Seed sources for future oak forests under climate change (ACORN)

Climate change poses major challenges for forest trees. Drought events are increasingly causing forest damage and even large-scale forest dieback. One possibility to increase the fitness of forest trees under climate change is to afforest with reproductive material from stands that currently grow under drier environmental conditions. In the ACORN project, we are investigating options for such a seed transfer in pedunculate, sessile and downy oak. All three species have a large natural distribution area in Europe. Within this range, they grow on dry sites, as well as on sites with a good water supply, which can also be in close proximity to each other. Therefore, the oaks might have developed a resistance to drought even on a small scale. We hypothesise that natural selection contributed to the establishment of the most drought-tolerant genotypes in sites with pronounced water deficit. We examine oak populations in two study areas, (i) central Europe and (ii) the eastern Mediterranean. For each species, the investigation includes 10 stand pairs within each of these areas. Each pair consists of one dry and one moist site. To check whether small-scale adaptation has occurred, we take the genetic fingerprint of the oaks. The analysis aims to provide information on whether natural selection for drought-tolerant individuals left traces in the genetic make-up of the oak stands. In addition, we are investigating the phenotype of seedlings from these stands in the field. In this way, we are testing whether offspring of oaks from dry sites also show physiological and morphological differentiation that can be inherited. The seedlings will also be genetically analysed. Thereby, correlations between phenotype and genotype should point to genes involved in drought adaptation. These approaches will be used to answer the question whether there is a genetic adaptation of oak to drought. Furthermore, possible signatures of adaptation will be compared on two levels: between different sites within the regions and between the two study areas, which differ climatically (Central Europe vs. Eastern Mediterranean). The results will serve as a basis for developing concepts for seed transfer to increase the fitness of oaks under climate change. This should provide a basis for adapting seed recommendations to the requirements of climate change. The ACORN project is a cooperation project of partners from Austria, Germany, Greece, Switzerland and Turkey within the BiodivERsA network. As it is a practice-oriented project, we will engage with stakeholders including policy makers, private companies (e.g. tree nurseries) and non-governmental organisations, among others, from the very beginning of the project.

The pace of climate change may be too fast for forest trees to adapt locally. Drought poses a particular challenge in this context. The use of planting material from drought-adapted stands could enhance the fitness of future forests under climate change. With their large distribution ranges and diverse adaptations, European oaks are among the tree species with high potential under climate change. The collaborative project ACORN focused on the pedunculate, sessile and downy oak to identify stands, aiming to identify stands that can serve as seed sources for climate-resilient forests. Over 120 stands and more than 3,000 trees were genetically analysed. These were distributed across a large study area in temperate Central Europe and the drier, warmer eastern Mediterranean region. Furthermore, they largely consisted of stand pairs, each comprising one dry site and one site with sufficient moisture in close spatial proximity. Using molecular markers, a genetic fingerprint was generated for each sampled oak. Data analysis provided key information about species identity, genetic exchange between species through hybridization and the spatial distribution of genetic diversity. Subsequently, DNA from approximately 20 trees per oak species per site was pooled in equal proportions to enable stand-level decoding of the entire genome through whole-genome sequencing. Correlations between genetic constitution and local climate/soil conditions suggested drought adaptation. Genetic differences at specific genomic regions between adjacent dry and moist sites demonstrated that drought adaptation can also occur at small spatial scales. Additionally, seeds from selected stands were collected to establish field trials at three sites. The resulting progenies differed in drought-stress-related physiological and anatomical traits. These differences often correlated with the original site conditions of the mother stands and further supported adaptation occurring both regionally and locally. Associations between trait expression and genetic variation support the assumption of genetic control over these traits. Thus, the ACORN project not only identified potentially drought-adapted oak stands but also delivered - through an interdisciplinary approach - crucial insights into the genetic adaptation to drought across different geographical scales. These findings were actively shared with stakeholders at national and international levels.