Northern red oak (Quercus rubra) was introduced to Europe in the late 17th century and is today the most important deciduous foreign tree species in Germany. Despite its importance little is known about the origin of local red oak stands and patterns of genetic variation in German red oak stands. To be able to make recommendations regarding the adaptive potential of red oak and the selection of provenances with respect to climate change, a better understanding of the genetic diversity and structure of German red oak stands is needed. Therefore, individuals from 62 stands in Germany and North America were genotyped at five chloroplast microsatellite loci to characterize chloroplast haplotype diversity and geographical structure. Compared to reference stands from the natural distribution range, German stands demonstrated a relatively low genetic differentiation among populations and represented only a fraction of the haplotype diversity found in North America. For several stands located in the south of Germany a considerably higher haplotype diversity was found. Therefore, we conclude that German stands originated from a limited geographic range within the natural distribution, which was possibly located in the northern part of the native distribution range. While most German stands showed signatures of founder effects due to originating from a small number of individuals within a limited range, particularly stands in the south of Germany could have benefited from admixture and multiple introductions of different North American provenances.

No abstract

European beech (Fagus sylvatica L.) is one of the most important forest tree species in Europe, and its genetic adaptation potential to climate change is of great interest. Saplings and adults from 12 European beech populations were sampled along two steep precipitation gradients in Switzerland. All individuals were genotyped at 13 microsatellite markers and 70 SNPs in 24 stress response and phenology related candidate genes. Both SSR and SNP markers had high genetic diversity in the studied populations and low but statistically significant population differentiation. Two approaches were used to discover SNPs with signatures of selection: search for FST SNP outliers and analyses of SNP associations with environmental variables such as temperatures, precipitation and humidity. Three (4.3%) SNPs were consistently identified as outliers in the adults by more than one method, and they were very likely under positive selection. Twenty (28.6%) SNPs in the saplings and 10 (14.3%) SNPs in the adults were associated with environmental variables found by more than one method. In general, there were 22 (31.4%) SNPs in 17 (70.8%) candidate genes in the saplings, and 16 (22.9%) SNPs in 10 (41.6%) candidate genes in the adults, consistently identified by at least two of the five methods used, indicating that they are very likely under selection. Genes with SNPs showing signatures of selection are involved in a wide range of molecular functions, such as oxidoreductases (IDH), hydrolases (CysPro), transferases (XTH), transporters (KT2), chaperones (CP10) and transcription factors (DAG, NAC transcription factor). The obtained data will help us better understand the genetic variation underlying adaptation to environmentally changing conditions in European beech, which is of great importance for the development of scientific guidelines for the sustainable management and conservation of this important species.

Despite the ecological and economic importance of European beech (Fagus sylvatica L.) genomic resources of this species are still limited. This hampers an understanding of the molecular basis of adaptation to stress. Since beech will most likely be threatened by the consequences of climate change, an understanding of adaptive processes to climate change-related drought stress is of major importance. Here, we used RNA-seq to provide the first drought stress-related transcriptome of beech. In a drought stress trial with beech saplings, 50 samples were taken for RNA extraction at five points in time during a soil desiccation experiment. De novo transcriptome assembly and analysis of differential gene expression revealed 44,335 contigs, and 662 differentially expressed genes between the stress and normally watered control group. Gene expression was specific to the different time points, and only five genes were significantly differentially expressed between the stress and control group on all five sampling days. GO term enrichment showed that mostly genes involved in lipid- and homeostasis-related processes were upregulated, whereas genes involved in oxidative stress response were downregulated in the stressed seedlings. This study gives first insights into the genomic drought stress response of European beech, and provides new genetic resources for adaptation research in this species.

Two subspecies of European beech (Fagus sylvatica L.) can be found in southeast Europe: Fagus sylvatica ssp. sylvatica L. and Fagus sylvatica ssp. orientalis (Lipsky) Greut. & Burd. (Fagus orientalis Lipsky). In a previous study, based on genetic diversity patterns and morphological characters, indications of hybridization between both subspecies were found in northeastern Greece, a known contact zone of F. sylvatica and F. orientalis. Nevertheless, potential genetic admixture has not been investigated systematically before. Here, we investigated genetic diversity and genetic structure of 14 beech populations originating from Greece and Turkey as well as of two reference F. sylvatica populations from Germany based on nine expressed sequence tag-simple sequence repeat (EST-SSR) markers. Very low genetic differentiation was detected among F. sylvatica populations (mean GST: 0.005) as well as among F. orientalis populations (mean GST: 0.008), but substantial differentiation was detected between populations of the two subspecies (mean GST: 0.122). Indications for hybridization between both subspecies were revealed for one population in Greece. One of the genetic markers showed specific allele frequencies for F. sylvatica and F. orientalis and may be used as a diagnostic marker in future studies to discriminate both subspecies.

Rapidly decreasing water availability as a consequence of climate change is likely to endanger the range of long-lived tree species. A pressing question is, therefore, whether adaptation to drought exists in important temperate tree species like European beech (Fagus sylvatica L.), a wide-spread, dominant forest tree in Central Europe. Here, five beech stands were selected along a precipitation gradient from moist to dry conditions. Neutral genetic markers revealed strong variation within and little differentiation between the populations. Natural regeneration from these stands was transferred to a common garden and used to investigate the expression of genes for abscisic acid (ABA)-related drought signaling [9-cis-epoxy-dioxygenase (NCED), protein phosphatase 2C (PP2C), early responsive to dehydration (ERD)] and stress protection [ascorbate peroxidase (APX), superoxide dismutase (SOD), aldehyde dehydrogenase (ALDH), glutamine amidotransferase (GAT)] that are involved in drought acclimation. We hypothesized that progenies from dry sites exhibit constitutively higher expression levels of ABA- and stress-related genes and are less drought responsive than progenies from moist sites. Transcript levels and stress responses (leaf area loss, membrane integrity) of well-irrigated and drought-stressed plants were measured during the early, mid- and late growing season. Principal component (PC) analysis ordered the beech progenies according to the mean annual precipitation at tree origin by the transcript levels of SOD, ALDH, GAT and ERD as major loadings along PC1. PC2 separated moist and drought treatments with PP2C levels as important loading. These results suggest that phosphatase-mediated signaling is flexibly acclimated to the current requirements, whereas stress compensatory measures exhibited genotypic variation, apparently underlying climate selection. In contrast to expectation, the drought responses were less pronounced than the progeny-related differences and the transcript levels were constitutively lower in beeches from dry than from moist sites. These results imply that beeches from dry origins may have evolved mechanisms to avoid oxidative stress.

No abstract

Studies of genetic variation underlying traits related to drought tolerance in forest trees are of great importance for understanding their adaptive potential under a climate change scenario. In this study, using a candidate gene approach, associations between SNPs and drought related traits were assessed in saplings of European beech (Fagus sylvatica L.) representing trees growing along steep precipitation gradients. The saplings were subjected to experimentally controlled drought treatments. Response of the saplings was assessed by the evaluation of stem diameter growth (SDG) and the chlorophyll fluorescence parameters FV/FM, PIabs, and PItot. The evaluation showed that saplings from xeric sites were less affected by the drought treatment. Five SNPs (7.14%) in three candidate genes were significantly associated with the evaluated traits; saplings with particular genotypes at these SNPs showed better performance under the drought treatment. The SNPs were located in the cytosolic class I small heat-shock protein, CTR/DRE binding transcription factor, and isocitrate dehydrogenase genes and explained 5.8–13.4% of the phenotypic variance. These findings provide insight into the genetic basis of traits related to drought tolerance in European beech and could support the development of forest conservation management strategies under future climatic conditions.