, is a crucial issue for the management of forest ecosystems. This note has clarified the components of risk, distinguishing hazard, vulnerability and the assets associated with forests. Concrete examples show that forests are increasingly exposed to multiple hazards, particularly in Europe, with varied effects on ecosystem services, the forest economy and biodiversity. Adaptation strategies, such as fuel management and the recovery of damaged timber, can mitigate these impacts by reducing forest vulnerability.

In the future, the development of a multi-hazard approach and proactive management practices will be crucial. To this end, it will be necessary to rely on clear conceptual frameworks in order to target the different components of risk appropriately.

However, uncertainties remain about the complex interactions between the different hazards and the scale of future changes. Further research is needed to refine our knowledge, particularly regarding the impacts of extreme events. Other elements beyond forests are exposed, and potentially vulnerable, to the hazards discussed here. These include populations (fires can harm health through air pollution) and infrastructure (transport, housing, etc.). Any strategy for adapting to “forest” hazards must therefore be cross-sectoral and systemic, looking beyond the forest itself and considering these multiple components.

Miguel Rivière
Centre for studies and strategic foresight

Notes de bas de page

1- Secrétariat général à la planification écologique, 2023, La planification écologique pour la forêt : principaux enjeux et leviers.

2 - IPCC, 2022, Climate Change 2022: Impacts, Adaptation and Vulnerability, Cambridge University Press.

3 - Lecina-Diaz J. et al., 2021, “Characterizing forest vulnerability and risk to climate-change hazards”, Frontiers in Ecology and the Environment.

4 - Département de la santé des forêts, 2024, Situation de l’épicéa commun liée aux attaques de scolytes en région Bourgogne-Franche-Comté. Clerget, V., 2024, Renouvellement forestier suite à la crise du scolyte typographe en Bourgogne-Franche-Comté.

5 - Senf C., et al., 2020, “Excess forest mortality is consistently linked to drought across Europe”, Nature Communications.

6 - Bowman D. et al., 2020, “Vegetation fires in the Anthropocene”, Nature Reviews Earth & Environment.

7 - El Garroussi S., et al., 2024, “Europe faces up to tenfold increase in extreme fires in a warming climate”, npj Climate and Atmospheric Science.

8 - Fargeon H., et al., 2020, “Projections of fire danger under climate change over France: where do the greatest uncertainties lie?”, Climatic Change.

9 - Pimont F., et al., 2022, “Future expansion, seasonal lengthening and intensification of fire activity under climate change in southeastern France”, International journal of wildland fire.

10 - Hlásny T., et al., 2021, “Bark Beetle Outbreaks in Europe: State of Knowledge and Ways Forward for Management”, Current Forestry Reports.

11 - Outten S., Sobolowski, S., 2021, “Extreme wind projections over Europe from the Euro-CORDEX regional climate models”, Weather and Climate Extremes.

12 - Seidl R. et al., 2017, “Forest disturbances under climate change”, Nature Climate Change.

13 - Seidl R., Turner, M. G., 2022, “Post-disturbance reorganization of forest ecosystems in a changing world”, Proceedings of the National Academy of Sciences.

14 - Viljur M. et al., 2022, “The effect of natural disturbances on forest biodiversity: an ecological synthesis”, Biological Reviews.

15 - Thom D., Seidl R, 2016, “Natural disturbance impacts on ecosystem services and biodiversity in temperate and boreal forests”, Biological Reviews.

16 - Johnston C. M., et al., 2024, “Unraveling the impacts: How extreme weather events disrupt wood product markets”, Earth’s Future.

17 - Hlásny T., et al., 2019, “Living with bark beetles: impacts, outlook and management options”, European Forest Institute.

18 - Wang Y., Lewis D. J., 2024, “Wildfires and climate change have lowered the economic value of western US forests by altering risk expectations”, Journal of Environmental Economics and Management.

19 - Seidl R., et al., 2014, “Increasing forest disturbances in Europe and their impact on carbon storage”, Nature climate change.

20 - Pilli R., et al., 2016, “Modelling forest carbon stock changes as affected by harvest and natural disturbances. II. EU-level analysis”, Carbon balance and management.

21 - Pilli R., et al., 2021, “Combined effects of natural disturbances and management on forest carbon sequestration: the case of Vaia storm in Italy”, Annals of Forest Science.

22 - Seidl R., et al., 2018, “Invasive alien pests threaten the carbon stored in Europe’s forests”, Nature Communications.

23 - Mauri E., Jankavić M., 2024, Wildfire risk planning and prevention – Innovations in the Mediterranean and beyond, European Forest Institute.

24 - Brown P., 2024, Cost-effectiveness of large-scale fuel reduction for wildfire mitigation in California, The Breakthrough Institute.

25 - Gardiner B., et al., 2013, Living with storm damage to forests, European Forest Institute.

26 - Caurla S., et al., 2015, “Store or export? An economic evaluation of financial compensation to forest sector after windstorm. The case of Hurricane Klaus”, Forest Policy and Economics.

27 - Carbone 4, 2023, Scénario de convergence de filière, résumé exécutif.