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‘Europe’s forests increasingly under pressure from climate-driven disturbances 

Every summer we see in the news flames burning down trees and houses, firefighters pouring water on mountain sides. In the winter we see massive windstorms blowing off entire forest landscapes. We read about very small insects that kill millions and millions of trees in few years. 

In parallel, we are also observing trees becoming political in Europe. Placed at the core of many policy documents and climatic pledges, forests and their climate mitigation potential are being increasingly recognised as key in the critical achievement of European climate and biodiversity targets, as well as for the many other services they provide to society.  

Media and policy attention underline that we urgently need more knowledge and sound research results on how disturbances develop, how they impact European forests and the so-called “ecosystem services” they provide, and how to respond to the seemingly increasing forest disturbance risks. A team of forest researchers from Wageningen University, the European Forest Institute and numerous research institutes across Europe investigated forest disturbances over the past 70 years and can now provide ground-breaking results in the paper “Significant increase in natural disturbance impacts on European forests since 1950” published in the journal “Global Change Biology”. 

Figure 1: Countries included in the study and their division in ecological zones (ecozones).

“The analysis of disturbance trends showed that damage by all forest disturbance causes clearly increased from 1950 to 2019”, says the leading author of the paper Marco Patacca (Wageningen Research). The researchers can state this based on a comprehensive update of the Database on Forest Disturbances in Europe (DFDE) they did in a joint efforts of European research projects called I-Maestro and RESONATE. This update resulted in a unique collection of ground-based, empirical observations of forest disturbances in Europe, counting more than 170,000 records in 34 European countries.  

Summary of Results 

  • The analysis of the updated DFDE data showed that the trends of all five investigated disturbance causes (wind, fire, bark beetle, other biotic and abiotic disturbances) increased between 1950 and 2019 at European scale. The timber volume damaged by disturbances in the last two decades accounted for 16% of timber harvest over Europe every year. This translates in ~80 million m3 of timber per year.

The reported increases differed between disturbance causes and regions: 

  • Wind (incl. large winter storms, thunderstorms and tornadoes) caused most of the total forest damage (46%), however, the increasing European trend is weaker than for other causes. Higher increases in the Pannonic and Northern/Baltic ecological zones document, however, that windstorms coming from the Atlantic are increasing their damages further East into Continental Europe. 
  • Fire is the second most damaging cause (24% of total damage). In the last two decades, we witnessed a shift of fire regimes. Whereas the overall trend of burned area was slightly decreasing due to improved fire detection and control efforts, we observed that most of the damages are caused by extreme, regional, climate-induced events known as mega-fires. Mega-fires can be far more destructive than normal wild-fires and once they pass a certain threshold there is nothing to do to control them, except waiting for a shift in climate conditions. Because their climate dependency, those events are expected to increase as climate changes unfolds (Gruning et al 2022). In 2022 (not included in the reported data) all previous records of fire damage in Europe were broken.  
  • Bark beetles accounted for 17% of the total damage, which is a share that doubled in the last 20 years. It’s important to state that bark beetle outbreaks were already reported throughout the 1970s, 1980s and 1990s. However, after 2000 the magnitude of this disturbance cause reached the epidemic phase, causing destructive, large-scale damages, especially in Central Europe. (It is noteworthy that the worst damages from bark beetles occurred in 2020 and 2021 and were thus not yet captured in the published records records). 
Bark beetle damage (photo: Rosa Castañeda)
Bark beetle damage (photo: Rosa Castañeda)
  • Other biotic disturbances (incl. fungi, other insects, pathogens, and animals damaging trees) reflect 8% of the total damage. However, this category shows the highest increasing trend of all disturbance causes. Because the agents grouped in this category (insects, pests and other living organisms) are very sensitive to the changing climate, they are expected to keep increasing at high rates, as climate change is unfolding.  
  • Other abiotic disturbances include a variety of drivers which account for a lower magnitude of damage at EU level (5%). However, this category also includes drought, one of the main climate-induced disturbances worldwide, and therefore expected to increase. Nevertheless, drought damage is difficult to identify as it often plays a key role as a predisposing factor to other disturbances (e.g., fire, pests), which are then ultimately responsible for tree mortality. 

For the past 2 decades, the internet provided a valuable resource for data gathering. However, further back in time disturbance reports are often more hidden in local archives and libraries. To overcome this difficulty a team of 20+ European and country experts in the field, gathered countries’ data in paper form and local languages, and combined expert’s knowledge with statistical models to reconstruct disturbance history. The results of our comprehensive disturbance data analysis stressed the importance of increasing efforts in disturbance reporting and monitoring, especially for: 

  • Small-scale disturbances (chronic damage) – only few countries gather and report such data;  
  • Biotic disturbance agents, like insects, diseases, and fungi many of these agents are poorly captured in current reporting;    
  • Drought impacts, which are difficult to disentangle from secondary disturbances.  
  • The increase of forest disturbances in Europe has also been observed using measurements from satellites or aircraft (called remote sensing data). Whereas the strength of remote sensing is the high geographic resolution of disturbance observations and the ability to frequently repeat observations, it remains challenging to correctly attribute remote sensing derived disturbance impacts to specific causes and to separate them from scheduled timber harvesting. This comprehensive new disturbance data set provides ground-based data with more reliable separation of disturbance causes at EU scale, thus corroborating and validating remote sensing data.  

Reflections and discussion 

Post windthrow damage in Slovenia (photo: Matteo Cerioni & Gal Fidej)
Post windthrow damage in Slovenia (photo: Matteo Cerioni & Gal Fidej)

From carbon sink to carbon source: The increasing mortality of forests due to disturbances is already causing the emission of a lot of carbon stored in the forest ecosystems in some European regions. This can easily shift the forest carbon sink (climate-change mitigation effect) into a carbon source, increasing CO2 emissions to the atmosphere. The subsequent emission’s increase is expected on the one hand to counter-balance the efforts that are being made to improve the sink of forests through climate-smart forest management; on the other hand, they might be hampering the achievement of the EU climate goals by 2050.

  • Severe impacts on local economies: Disturbances can have severe consequences at regional and local scale because special logging interventions to collect damaged wood disrupt the planned forest management cycles. This makes the achievement of long-term management objectives very challenging. Moreover, mid-term shortages in resource supply may weaken the local forest sector driven rural economies, threatening people well-being and economic security.  
     
  • Disruptions on international timber markets: Disturbance-induced, regional over-harvesting can cause instability, severe fluctuations or disruptions on the international timber markets, potentially causing negative effects on the implementation of the European bioeconomy.

Recommendations

  • We need to improve data collection on small disturbance impacts. Our analysis of country data revealed that more consistent data gathering on disturbance events is required, particularly regarding the reporting of smaller, dispersed damages.  
  • Moreover, to adapt to shifting disturbance regimes and to assess potential policy trade-offs, a harmonized, consistent and close-to-real-time European monitoring system of forest disturbances should be established. This would likely consist of a combination of remote sensed and ground-collected data.

The final complete reference for the publication is:  

Patacca, M., Lindner, M., Lucas-Borja, M. E., Cordonnier, T., Fidej, G., Gardiner, B., Hauf, Y., Jasinevičius, G., Labonne, S., Linkevičius, E., Mahnken, M., Milanovic, S., Nabuurs, G.-J., Nagel, T. A., Nikinmaa, L., Panyatov, M., Bercak, R., Seidl, R., Ostrogović Sever, M. Z. … Schelhaas, M.-J. (2022). Significant increase in natural disturbance impacts on European forests since 1950.  Global Change Biology, 00, 1–18.

DOI:  https://onlinelibrary.wiley.com/doi/10.1111/gcb.16531

Acknowledgements

The study has been founded by the I-Maestro project, grant no. 773324 for the Forest Value EU funding and national funding through Fachagentur Nachwachsende Rohstoffe (FNR) grant 2219NR189 and ADEME grant no. 1903C0009; and the EU H2020-RUR-2020-2 project RESONATE (grant no. 101000574). The lead author M. Patacca acknowledges financial support from Lister Buildings, CoC n. 68345836.


Featured image: Bark beetle damage in German Sauerland (photo: Gesche Schifferdecker)

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