Genomic diversity for climate change resilience

Background

The share of European Beech in German forest stands has been actively increased in the past decade with the goal to make forests more resistant to changing climatic conditions. However, recent hot and dry summers showed that beech forests with older trees (>60 years) are also vulnerable to drought stress resulting in thinned crowns, severe leaf browning and reduction in basal area increment. Consequently, doubts are rising, about whether European Beech is still a suitable tree for future German forests. Diverse symptoms were reported, including Phythophthora infections and the beech complex disease, that mostly describe infections of the main stem, or the above-described vitality loss that first manifests in the tree crown. What is missing so far is a combined assessment of these symptoms in population wide screenings, and studies testing these symptoms in an evolutionary context.

Goals

We aim to provide a basis to better understand the distribution of beech disease symptoms in German forests and their interrelatedness and to add an evolutionary perspective to the discussion on the future of beech as an important forest tree in Germany.

Hypotheses

H1 | Susceptibility to beech complex disease in Fagus sylvatica is influenced by genetic variation.
H2 | Variation in symptoms of vitality loss in Fagus sylvatica reflects underlying genetic differences in drought response.
H3 | Trees exhibiting vitality loss have a higher likelihood of secondary infection, partially mediated by environmental conditions.
H4 | Crown dieback in Fagus sylvatica has increased substantially between 2008–2010 and 2024, with the strongest decline in the driest region.

Methods

In TREEvolution II we use a combined assessment of stem and crown health status of 1500 beech trees and relate the disease symptoms to the genotype and environmental factors. We will first assess the health status of the tree stems and the tree crowns in the leaf-off status and combine this assessment with a controlled analysis of the bark microbiome of healthy and diseased trees using ITS and 16S amplicon sequencing. This will be followed by a late summer assessment of the tree crown, both by classifying the crown from the ground and secondly by imaging the crown using a UAV carried LiDAR to assess crown thinning thorough the analysis of laser return ratios from different crown layers.

In addition, we will use a UAV based multispectral sensor to measure indices such as NDVI, GNDVI, NDRE, to extrapolate values measured from individual leaves to the tree crown. We will then test to what extent the obtained phenotypes can be explained by genomic variation among trees using genome-wide association methods (GWAs) or by the environmental variation among experimental plots (Eps) in the Schorfheide-Chorin, Hainich, and Swabian Alb.

Results

In the first phase of the project TREEvolution, we compared the genomes of adult trees with paired seedlings across all three Biodiversity Exploratories. We found that in the population of seedlings the allele frequencies shifted at specific loci compared to the population of adult trees, in a pattern comparable to a selective sweep, which indicates strong selection in the very early life stages. The loci putatively under selection were enriched in GO terms that highlight the involvement of plant stress response pathways. Furthermore, when we correlated the single nucleotide polymorphisms (SNPs) with environmental variables using genome environment-associations, these associations were stronger in the seedling population, indicating a stronger environmental filtering in seedlings compared to their parents. Together our results indicate that our beech forests have the potential to respond to the selection pressure imposed by the changing climatic conditions.