Increasing forest disturbances in Europe and their impact on carbon storage

Abstract

Disturbances from wind, bark beetles, and wildfires have increased in Europe’s forests throughout the 20^th century ¹. Climatic changes were identified as a main driver behind this increase ², yet how the expected continuation of climate change will affect Europe’s forest disturbance regime remains unresolved. Increasing disturbances could strongly impact the forest carbon budget ^3,4, and are hypothesized to contribute to the recently observed carbon sink saturation in Europe’s forests ⁵. Here we show that forest disturbance damage in Europe has continued to increase in the first decade of the 21^st century. Based on an ensemble of climate change scenarios we find that damage from wind, bark beetles, and forest fires is likely to increase further in coming decades, and estimate the rate of increase to +0.91·10⁶ m³ of timber per year until 2030. We show that this intensification can offset the effect of management strategies aiming to increase the forest carbon sink, and calculate the disturbance-related reduction of the carbon storage potential in Europe’s forests to be 503.4 Tg C in 2021-2030. Our results highlight the considerable carbon cycle feedbacks of changing disturbance regimes, and underline that future forest policy and management will require a stronger focus on disturbance risk and resilience.

Natural disturbances, i.e., large pulses of tree mortality from agents such as wildfire, insect outbreaks, or strong winds, are integral drivers of forest dynamics ⁶ and contribute to the diversity and adaptive capacity of ecosystems ⁷. Yet, forest disturbance regimes have changed considerably in recent years. The frequency and severity of large wildfires, for instance, has increased around the globe in past decades ^1,8,9. Also, recent bark beetle outbreaks, e.g., in North America and Central Europe ^10,11, have reached unprecedented levels. A continuation of this trend towards more frequent and severe disturbances is also presumed for the coming decades. Bark beetles are, for instance, expected to colonize previously unsuitable habitats in higher latitudes and mountain forests ¹², and large wildfires occurring only rarely in the past are predicted to return with higher frequency under climate change ¹³.

Intensifying disturbance regimes are thus expected to be among the most severe impacts of climate change on forest ecosystems, raising concerns that disturbances might increasingly interfere with a continuous and sustainable provisioning of ecosystem services to society ^14,15. With regard to forest carbon (C) storage, which is an increasingly important ecosystem service in the context of climate change mitigation, forests are “slow in, rapid out” systems ¹⁶, with disturbance being a major pathway of fast, large-scale ecosystem C loss ³. Forest-related climate change mitigation policies are thus highly sensitive to disturbance regimes ⁴. Intensifying disturbance regimes have already been associated with a weakening of the European forest C sink recently ⁵. A further increase in disturbance damage in the future might thus pose a major risk for Europe’s climate change mitigation efforts, as it could counteract the strive to offset anthropogenic climate change through enhanced C storage in forest ecosystems ⁴. Yet, consistent continental-scale assessments of potential changes in the forest disturbance regime under climate change are still missing to date. Furthermore, it is as of yet unclear, how alternative European forest policies ¹⁷ will influence disturbance regimes, and to which degree increasing disturbances might offset the potential of these policies to enhance Europe’s forest C storage capacity.

Using a novel combination of Europe-wide disturbance observations (>29,000 records), scenario simulations of future forest development, and statistical disturbance modeling, we here report a continental-scale disturbance time series from 1971 to 2030 (see Supplementary Information). Our analysis focused on an area of 131.6·10⁶ hectares of forests available for wood supply in 2005, covering 29 European countries in eight different ecoregions. We address the three most detrimental forest disturbance agents in Europe, which are wind, bark beetles, and forest fires. Future disturbance damage for 2011–2030 is projected under four alternative forest management strategies ¹⁷ for an ensemble of climate scenarios (continental-scale ensemble median warming of +1.1°C, precipitation increase of +2.8%, and an increase in maximum daily windspeed by +0.7% until 2030 compared to 1971-2001, see Supplementary Table 4). To investigate potential impacts of disturbance on climate change mitigation efforts we quantified the impact of future disturbance regimes on the C storage capacity of Europe’s forests by means of an analytical C cycle model ¹⁸. We evaluated how the effects of changing climate and disturbance regimes interact with alternative management strategies with regard to their implications on forest C storage.

We found that the disturbance intensification previously reported for the second half of the 20^th century ^1,2 accelerated in the first decade of the 21^st century. With damage of 32.3·10⁶ m³ yr⁻¹ (wind), 14.5·10⁶ m³ yr⁻¹ (bark beetles), and 9.4·10⁶ m³ yr⁻¹ (forest fires) the disturbance levels observed for the first decade of the 21^st century were the highest of the last 40 years for all three disturbance agents (increasing by +139.6%, +601.9%, and +231.1% relative to 1971-1980, respectively). The total disturbance damage from these three agents increased on average by +1.06·10⁶ m³ yr⁻¹ between 1971 and 2001. This rate of increase rose to +1.60·10⁶ m³ yr⁻¹ in 2002-2010.

Assuming a continuation of current forest management (reference strategy), projections under climate change resulted in a further increase in disturbance damage in all scenarios and for all agents (Figure 1). The ensemble median (and interquartile range, IQR) wind damage for Europe was estimated to 44.5·10⁶ m³ yr⁻¹ (IQR: 6.5·10⁶ m³ yr⁻¹) in 2021-2030 (+229.4% compared to 1971-1980). Bark beetle damage increased to 17.9·10⁶ m³ yr⁻¹ (IQR: 1.4·10⁶ m³ yr⁻¹) in 2021-2030 (+763.7% relative to 1971-1980). For the same period, the timber volume damaged from forest fires was predicted to increase to 11.7·10⁶ m³ yr⁻¹ (IQR: 0.6·10⁶ m³ yr⁻¹) (+313.9% relative to 1971-1980). Over all agents, the annual increase in disturbance damage predicted for 2011-2030 was +0.91 Mill. m³ yr⁻¹ (median over the ensemble of climate change scenarios). If stable climatic conditions were assumed, however, disturbance damage remained constant or decreased moderately (Supplementary Figure 9). This documents that climate change is the key driver of the disturbance changes projected for the coming decades (see also Supplementary Information).

Figure 1. Forest disturbance damage in Europe 1971 – 2030.

Values are disturbance damage in Millions of cubic meters of timber per year – please note the different scales on the individual panels. Predictions (lighter hues) assume a continuation of business-as-usual forest management (reference strategy) and represent the median over an ensemble of scenarios of future climate and forest growth. Missing agents – such as bark beetles in the Northern and Atlantic ecoregions – indicate that they were not modeled in these particular ecoregions due to a lack of historical data and/ or relevance of the agent in these areas (see Supplementary Table 5).

Ecoregions across Europe were affected by the predicted intensification of disturbance regimes (Figure 1). Forest fires, the dominant disturbance agent in the Mediterranean region, were projected to increase particularly on the Iberian Peninsula, while bark beetle damage increased most strongly in the Alps. Wind damage, being the disturbance most strongly driven by individual events of extreme weather, showed the highest variation over time, but was also projected to increase throughout the study period, particularly in mid-latitude ecoregions. In general, predicted increases in disturbance percentage (i.e., the annual disturbance damage relative to growing stock, Figure 2) were smaller than changes in absolute damage levels. Yet, also the continental-scale disturbance percentage over all three agents increased considerably, from 0.126% yr⁻¹ (1971-1980) to 0.264% yr⁻¹ in 2002-2010 and 0.311% yr⁻¹ in 2021-2030.

Figure 2. Annual disturbance percentage (relative to growing stock) in Europe’s forest ecosystems.

Predictions beyond 2010 assume a continuation of business-as-usual forest management (reference strategy). The median, interquartile range (dark grey), and minimum – maximum range (light grey) over an ensemble of scenarios of future climate and tree growth is indicated. Please note that the y-axis is logarithmically scaled.

The projected intensification of disturbance regimes has considerable impacts on the C storage capacity of Europe’s forests. In 2021-2030, Europe’s forest ecosystems could potentially store a total of 22,421 Tg C (ensemble median under reference management), with an annual C uptake (net ecosystem productivity, NEP) of 99.2 Tg C yr⁻¹ (Supplementary Table 6). This potential C stock (i.e., the C stored when disregarding the effect of natural disturbances) is lowered by 503.4 Tg C (IQR: 36.7 Tg C) when damage by wind, bark beetles, and forest fires is considered (Table 1). The continental-scale C effect of disturbance is thus in the same order of magnitude as the total amount of C stored in the forest ecosystems of the Atlantic ecoregion of Europe (Supplementary Table 9). More than one third of this disturbance effect (183.6 Tg C) can be attributed to the changes in the climate and disturbance regimes predicted for 2011-2030. Disturbances lead to the reversal of an otherwise positive climate change effect on the forest C balance: While climate change alone was projected to increase forest C stocks in Europe (+126.3 Tg C), the combined effect of changing climate and disturbance regimes resulted in a C loss of −57.4 Tg C in 2021-2030 compared to simulations under stable climate (Table 1).

Table 1.

The impact of climate change and natural disturbances on the carbon stored in Europe’s forest ecosystems (in Tg C). Results are for the period 2021-2030, and effect sizes relate to a 20-year analysis period. Projections are for a continuation of reference management, and the median over an ensemble of climate scenarios is reported for assessments under climate change.

			disturbance
		no	yes	effect size
	no	22,295	21,975	−319.8
climate change	yes	22,421	21,917	−503.4
	effect size	+126.3	−57.4	−183.6

Alternative management strategies can increase the forest C sink and enhance the climate change mitigation function of forests, e.g., through increased rotation periods and optimized thinning regimes. Relative to the continuation of business-as-usual management (reference strategy) implementing a dedicated carbon management strategy would increase the C stored in Europe’s forests by 562.7 Tg C in 2021-2030, while shifting towards biodiversity conservation would still increase C stocks by 153.4 Tg C (undisturbed simulations, ensemble median values, Table 2). However, these management strategies at the same time alter the susceptibility of forests to disturbance (e.g., older forests have a higher predisposition to wind and bark beetle damage), which can lead to increased disturbance impact and disturbance-related C loss. In the carbon and biodiversity strategies the absolute level of timber volume damaged by disturbance increased by +12.4% and +7.6% relative to reference management (see also Supplementary Table 8). This increase in disturbance diminishes the C gain from management. In the biodiversity strategy, for instance, the total C gain relative to reference management is reduced to 72.6 Tg C (−52.7%) when the effect of disturbance is accounted for (Table 2).

Table 2.

The effect of alternative forest management strategies on the C stored in Europe’s forests (in Tg C) relative to reference management in 2021-2030 (i.e., after a 20-year analysis period). Management effects are reported for all combinations of climate and disturbance effects, and the median over an ensemble of climate scenarios is reported for assessments under climate change.

climate change effect	disturbance effect	effect of alternative management strategies
		carbon	biodiversity	wood energy
no	no	+561.9	+155.2	−268.1
no	yes	+449.1	+3.1	−262.9
yes	no	+562.7	+153.4	−268.4
yes	yes	+510.1	+72.6	−259.1

We generally found the effect of disturbance on C stocks to be in the same order of magnitude as the C signal from alternative management strategies. This indicates that a changing disturbance regime has the potential to thwart desired management effects in Europe’s forests. Our continental-scale findings of disturbance impacts on C storage are in congruence with previous studies on particular disturbance agents and events in Europe ^19-21. Also the considerable interactions between management strategies and disturbance impacts on C stocks found in our analysis are well in line with theory and previous assessments ^4,22. However, an in-depth evaluation of our methodology also revealed uncertainties regarding the future trajectories of Europe’s disturbance regimes. Particularly for wind disturbance – which currently amounts to more than half of the total damage from the three agents addressed here – some regional disturbance models considerably underestimated damage in selected periods (see Supplementary Figure 4). This suggests that not all relevant processes might be captured in these models, and underlines the need for more process-oriented analyses in the future ²³. Moreover, regionally important disturbance agents not addressed here (e.g., insect herbivores, pathogens) might also be sensitive to climatic changes. Furthermore, considering the extended time horizon of forest dynamics, future work should aim at going beyond the temporal horizon addressed here, as changes in forest structure and composition might become more important drivers of disturbance regimes than direct effects of climate change over longer time scales ²⁴.

Generally, it is important to note that intensifying forest disturbance regimes will not only affect the forest C sink, but will also have implications for a wide variety of other ecosystem services. Besides devaluating timber through burning, breakage, and post-disturbance fungal infections, the predicted rise in disturbance damage will increase management costs (e.g., through salvage operations, fire suppression, and insect prevention measures) and fan the volatility of timber markets ²⁵. This will further amplify the negative effects of climate change on the timber-based forest economy in Europe ²⁶. Also the provisioning of drinking water could be negatively affected by intensifying disturbance regimes ²⁷, as water filtering and retention strongly rely on the maintenance of a continuous forest canopy. However, not only provisioning services of forest ecosystems but also cultural, regulating, and supporting services (e.g., recreational value, protection against soil erosion, air quality, and primary productivity) might be adversely affected by the continued increase in natural disturbances projected here. Since intensifying disturbance regimes have the potential to strongly interfere with management objectives, considerations of disturbance risk and resilience will require a more central role in Europe’s forest policy and management in order to sustain ecosystem functions and services in the future.

Methods

Assessment tools

To project trajectories of future forest development under different management strategies and future climates we used the European Forest Information SCENario model EFISCEN. EFISCEN is a large-scale forest simulation model that projects forest resource development at regional to European scale ²⁸. In EFISCEN, the state of the forest is described as an area distribution over age- and volume-classes in matrices, based on national forest inventory data. Transitions of area between matrix cells represent different natural processes in the simulation (e.g., growth, mortality), and are influenced by management regimes and scenario changes. The effects of climatic changes are implemented in EFISCEN through deriving response functions for key processes such as forest growth from detailed process-based models.

We used empirically parameterized disturbance models to project the future damage by wind, bark beetles, and wildfires in Europe’s forest ecosystems. The empirical relationships were developed using country-scale disturbance data for the period 1958-2001 ¹, and are based on a compilation of >29,000 disturbance records across Europe ²⁹. The disturbance agents wind, bark beetles, and forest fire were represented by individual models at country scale. Using unsupervised machine learning, indicators of climate as well as forest extent, structure and composition were selected as predictor variables ². Their influence strength on disturbance damage was determined via structural equation modeling (SEM) ². For prediction, we used climate scenario data in combination with the respective forest structure and composition projections of EFISCEN.

To estimate the impact of disturbance on the forest C budget we used the REGIME model ¹⁸, which provides a general framework to quantitatively assess disturbance effects on ecosystem carbon storage capacity at large spatial scales. The main constituents of the C cycle in REGIME are net primary production, the size of the ecosystem C pools, and their residence times. The disturbance regime is characterized by the fraction of live biomass C removed per unit of time. We used simulation results of undisturbed EFISCEN runs to parameterize the REGIME model, and estimated disturbance levels by means of SEMs. The overall C effect of disturbance in Europe’s forests was calculated by additively combining the three disturbance agents studied. Details and evaluations of the applied approaches can be found in the Supplementary Information.

Scenario analysis

For the period 2011 to 2030 we studied 14 different scenarios of future climate change and tree growth (Supplementary Table 4). These scenarios cover three different storylines of future global development as outlined by the IPCC (A1B, B1, and B2). The corresponding changes in the climate system were derived from runs with three different sets of global circulation models and regional climate models, and climate change signals were calculated by standardizing the prediction period to a past baseline period. For each of these scenarios (with the exception of the B2 storyline) two alternative assumptions with regard to the fertilizing effect of CO₂ on tree growth were studied, ranging from full acclimation (no fertilizing effect) to a persistent growth enhancement through elevated atmospheric CO₂ levels ³⁰. For the ensemble analysis of future trajectories we focused on the nine unique (non-replicated) scenarios of future climate change and tree growth, and report the ensemble median except where stated otherwise. A scenario of stable climate (assuming the climate conditions of 1971-2001) was used to isolate the role of climate change in our projections.

For all climate scenarios, four different management strategies were simulated with EFISCEN ¹⁷. The reference strategy describes the future trajectory of Europe’s forests under a continuation of current forest policies (business as usual management). Both forest area and growing stock increase distinctly under this strategy, while the proportion of conifers on growing stock increases only slightly and the median forest age decreases. The three alternative management strategies represent a shift in forest policy towards carbon management, biodiversity conservation, and wood energy production, respectively ¹⁷. By the end of the study period in 2030 these alternative strategies considerably alter the forest extent, structure, and composition relative to reference management (Supplementary Table 7). The strategies with the most profound changes in forest structure, composition, and extent are the biodiversity and carbon strategies. The former assumes that priority is given to the protection of biological diversity, resulting in less wood removals, longer rotation periods and increasing growing stock. In addition, a sizeable percentage of the forest area is taken out of management. As the focus of this study is on forests available for wood supply ¹⁷, the forest area is decreasing under the biodiversity strategy in our analysis. In the carbon strategy rotation periods were increased and thinning regimes were optimized for increased C storage. The fourth strategy, promoting wood energy, showed comparatively little contrast to reference management with regard to indicators relevant in the context of disturbance damage, as many of its assumptions relate to increased removal of harvest residues as well as market changes and policy responses.