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Rapid human-induced environmental changes like climate warming represent a challenge for forest ecosystems. Due to their biological complexity and the long generation time of their keystone tree species, genetic adaptation in these ecosystems might not be fast enough to keep track with conditions changing at such a fast pace. The study of adaptation to environmental change and its genetic mechanisms is therefore key for ensuring a sustainable support and management of forests. The 4-day conference of the European Research Group EvolTree (https://www.evoltree.eu) on the topic of “Genomics and Adaptation in Forest Ecosystems” brought together over 130 scientists to present and discuss the latest developments and findings in forest evolutionary research. Genomic studies in forest trees have long been hampered by the lack of high-quality genomics resources and affordable genotyping methods. This has dramatically changed in the last few years; the conference impressively showed how such tools are now being applied to study past demography, adaptation and interactions with associated organisms. Moreover, genomic studies are now finally also entering the world of conservation and forest management, for example by measuring the value or cost of interspecific hybridization and introgression, assessing the vulnerability of species and populations to future change, or accurately delineating evolutionary significant units. The newly launched conference series of EvolTree will hopefully play a key role in the exchange and synthesis of such important investigations.

Douglas-fir (Pseudotsuga menziesii) is a conifer tree native to western North America. In central Europe, it shows superior growth performance and is considered a suitable substitute for tree species impaired in vitality due to climate change. Maintenance and improvement of growth performance in a changing environment is a main challenge for forest tree breeders. In this context, genetic variation as a factor underlying phenotypic variation, but also as the basis for future adaptation, is of particular interest. The aims of this study were to analyse (1) genetic diversity of selected Douglas-fir provenances, (2) variation in height growth among provenances, and (3) to assess the link between genetic and phenotypic variation in height growth. Genotyping was done on microsatellite loci. Effects of ‘provenance’, ‘genotype’, and ‘site’ on height growth were assessed by fitting mixed linear models. The most significant genetic differentiation was observed between provenances of the coastal variety, versus a provenance of the interior variety originating from British Columbia. Although genetic differentiation among provenances of the coastal variety was lower, genetic structures within this variety were identified. Moreover, genetic diversity showed a latitudinal gradient with the southernmost provenances being more diverse, probably reflecting the species’ evolutionary history. The modelling approach revealed that height growth differed significantly by ‘provenance’, ‘site’, and the interaction between ‘site’ and ‘provenance’. Additionally, this analysis showed that genetic variation captured by the genotyped microsatellite loci was significantly related to variation in height growth, providing statistical evidence for a genetic component in the observed phenotypic variation.

Douglas-fir (Pseudotsuga menziesii) is a conifer tree native to western North America. In central Europe, it shows superior growth performance and is considered a suitable substitute for tree species impaired in vitality due to climate change. Maintenance and improvement of growth performance in a changing environment is a main challenge for forest tree breeders. In this context, genetic variation as a factor underlying phenotypic variation, but also as the basis for future adaptation, is of particular interest. The aims of this study were to analyse (i) genetic diversity of selected Douglas-fir provenances, (ii) variation in height growth among provenances, and (iii) to assess the link between genetic and phenotypic variation height growth. Genotyping was done on microsatellite loci. Effects of provenance, genotype, and site on height growth were assessed by fitting mixed linear models. The most significant genetic differentiation was observed between provenances of the coastal variety, versus a provenance of the interior variety originating from British Columbia. Although genetic differentiation among provenances of the coastal variety was lower, genetic structures within this variety were identified. Moreover, genetic diversity showed a latitudinal gradient with the southernmost provenances being more diverse, probably reflecting the species' evolutionary history. The modelling approach revealed that height growth differed significantly by provenance, site, and the interaction between site and provenance, demonstrating that height growth is under strong genetic control. Additionally, this analysis showed that genetic variation captured by the genotyped microsatellite loci was significantly related to variation in height growth, providing statistical evidence for a genetic component in the observed phenotypic variation.

Oak species (Quercus spp.) in Central Europe grow on a relatively wide range of sites. Due to the economic importance of oak for its wood and other products, oak forests have long been managed by humans. This raises the question whether adaptation and/or human activities—especially the moving of propagules—have left their footprints on the genetic variation of oak populations. To address this question, we focused on the Upper Rhine Valley, a densely populated area today that was settled by humans early on. Here, the three most common native Central European oak species can be found. We studied their genetic variation across a large number of oak stands, growing on different sites and having different silvicultural histories, using neutral and EST-derived microsatellite markers. At the interspecific level, we showed that Quercus robur is relatively well delimited, while Quercus petraea and Quercus pubescens are more closely related. Natural hybridization might explain the increased genetic introgression between these two species. Within species, we found a low differentiation among populations of Q. robur and Q. petraea. In spite of forest fragmentation, we detected no spatial genetic barriers. However, we found that populations of Q. pubescens, a species with a marginal distribution in the study area were spatially structured. Genetic drift but also unidirectional introgressive hybridization with Q. petraea may account for this. Regarding the question of adaptation, we considered soil flooding, texture, drainage, and calcium carbonate in the upper horizons as physiologically important site condition variables. But with multivariate statistics, we could not find any significant effects of these parameters on genetic differentiation. Although there was no evidence for natural selection due to adaptation in stands of Q. robur, we demonstrated that age had a significant effect on their genetic variation and that stands established after the end of the Second World War had higher genetic diversity. We interpret these findings as being the result of an increase in large-scale transfers of reproductive materials during this time period and discuss arguments supporting this hypothesis. Finally, we consider the implications of these results for forest management.

Hybridization and introgression between Quercus alnifolia Poech and Q. coccifera L. is studied by analyzing morphological traits, nuclear and chloroplast DNA markers. The study site is a mixed stand on Troodos Mountains (Cyprus) and the analyzed material includes both adult trees and progenies of specific mother trees. Multivariate analysis of morphological traits shows that the two species can be well distinguished using simple leaf morphometric parameters. A lower genetic diversity in Q. alnifolia than in Q. coccifera and a high interspecific differentiation between the two species are supported by an analysis of nuclear and chloroplast microsatellites. The intermediacy of the four designated hybrids is verified by both leaf morphometric and genetic data. Analysis of progeny arrays provides evidence that interspecific crossings are rare. This finding is further supported by limited introgression of chloroplast genomes. Reproductive barriers (e.g. asynchronous phenology, post-zygotic incompatibilities) might account for this result. A directionality of interspecific gene flow is indicated by a genetic assignment analysis of effective pollen clouds with Q. alnifolia acting as pollen donor. Differences in flowering phenology and species distribution in the stand may have influenced the direction of gene flow and the genetic differentiation among effective pollen clouds of different mother trees within species., Comment: 30 pages, 7 figures, author's accepted manuscript