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    Spotlight on genetic diversity: an unseen ally in adapting forests to drought

    Drought events are expected to be more frequent and more extreme in the forthcoming decades due to climate change. Already in the past decade we have seen in Europe some of the most severe droughts on record. Leading to dramatic forest tree dieback in Europe. Many forest tree species are affected, even rather drought-resistant sessile oaks, and especially when they grow under limiting environmental conditions.

    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 17th, 2024.