With this second FoRISK webinar on the topic of drought, “Drought Adaptation of Forests in Europe – Practical Strategies,” FOREST EUROPE, in collaboration with the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), continues the journey to delve into measures we might take at regional, national or international level to support Europe’s forests to adapt to drought, to unravel the science behind these recommendations and provide practical guidance on the measures.

Extreme drought event in Central Europe in 2018: Its impact on beech forests and their recovery

The first presentation by Ansgar Kahmen, Professor of Physiological Plant Ecology and Head of the Department of Environmental Sciences at the University of Basel, Switzerland, focused on the results of his research on the extreme drought in Central Europe in 2018 and its devastating impact on forest trees. In particular, the vulnerability of European beech was discussed, but also whether recovery is possible. First, he explained that 2018 cannot be considered an isolated event, but is part of a trajectory towards a hotter and dryer climate. Six of the nine hottest summers of the past 120 years occurred in the last decade.

Since the 1980s, an increasing evaporative demand in the atmosphere has been observed, accompanied by a progressive decline in tree health. In beech trees, increasing crown damage became visible, which became even more severe after the extreme years of 2003 and 2018. Measurements of hydraulic conductivity in 2018 showed that vascular transport in beech trees was reduced or interrupted by up to 80%, and had been causing canopy decline. The higher susceptibility of beech compared to oak or maple can be explained by the differences in the rooting depth of the tree species: beech has only a shallow rooting system and is the first to run out of water.

But what about recovery? Severe symptoms of canopy defoliation were also present in the following year, in spring 2019, and thereafter. It was found that hydraulic conductivity had not been restored in symptomatic branches and that the amount of intact conducting tissue (or xylem area), directly relates with the foliage area of a branch. Drought is causing partial and irreversible loss of hydraulic conductivity, and without a fully developed canopy, the risk for tree mortality is increasing. He concluded his talk with an appeal to reason that, given the unceasing trend towards a new, warmer climate, we better study the acclimation and adaptation potential to future climate, rather than trying to understand a tree’s recovery potential to a past climate.

How to choose climate-adapted tree species and compositions for future mixed forest stands

The webinar continued with a presentation by Heidi Döbbeler, forest scientist at the Northwest German Forest Research Station (NW-FVA), working in the department of Forest Growth on the development of a decision support tool for hessian forest owners. The project “Maps for climate risk and forest development types as an improved consulting foundation for new challenges for Hessian forest owners” is part of the integrated climate protection plan and funded in collaboration with the state-owned forest and the German Forest Owners Association by the ministry of Hesse in Germany.

In Hesse, a temperature rise from 15.4 to 17.3 °C is expected, which will increase evapotranspiration but also extend the growing season. Accompanied by a decrease in precipitation during summer, this will have a negative impact on the climatic water balance and, thus, on the water available for plants during the growing season. In addition to these regional climatic data, information on soil property is a prerequisite for calculating the site-specific water balance, which is an important indicator of drought stress for trees under future climatic conditions at a given forest site in Hesse.

In a next step, tree species and their potential role in the future were categorized according to their tolerance to drought stress depending on the site water balance. The categories were defined as leading, mixed, accompanying, or excluded tree species. As an example, even if European beech will have problems in Hesse in the future and its distribution will be limited as a result, it will still remain part of the forest. Recommendations for forest development types (FDT) for mixed forests were compiled by intersecting ecograms of leading and mixed tree species. As an example, the oak-beech/hornbeam FDT consists of 80-60 % oak, 20-10 % beech, and 10% accompanying other tree species. 32 FDTs have been defined and include information on management practices, their socio-economic potential and closeness to nature. Every forest owner in Hesse is provided with site-specific recommendations and can freely access the information via a web portal or mobile application. Under the assumption that disturbances have a species or structure-specific effect, facing them by mixed, site-adapted, and structurally rich forests will reduce the susceptibility of forests and instead increase stability and resilience.

Potential of thinning to increase drought tolerance

Following was a presentation by Julia Schwarz, working at the Swiss Federal Research Institute as a researcher for silviculture and climate change and expert in the field of ecology and management of forest ecosystems. She provided insights into whether thinning can offer an adaptation strategy to drought. While long-term adaptation approaches aim at creating structurally and species-rich mixed forests in the future, short- to medium-term adaptation approaches must work with the existing forest stand and thinning might be a way of adaptation to drought stress.

Thinning reduces the amount of trees per area and thereby promotes the growth and vitality of the remaining trees by reducing the competition for natural resources, such as light, water, and nutrients. Thinning regimes may vary in intensity, frequency, by the age of the trees and the type applied (e.g. crown thinning). The short-term impacts at the stand level have been found to range from positive effects on the water balance due to fewer trees and less leaf area, lower evapotranspiration and higher soil moisture to negative effects due to an increased understory growth, increasing solar radiation and higher soil evaporation. At tree level, trees have more space, increased fine root growth with no change in transpiration or increasing transpiration. In medium terms, trees have been found to increase their crowns, their overall root system, the water extraction capacity and their growth rates. At stand level this means that evapotranspiration is reduced after thinning only for a short period of time before it returns to pre-thinning levels.

But can the advantages of thinned stands outweigh the disadvantages of un-thinned stands during drought? What are the effects on tree growth during and after drought? Clear answer: thinning can mitigate drought-induced growth decline and mortality due to higher soil water availability and water extraction capacity during moderate droughts, while trees in denser stands experience drought earlier and more extreme. But during an extreme event where soil water is depleted thinning will have no effect. Thinning effects are highly dependent on drought severity. She concluded her presentation with the recommendation for regular, moderate thinnings that will contribute to mitigating growth decline and mortality during moderate droughts.

Pine and mistletoe– insights from Poland and Switzerland

Concluding the presentations were Andreas Rigling, Professor of Forest Growth at ETH Zurich and the Swiss Federal Research Institute, with his presentation “The impact of mistletoe on Scots pine performance in a dryer future” together with Wojciech Kędziora, assistant professor at the Institute of Forest Sciences at Warsaw University of Life Sciences and his presentation “Problem with mistletoe in Poland.”

Mistletoes are hemiparasitic plants, well known for using the host’s water supply for their own photosynthetic capacity. When growing on a thriving host plant, they can get as old as 30 years. In central Europe Scots pine is a known host tree. In the host tree, the infection causes a reduced growth rate of branches, and with increasing infection numbers, branch diameter, and length, but also needle length and age, have been found to be negatively affected until even crown architecture and assimilation capacity are affected.

But what if a tree’s water balance is put under additional pressure by drought stress? While trees are able to reduce their water loss by closure of their stomata during a drought event, the mistletoes continue to transpire, thereby increasing the tree’s drought stress. With increasing infection numbers, differences in growth are exacerbated, causing defoliation, growth decline and a reduction in carbon assimilation up to 80%, and finally, increasing significantly the risk of tree mortality in combination with drought stress.

On the positive side, mistletoes not only indirectly contribute to reshaping forests in structure and tree species composition in the long term but also directly improve soil nutrient availability due to their nutrient-rich litter.

In Poland, mistletoe has only recently gained higher attention, and only after 2015-18 has it been perceived as a major problem. To assess the extent of the forests, ground-based monitoring programs were started, but were soon found to be greatly underestimating the actual infection numbers on trees. Most of the mistletoes are growing in the middle of the tree, which is not visible from the ground. The observation has been that mistletoes prefer older stands, stands with lower soil fertility, taller trees with sparse crowns, and more likely to appear at forest edges. There is great hope in the use of drones for inventory programs, but really, only a few management options are available. While pruning, thinning, or clear-cutting promise only limited success, long-term strategies will rely on rebuilding the forests with more diverse tree species compositions. Wider discussions between forest owners, forest industry, management, and public society, as well as cross-border communication, are needed.

Cross-border communication and sharing of experiences is going to be the core business of a future FoRISK. This webinar gave an outlook on a future FoRISK in action that promotes cross-border exchange and cooperation. Here, you can find all the presentations shared during the webinar.

The recording of the event can be found here:

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Understanding of the genetics underlying drought adaptation and wise use of forest genetic diversity has unseen power to improve forest stand vitality and guide management decisions to adapt Europe´s forests to climate change. With our new webinar “SPOTLIGHT ON GENETIC DIVERSITY: An unseen ally in adapting forests to drought” FOREST EUROPE together with the European Forest Resources Genetic Resources Programme (EUFORGEN), the EU-funded H2020 project FORGENIUS (FORest GENetic Resources Information and Services for End-USers) and EFI did focus on both the potential and limits of genetic adaptation of forest tree stands to drought, presenting on the latest science and how it can support management strategies and policies at regional, national and international level to foster adaptation to drought in European forests.

The first presentation by Maurizio Mencuccini, ICREA Research Professor at the Ecological and Forestry Applications Research Centre – CREAF (Barcelona, Spain) and Honorary Professor of Forest Science at the University of Edinburgh (UK) provided insights into the impacts of climate change on the physiology of trees and forests and illustrated “What happens to trees during and after drought?”. He started by explaining the impact of climate change on European forests: the long-term trend of hotter, more severe droughts and the probability of very long mega-droughts is increasing. While this is the general expectation for European forests, he reminded that no two droughts are the same; they differ in intensity and duration and vary for each location and so will the responses of the trees and the reasons by which trees can die. Severe droughts, especially if they occur repeatedly may lead to permanent damage causing tree mortality. The traits and their variation in tree species that decide on a tree’s vulnerability to mortality are under investigation; hydraulic systems in trees allow also to transport water from the roots to the shoots during drought; however, hydraulic safety defines the level of resistance, how long a tree continues to do so, and varies greatly in trees. This is why some species die of drought because of either thirst (desiccation) or hunger (running out of sugars). The efficiency and the safety of the tree hydraulic system are among the most important traits controlling how fast a tree can die of drought.

The webinar continued with a joint presentation by Delphine Grivet and Aida Solé-Medina, both researchers at the Institute of Forest Sciences (ICIFOR), INIA-CSIC, Madrid, Spain, taking us on a journey investigating the genetic diversity among and within tree species and unravelling the genetic basis of such variation (presentation: Genetic bases of drought response in Mediterranean pines: why variation among populations and individuals matter?. Since trees are long-lived and sessile organisms, tree species need to ensure their survival and adapt to environmental changes either via changes in the phenotype (phenotypic plasticity) or through intra-specific genetic variability. Looking at stone pine (Pinus pinea) and Mediterranean pine (Pinus pinaster) differences in trees adaptation strategies can be observed, the former showing greater phenotypic variability, the latter higher intra-specific variability when adapting to the local climate. In a next step it is important to find a way to identify the basis of the genetic variation enabling climate change adaptation. Here, the identification of molecular markers, genes that are e.g. involved in heat stress tolerance, and associating the genotype with a specific phenotype (e.g. higher survival) will not only help disentangle the phenotypic and genetic responses of forest populations, but could also lead to predictions of future species distributions or guide restoration programmes. Another way to understand the response of forest tree populations is by use of common garden experiments, e.g. planting trees from different tree populations in the same environment and observe the variations occurring. Populations from drier climates tend to grow slower and invest more in the belowground biomass for drought survival. Conserving and supporting intraspecific genetic diversity is crucial for forest management as it enhances the potential of the species to adapt to new environmental conditions imposed by climate change, there may very well be pre-adapted specimen to future or yet unknown conditions.

Following was a presentation by Stéphane Maury, Professor of Plant Physiology and Epigenetics at the University of Orléans in France and a driving force in understanding the evolutionary and functional impact of epigenetic variation in forest trees. He focused on the topic Epigenetics: what’s in it for tree adaptation?.

Epigenetic mechanisms facilitate rapid phenotypic modifications in response to environmental changes without altering the DNA sequence and poses a way for forest trees to adapt quickly to local environmental changes even during their long life-cycles. Surprisingly, epigenetic variability that allows acclimation or tolerance of stress has remained largely unstudied in forest trees. Stéphane Maury developed a new epigenetic axis on poplar, the genetic model tree, deciphering phenotypic plasticity and adaptation in response to water deficit in forest trees. He explained that when studying clonal propagation material of trees generated either under cold or warm growing conditions, the saplings will memorize the treatment even after plantation and this effect, called epigenetic memory, will continue to control their phenology and planting material could be planted according to this imprinting.

Concluding the presentations, Iskender Demirtas from the General Directorate of Combating Desertification and Erosion in Türkiye offered a vision of the ongoing genetic research carried out in Türkiye Increasing climate change emphasizes the importance of species adapted especially to arid conditions. Research studies focused on species with the ability to adapt to the arid and semi-arid conditions in forest areas in Türkiye. Research was carried out on pear and hawthorn tree species between 2018 and 2022. Throughout these two studies, wild pear genotype samples were collected from 96 different points of Turkey and hawthorn genotype samples were collected from 94 different points of Türkiye, depending on certain criteria, and were examined in the laboratory and nursery conditions. As a result of the two research studies in question, the possibilities of using tree species genotypes in afforestation and rehabilitation studies in arid-semi-arid areas were tried to be determined. Similar studies are ongoing with poplar.

The webinar was closed with a panel discussion and questions by the audience. You can see the recording of this webinar here:

We will continue with the important work providing insights into drought stress and offering practical strategies for adapting forests in Europe to droughts. Join our second webinar on May 17^th, 2024.

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Relevant topics we will cover in this second webinar on the topic of drought stress are:

• Look at the extreme drought event in 2018 in Central-Europe and its devastating impact on beech forests and discuss about recovery from drought stress.
• Secondly, introduce a decision support tool used in Germany for providing forest owners with science-based choices for future mixed forest stands.
• Look at the positive effects thinnings have on drought tolerance of forests, minimizing vulnerability to drought of existing forests in the short term.
• Talk about pine and mistletoe, the latest scientific insights on the parasitic relationship, and measures taken by forest management in Switzerland and Poland.

This webinar is open to everyone interested and is part of the FOREST EUROPE’s “From the roots to the canopy” series, this time organized in collaboration with the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL).

See the agenda here.

Register here and participate!

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Therefore, FOREST EUROPE recently organized a three-day workshop on “Managing biotic threats in forests – lessons learned from bark beetle calamities” (see programme here) in Breznice, Czech Republic, jointly with the Ministry of Agriculture of the Czech Republic, the Forest Management Institute (UHUL), and the Forestry and Game Management Research Institute of the Czech Republic (FGMRI). This event is part of FOREST EUROPE’S work toward the establishment of a pan-European Forest Risk Facility (FoRISK).

During the workshop, experts from 16 countries gathered to exchange good and bad experiences of bark beetle management in Europe, aiming to prevent and better prepare for future outbreaks of bark beetles and other biotic threats.

Different countries are facing different challenges:

In the first session, insights from the UK (Max Blake, Forest Research – Alice Holt Lodge UK) gave an idea on the ability of spruce bark beetles to spread over distances of 300-400 km, being blown over from outbreak areas in Belgium and France to the south-east of England. Spruce bark beetles are a non-native forest pest in the UK, and strict eradication protocols apply if an outbreak of this invasive species is detected. Gernot Hoch (BfW Austrian Research Centre for Forests) presented on the new and unexpected developments of spruce bark beetle outbreaks in natural spruce forests at higher elevations in Austria. Recently, higher temperatures have allowed for two beetle generation and population dynamics of unusual intensity also at higher elevations. The cooler climate in Finland and Norway has yet prevented the establishment of large-scale breeding populations of the spruce bark beetle, as presented by Tiina Ylioja (Luke – Natural Resources Institute Finland) and Paal Krokene (NiBio – Norwegian Institute of Bioeconomy Research). But also, these northern countries are preparing for more intense outbreak seasons due to increasing temperatures and are running national monitoring programs and setting-up preparedness plans. For some countries, the worst is likely still ahead, and we need more workshops connecting countries with different levels of experience and fostering cooperation.

Participants from Bulgaria (Petya Dimitrova-Mateva, Executive Forest Agency Bulgaria), Ukraine (Kateryna Davydenko, Ukrainian Research Institute of Forestry & Forest Melioration), and from the Czech Republic (Jan Lubojacký, Forestry, and Game Management Research Institute Czech Republic) also presented on bark beetle damages in pine forests. It is important for any management guideline to consider that different biotic and abiotic disturbance agents can also interact and result in further weakened tree stands. Bark beetle gaps in forest stands could create vulnerable edges that could also have cascading impacts, i.e., further facilitate the risk of increased wind disturbance.

Disturbances are interrelated and disturbance management has to consider the whole risk cycle:

The next session continued in this direction but also provided guidance for forest management in an uncertain future. Interrelations with other forest disturbances are possible at any time during a bark beetle outbreak – before (predisposition because of recent storm or drought events), during, or after (potentially flammable material of dead wood increasing the risk for fire). Eventually, one has to remember that after the disturbance is before the next disturbance, as Marcus Lindner (EFI Bonn) also reminded the participants during his presentation. He also highlighted that disturbance risk management should consider the full disaster risk management cycle with proactive and reactive management measures for prevention, preparedness, response, and recovery.

Marcus Lindner from the European Forest Institute.

The same rules apply for disturbances due to extreme climatic events, as participants learned in the presentation of Nenad Petrovic (University of Belgrade) on the severe damages of the Ice Break event in Serbia in 2014 and the regeneration process after almost ten years. Forest management strategies that ignore the uncertainties associated with climate change are unlikely to meet future expectations. Restoration of post-calamity areas was also the topic of Jan Leugner’s (Forestry and Game Management Research Institute Czech Republic) presentation. Large areas of bark beetle calamities often result in large areas of thickets (young forest stands) in the next decade from planting or natural regeneration of trees. Dissemination of new knowledge and experiences is of the utmost importance to avoid loss of species admixtures, loss of stability, and overall functionality of the ecosystems of the next forest generation.

Subsequently, the participants were introduced to the policy brief “Managing bark beetle outbreaks in the 21st Century” by the author Tomas Hlasny (Czech University of Life Sciences in Prague). This short and concise brief with recommendations for policymakers emphasizes that resilience thinking when adopted in policy and management frameworks, will help deal with increasing risks and enable us to see natural disturbances also as opportunities for creating new, resilient forests to adapt and recover from future shocks more efficiently. The policy brief also comes with a toolbox addressing the full disaster risk management cycle that can be directly implemented in national crisis plans. (Policy brief and a short summary video on the policy brief will be available soon).

In the afternoon of the second day, a field trip was organized for the participants in the southwestern Brdy Mountains with demonstrations of forest sites in the state and privately owned forests affected by spruce bark beetles at large-scale and measures taken to deal with the damage and for restoration explained. It was organized in cooperation with the Military Forests and Estates of the Czech Republic and the Forest Administration of the Archbishopric of Prague.

Risk management for forest disturbances: what is needed – towards FoRISK and international cooperation

During the last day, participants had a chance to learn about the activities of FAO on the management of bark beetle outbreaks from a global perspective by Shiroma Sathyapala (FAO), the initiative to establish a nationwide forest damage database in cooperation with the federal states in Germany at the new Institute for Forest Protection and the vision to extend it to pan-Europe by Henrik Hartmann ( Julius Kuehn-Institute for Cultivated Plants). Strengthening international cooperation for risk management is the idea behind the pan-European forest risk facility (FoRISK). FoRISK’s role will be to provide information about transboundary problems, get the leading countries involved, and enable local exchange. It should further be a platform for communication between science, policy, and practitioners. These were some of the results of a group work activity wherein the present experts were asked to provide feedback on the achievements of the FoRISK pilot so far and give guidance on further work before the Ministerial conference in Bonn 2024 makes a decision on the continuation of the FoRISK.

From FOREST EUROPE, we would like to thank all participants, co-organizers, and all involved speakers for their support and valuable insights provided, including also presentations on the experiences with spruce bark beetle from Germany (Ralf Petercord, Ministry of Agriculture and Consumer Protection, North Rhine-Westphalia), Czech Republic (Vit Sramek and Milos Knizek, Forestry and Game Management Research Institute), Poland (Kamil Szpakowski, General Directorate of State Forests Poland) and Lithuania (Glazko Zbignev, Ministry of Environment Lithuania). We hope to continue with this important work to safeguard the future of our forests.

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