Drought events are becoming longer and more severe in Europe. Alerts over low soil moisture are now being issued also in winter and drought has overtaken wind as the most important disturbance agent on the continent. As a result, the structure and resilience of Europe’s forests are starting to face long-term impacts. In countries like Germany, popular terms such as “Waldsterben 2.0” (forest dieback 2.0) are becoming frequent in the media to describe catastrophic tree mortality levels and secondary results from drought, such as bark beetle outbreaks.

But while pest outbreaks and more frequent forest fires are hotly debated in society, their connections to drought are often under the radar. Even less discussed are the consequences of drought on forest biodiversity, which are sometimes hard to understand, describe and quantify.

On the one hand, canopy defoliation and discolouration are well monitored in inventories across Europe. On the other, the causal relationship between soil moisture measurements, defoliation and discolouration is not well understood, and much less so the link with forest biodiversity, which in itself is very difficult to measure. With current data and understanding, clear links between drought, defoliation and the impact on canopy-dwelling animals cannot be straightforwardly concluded.

To further complicate things, different animal taxa and species show opposing reactions to drought, with some benefitting and others suffering. Still, despite these challenges, recent research has provided valuable conclusions on how to steer forest management to foster forest biodiversity.

How does drought affect forest ecosystems?

Drought has a strong positive correlation with larger and more severe forest fires and has been statistically linked to excess forest mortality across Europe, thus leading to decreased ecosystem productivity. Dry conditions, especially when combined with high temperatures, cause a more negative water balance in the soil, forcing plants to regulate evapotranspiration by closing the stoma on their leaves (a type of pore). This reaction reduces photosynthesis, decreasing the absorption of carbon, and over time may cause tissue dehydration and hydraulic failure. Plant tissues then start to die from starvation, which leads to leaf shedding and defoliation. As trees suffer from lower vitality, their defence mechanisms begin to wane – such as the production of chemicals or resin that would normally help trees resist bark beetle attacks in the case of spruce trees or fungus in the case of beech.

Several hotspots of canopy mortality have been identified through remote sensing data across Europe, from Spain to central-eastern Europe and Finland. In France, parts of the Iberian Peninsula and eastern Europe, drought was responsible for 30% or more of the total mortality of forest canopy in the past three decades. Between 1987-2016, approximately 50,000 ha of excess canopy mortality was caused by drought, with four out of five of the largest drought pulses occurring in the 21st century. Consequences might extend over multiple seasons, years and even decades.

What can we expect as long-term outcomes of this process? Not the generalised death of entire forests as suggested by the German expression “Waldsterben” - but tremendous changes in forest composition are likely. The extreme 2018 summer drought offers a glimpse of the tree species most likely to be affected in Central Europe: in the aftermath of the drought, Norway spruce and European beech faced the highest mortality, while Scots pine, silver fir and sessile and pedunculate oak also suffered. Over the course of time, more vulnerable tree species may face local extinction and/or be replaced by other more climate-adapted ones. The process of adapting to a new climate, which might take decades or centuries to occur naturally, can be accelerated by climate-smart forestry that assists tree “migration” to new areas.

The Norway spruce is one of the tree species facing the highest mortality in Central Europe in the aftermath of drought events.

Can research help us understand how biodiversity is affected by drought?

As forest ecosystems undergo a long-term transition, identifying general trends of how biodiversity levels will respond to future disturbance scenarios is tricky, both in terms of species composition, community structure and functions. But recent studies provide anecdotal evidence on specific responses to past drought events.

Insects in general are a potential indicator of how drought might impact biodiversity and higher levels of the food chain, as they serve as prey for other animals and contribute to plant pollination. More specifically, canopy-dwelling insects are a directly affected group whose habitat is being dramatically altered by changes in tree canopy structure.

In 2020, a study looking at beetles inhabiting French forests found that wood-boring and saproxylic species (which depend on dead or dying wood), as well as generalist leaf-eating species, tended to benefit from climate-induced oak decline. However, specialised leaf-eating species suffered a negative effect.

Other research focused on flying insects in Pyrenean forests found that different tree dieback levels can have different effects on insect communities, with winning or losing species depending on dieback intensity. For example, this is explained by complex dynamics such as the one between parasitoid wasps and their hosts. Parasitoid wasps may benefit from tree diebacks that create more complexity in a forest, but those who feed on tree sap might be affected by the poorer quality of their host trees.

When it comes to plant species, tree canopy opening after drought may cause several shifts in community composition, creating effects comparable to those of thinning treatments such as an increase in number, abundance and flowering rate of understory plants. Drought might also reduce the cover of perennial plants and allow colonisation by annual or biannual plants. Effects might be felt throughout several years if dominant plant species are not able to recover and are overtaken by species with higher regenerative capacity.

In a nutshell, while certain insect and plant species benefit from natural disturbances, which might help create dynamic habitats by changing forest structure, others suffer from it, in particular those dependent on later forest development stages and factors such as the density of large trees. But although these findings corroborate the belief that natural disturbances create “winners” and in general lead to more heterogeneous habitats, especially in managed forests, they must be taken with a grain of salt - accompanied by a closer look at who the “losers” are.

Forests with varied dieback levels can promote different animal and plant communities and initiate a process of post-disturbance natural succession, with a balanced response of winner or loser species throughout the dieback intensity gradient. However, widespread diebacks without a diversity of patterns hold the risk of homogenising environmental conditions, thus favouring generalist species and threatening specialised, rare species.

The generalist leaf-eating beetle Phyllobius pyri tends to benefit from oak decline, while species feeding on sap might suffer from it.

How can forest management be adapted to safeguard biodiversity?

Much more research is necessary for a deeper understanding of how biodiversity responds to drought and climate change. The most pressing research needs in the field have been outlined in a recent review, calling for studies adopting whole-tree approaches, from canopy to the forest floor, and looking at how tree dieback plays out at various spatial scales, from microhabitats to landscapes. Additional studies on different species and taxonomic groups are also needed, as well as assessments of how biodiversity is affected by forest management.

Despite that, a few recommendations can already be put into practice: While non-intervention strategies may appear advantageous for preserving biodiversity, they may result in significant forest dieback and put rare and specialized species at risk. Moreover, non-intervention may enable invasive alien species to dominate native and rare species. Therefore, it is crucial to adopt management strategies that strike a balance between conserving biodiversity and mitigating harmful effects on rare species.

Adopting mixtures of tree species, in particular by incorporating species and genotypes tolerant to drought, as well as considering their relationships with water-related climatic features of forest sites, is also necessary to prevent increased tree mortality. There are also opportunities for enhancing biodiversity through closer-to-nature forest management, with approaches that include elements of nature conservation management, such as maintaining tree species and structurally diverse forest stands, using long production cycles and controlling ungulate populations. Other relevant recommendations on drought-tolerant and biodiversity-oriented forest management are being developed by several EU-funded projects, such as OptFORESTS, ForestPaths and initiatives such as the Integrate Network, INFORMA, OptFORESTS, ForestPaths and initiatives such as the Integrate Network.

The importance of such recommendations is not to be underestimated, both for biodiversity and forest management in general. While biodiversity may suffer from climate change, biodiverse forests are becoming a “must” and not just a “nice to have”, as they support high levels of forest functioning, especially when water becomes more limiting.