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The accurate assessment of forest persistence under environmental change is dependent on the fundamental understanding of the genetic consequences of human intervention and its comparison to that of natural processes, as declines in genetic diversity and changes in its structuring can compromise the adaptive ability of a population. The European beech, Fagus sylvatica, has experienced prolonged human impact over its 14 million ha range with contemporary forests harbouring high ecological, economic, and cultural value. Historical traditional management practices, such as coppicing and pollarding, have impacted a large portion of Europe’s forests. This form of management encouraged vegetative regeneration, prolonging the longevity of individual trees. In several cases, the structure and function of managed trees and their associated ecosystems were significantly altered. Specifically, coppiced beech forests in Europe displayed significantly larger extents of spatial genetic structuring compared to their natural counterparts, revealing a change in the genetic composition of the population due to decades of management. Humans have also aided in the dispersal of beech within and outside of its natural range. In Great Britain, the putative native range retained signals of past colonisation dynamics. However, these signals were obscured by the wide-spread translocation of the species throughout the country. Evidence of post-glacial colonisation dynamics can be found in Sweden as well. In contrast to Britain, the structure of this natural leading range edge displays a gradual reduction in population size where isolation was found to have acted as an effective barrier to gene flow reducing the genetic diversity of populations.

Increases in temperature, altered precipitation patterns, and the occurrence and severity of extreme climatic events have been important characteristics of the climate change observed to date. This has had many and diverse impacts upon the living world, with one recent observation being a global reduction in the net primary production of all terrestrial vegetation. Increases in temperature and the frequency of extreme events are predicted to continue throughout the 21st century, and can be expected to have far reaching effects on global terrestrial ecosystems. Increases in temperature and drought occurrence could fundamentally impact upon the growth rates, species composition and biogeography of forests in many regions of the world, with many studies indicating that this process is already underway. European beech, Fagus sylvatica, is one of Europe’s most widespread and significant broadleaved tree species, forming an important and frequently dominant component of around 17 million hectares of forest. However, the species is also considered to be drought sensitive. Thus, much research interest has focused on eliciting the details of its physiological response to increased water stress, whilst dendroecological studies have attempted to identify sites and regions where reductions in growth might be found. A significant knowledge gap exists regarding a multi-regional, range-wide view of growth trends, growth variability, climate sensitivity, and drought response for the species. Predicting the potential effects of climate change on competition and species composition in mixed species forests remains an important challenge. In order to address this knowledge gap, a multi-regional tree-ring network was constructed comprising of 46 sites in a latitudinal transect across the species’ Western European range. This consisted of 2719 tree cores taken from 1398 individual trees, which were used to construct tree-ring chronologies for each site in the network. As a first step in a multi-regional assessment for F. sylvatica, a combination of the tree-ring chronologies and environmental data derived from a large scale gridded climate dataset were used in a multivariate analysis. Sites in the latitudinal transect were partitioned into geographically meaningful regions for further analysis. The resulting regions were then studied using climate-growth analysis, pointer year analysis of drought years, analysis of growth trends and growth variability, in order to examine regional variation in the response of the species to climate. Furthermore, a combination of long-term monitoring data from one specific site was combined with tree-ring sampling of multiple cohorts of F. sylvatica and one co-dominant competitor, Quercus petraea, to study the effects of an extreme drought event in 1976 on mortality and subsequent recovery. Key results of the multi-regional analysis are that large scale growth reductions are not evident in even the most southerly and driest portions of the species’ range. Radial growth is increasing, both in the north and in the core of the species’ range, with southern range edge forests maintaining stable growth. However, the variability of growth from year to year is increasing for all regions, indicative of growing stress. Crucially, the southern range edge, which previous studies had identified as an ‘at risk’ region, was shown to be more robust than expected. Climate sensitivity and drought impacts were low for this region. Instead, forests in the core of the species range, both in continental Europe and in the south of the UK, were identified as having the highest climate sensitivity, highest drought impacts, and experiencing periodic reductions in growth as a result. Northern range edge forests showed little sign of being affected by drought, instead having low climate sensitivity and strongly increasing growth trends. Extreme drought was found to affect species differently: the dominant species (F. sylvatica) failed to recover pre-drought levels of growth, whilst a transient effect of competitive release occurred for the co-dominant species (Q. petraea). There was also a long term effect on the relative abundance of the two species within the woodland, due to differences in the levels of drought induced mortality experienced by the species. This shows that in the case of extreme climatic events where thresholds in the ability of species to tolerate water stress are breached, the effects of drought can be rapid and long lasting. Drought impacts can cascade beyond that experienced by the most drought sensitive species, due to changes in competitive interactions between species in mixed species forests. The implications of this work suggest opportunities, risks and strengths for F. sylvatica. In the northern portion of the species’ range, predicted increases in productivity are confirmed by recent growth trends, indicating a good outlook for the species. At the southern range edge, F. sylvatica forests exist either in locations where precipitation is high or locations where local environmental conditions buffer them from an inhospitable regional climate. These factors result in southern range edge forests which are highly resilient to the effects of increasing climate stress. It is instead in the core of the species’ range where the most sensitive forests are found. The effects of extreme drought on a range core forest demonstrated here provide a cautionary note: where drought stress tolerance thresholds are breached, rapid and long lasting effects on growth and mortality can occur, even in regions where drought has not previously been considered to pose a strong risk to the species.

Forests dominated by beech (Fagus sylvatica L.) cover a large part of Europe and occur on a wide variety of soils, realizing a broad ecological niche in terms of soil chemical properties including pH, base saturation, and plant-available phosphorus (P) pools in the mineral topsoil. On the other hand, a decrease in foliar P concentrations and a corresponding increase in the nitrogen (N) to P ratio during the last decades all over Europe raised questions about a possible reduction in forest health and productivity due to P limitation. These changes in leaf nutrient status have been attributed to continuing high N deposition and increasing atmospheric carbon dioxide concentrations. Both accelerated tree growth due to high N and carbon (C) input and adverse effects of elevated N on fine root biomass, mycorrhization, and litter mineralization appear to have created a higher need for other nutrients that cannot be met by the supply from the soil. Surprisingly little is known about internal P allocation of beech and its capacity to deal with changing soil P availability in the short-term. Considering the distribution of beech forests on soils encompassing a broad range of nutrient availability, and thus proven ability to adapt to different soil conditions in the long-term, we hypothesized that this tree species exhibits a high phenotypic plasticity allowing it to alter multiple traits in response to local nutrient availability leading to adaptive acclimation and resulting in a phenotype tailored for local soil conditions. This PhD-thesis aimed to investigate acclimation of beech from high to low and from low to high soil P availability in terms of plant internal P cycling and plant-soil interactions in the rhizosphere. For this purpose, we grew two groups of 12–15-year-old beech saplings originating from sites with high and low soil P availability in mineral soil (Bh/Bv horizon) from their own site and soil from the other site in rhizoboxes for two years in the greenhouse. We assumed that our saplings were adapted to their soil of origin and that their P nutritional status thus reflected the nutrient availability in the soil of origin. First, we assessed the plasticity of mass and P allocation in the beech saplings aiming to identify the traits most responsive to current soil conditions. After two growing seasons, P concentrations in leaves and stem, as well as mass allocation to leaves and fine roots were affected by both soil and plant origin. By contrast, P allocation to leaves and fine roots, as well as P concentrations in fine roots, were determined almost entirely by the experimental soil. Independent of the P nutritional status defined as average concentration of P in the whole plant, which still clearly reflected the soil conditions at the site of plant origin, P allocation to leaves was a particularly good indicator of P availability in the experimental soil. Furthermore, the high plasticity of this plant trait was indicated by a large difference between plants growing in the two experimental soils. This suggests a strong ability of beech to alter resource allocation in response to specific soil conditions. Second, we investigated the plasticity of root traits and rhizosphere properties of young beech trees from populations, that are adapted to either high or low nutrient supply, when growing in soils differing in their fertility. Fine root traits related to growth (biomass, length), architecture (branching) and morphology (diameter) responded strongly to the properties of the soil. This response was modified by an effect of provenance, that was consistent with an influence of the plant status in those nutrients, which were not in sufficient supply in the soil. However, an additional genotypic difference in the sensitivity of the beech saplings to different soil nutrient supply could not be excluded. Fine root parameters normalized for length, mass, or volume (root tip density and frequency, specific root length and area, root tissue density) did not differ among the treatments. Differences in mycorrhizal colonization and rhizosphere parameters related to phosphorus mobilization potential (pH, an abundance of organic acid anions, phosphatase activity) were small and mainly determined by the soil. Provenance had only a minor modifying effect, possibly due to differences in the ability to transfer carbon compounds from the shoot to the root and the fungal partner. Our results indicate the high plasticity of young beech trees to adapt root growth, architecture, and morphology to different soil nutrient supply, thereby also taking into account internal nutrient reserves. Third, amplicon sequencing was used to assess to which degree beech saplings with a different P nutritional status can shape their rhizosphere microbial community along the root axis. After the first growing season, we determined the community composition and relative abundance of bacteria and fungi in bulk soil and different zones of the rhizosphere of freshly grown roots defined by root segments potentially differing in their functionality (root tip, elongation zone, side root, old roots). We found that soil was the main factor determining microbial diversity and composition irrespective of plant origin and zone. Overall, OTU (operational taxonomic unit; an operational term for groups of closely related biota) numbers of bacteria and fungi, correlating well with soil microbial biomass, were higher in the soil with high P availability than in the soil with low P availability. Based on the relative abundance of OTUs, the bacterial and fungal communities in the different rhizosphere zones grouped distinctly indicating a succession from bulk soil to older roots via root tip, elongation zone, and side root zone demonstrating the often-postulated gradual development of rhizosphere microbial communities along the root axis. There was a significant effect of plant nutritional status on bacterial community diversity (beta diversity) differentiating elongation zone and on fungal community in the elongation and root tip zones. Furthermore, local average microbial species diversity (alpha diversity quantified with Shannon index) in the bulk soil and active rhizosphere zones (root tip, elongation zone) was higher in treatments with beech saplings acclimating to altered soil P availability than in treatments with plants adapted to the respective conditions. The only indication of a plant effect potentially offering an advantage concerning P nutrition, was a higher relative abundance of the bacterial order Solibacterales, potentially important for the mobilization of inorganically bound P, in some zones in treatments with beech saplings exhibiting a low P nutritional status. We conclude that overall, the P nutritional status of beech saplings had only a small influence on the soil microbial community in the rhizosphere. However, the related plant effects were strongest in the rhizosphere of the elongation zone, i.e., the potentially most active root segment in terms of root exudation. This Ph.D. thesis demonstrated that beech saplings are capable of fast acclimation to changes in soil P availability involving alterations of internal P allocation patterns, root architecture, and interactions with local ectomycorrhizal fungi and bacteria in the rhizosphere. Comparing the results for treatments representing plants adapted to low or high soil P availability and for treatments representing plants with a given P nutritional status acclimating to altered soil P availability suggests that alterations of the same sets of multiple traits are involved in both long-term adaptation and short-term acclimation to given soil conditions. Together, the traits define respective growth strategies of adapted plants, i.e., a conservative strategy in soils with low P availability prioritizing nutrient storage and internal recycling and thus limiting losses, and a strategy prioritizing fast growth in soils with high P availability leading to high P acquisition rates but also high losses. These strategies can be considered analogous to and being part of “recycling” and “acquiring” strategies on the ecosystem level. However, our results also indicate that during the first phases of acclimation to lower soil P availability, beech saplings with a high P nutritional status use their reserves to grow as much as possible.