Dr. Christoph Sperisen

Abies alba Mill. is a prominent European tree species predominantly inhabiting cool and humid montane environments. However, paleoecological evidence reveals that during the Eemian and mid-Holocene, A. alba thrived in much warmer and drier climates. This capacity is nowadays reflected in cryptic meso- and sub-Mediterranean lowland populations. To link A. alba populations across diverse climates spanning from the Mediterranean lowlands to the Alpine timberline, we genotyped 421 specimens from Italy and Switzerland at 190 single-nucleotide polymorphisms (SNPs). Population genetic structure analyses indicate that isolated meso- and sub-Mediterranean lowland populations in Tuscany and Ticino align genetically with higher elevation populations in each region, suggesting that their capacity to thrive in warmer, drier conditions cannot be attributed to plantations with planting stock originating from different climates or to the occurrence of a single warm-adapted lineage showing a disjunct distribution, unless migration between Tuscany and Ticino stands occurred. Genotype–environment associations reveal that temperature seasonality, precipitation during critical seasons, and relative humidity are important for explaining the species' genetic variation. With genotype–environment and genotype–phenotype associations, we identified candidate adaptive genes potentially linked to climatic conditions and drought response. While certain adaptive alleles may have spread from Tuscany and Southern Italy or could be explained by a shared ancestry of Ticino and Tuscan populations, local adaptation may have occurred at specific loci. These findings underscore the importance of considering the hitherto overlooked lowland Mediterranean populations of A. alba to better understand the species' climatic niche and its potential for forest conservation and management under global warming.

Abies alba is an important European tree species currently mostly found at cool and humid sites in the montane zone. In the past, it grew under markedly warmer and drier climates during the Eemian and mid-Holocene, and cryptic Mediterranean populations confirm the species' capacity to grow under warm, summer-dry conditions. However, it is unknown if warm-loving Mediterranean occurrences are related to specific genetic properties (e.g., subspecies or ecotypes). Investigating the genetics of cryptic warm-loving populations is crucial for a better understanding of past and future population dynamics of A. alba. We genotyped 478 A. alba samples at 174 single-nucleotide polymorphisms (SNP), covering a broad latitudinal range from Southern Italy to Switzerland while accounting for local altitudinal gradients, and combined these newly introduced genotypes with those of other European Abies populations from the literature. Population genetic structure analyses grouped the warm-loving meso- and sub-Mediterranean populations into the same genetic cluster as the mountain populations of each region. The occurrence of three genetic clusters from Northern to Southern Italy is in line with the glacial refugia history. The inferred evolutionary and demographic history suggests a northward expansion of A. alba after glaciation, as well as a trans-Adriatic gene flow between Balkan and Southern Italian populations. Collectively, the combined genotypic data from individuals across the species' range demonstrate that cryptic Mediterranean populations of A. alba align with the local and large-scale genetic structure of populations from its main range, suggesting that the species is able to thrive in a warmer and drier environmental range than hitherto anticipated. This finding implies that it is unneeded to postulate extinct subspecies or ecotypes to explain the occurrence of meso- and sub-Mediterranean Eemian or mid-Holocene silver fir forests, with important implications for future A. alba population dynamics.

Climate change-related extreme drought events already have a significant impact on the productivity and mortality of Central European forests. European beech (Fagus sylvatica ssp. sylvatica), one of the most important European broadleaved species, has responded to such drought periods with increasing mortality and reduced volume increment. This has raised concerns about its suitability and adaptive capacity in relation to future climatic conditions and motivated the search for alternative tree species that are suitable for assisted migration into European beech forests. One of the candidates is the Oriental beech species complex (F. sylvatica ssp. orientalis), whose range extends from the Balkan to Iran and, at least in some parts of its range, grows under a warmer and drier climate. In order to evaluate whether Oriental beech is more drought tolerant, we compared the radial growth response to droughts between 1920 and 2018 of a total of 138 European and 122 Oriental beeches growing under identical site conditions in eight different locations in Germany and France. The species identity of all analysed trees was verified by microsatellite analyses, and the origin of the introduced Oriental beech was traced to the Greater Caucasus (7 stands) and the Black Sea coast (1 stand). The drought responses of radial growth were quantified using the indices resistance, resilience, and recovery as suggested by Lloret et al. (2011) and growth recovery time (GRT) (Thurm et al., 2016) and used as response variables in generalized linear mixed effect models. Considering only the average radial growth response to severe and extreme drought events, both the different Lloret indices and the GRT did not show prominent difference between Oriental and European beech. However, the mixed model analyses, which also included interaction terms, revealed interspecific differences in drought tolerance, depending on the intensity and timing of the drought. In extreme summer drought years, values of resistance predicted by the mixed-effect models were significantly higher in Oriental beech than in European beech, whereas its resilience was only slightly better than in European beech, regardless of drought intensities. In contrast, Oriental beech was much more susceptible to spring drought with significantly weaker growth recovery and distinctly longer growth recovery times. Based on these results, Oriental beech provenances from the Caucasus do not appear to be sufficiently more drought tolerant than European beech to justify an assisted migration approach to adapt Central European forests to climate change. To analyse the drought tolerance of Oriental beech more comprehensively, introduced trees representing other genetic clusters need to be analysed, as well as the effects of repeated drought events on growth and mortality.

The benefits and risks of human-aided translocation of individuals within the species range, assisted gene flow (AGF), depend on the genetic divergence, on the rate and direction of hybridization, and on the climate transfer distance between the host and donor populations. In this study, we explored the use of Oriental beech (Fagus sylvatica subsp. orientalis), growing from Iran to the Balkans, for AGF into European beech populations (F. sylvatica subsp. sylvatica) that increasingly suffer from climate warming. Using samples from natural populations of Oriental and European beech and microsatellite loci, we identified 5 distinct genetic clusters in Oriental beech with a divergence (F_ST) of 0.15 to 0.25 from European beech. Using this knowledge, we traced the origin of 11 Oriental beech stands in Western Europe established during the 20th century. In two stands of Greater Caucasus origin, we found evidence for extensive hybridization, with 18% and 41% of the offspring having hybrid status. Climate data revealed higher seasonality with warmer and drier summers across the native Oriental beech sites in comparison to the planting sites in Western Europe. Accordingly, we found that bud burst of Oriental beech occurred four days earlier than in European beech. Overall, our results suggest that AGF of Oriental beech could increase the genetic diversity of European beech stands and may foster introgression of variants adapted to expected future climatic conditions. Our study showcases the evaluation of the benefits and risks of AGF and call for similar studies on other native tree species.

Klimawandel, Trockenheit, Tannensterben: Forscherinnen und Forscher debattieren über die Zukunftsaussichten der Weisstanne. Jüngste Forschungsergebnisse deuten an, dass diese Baumart grösseres Potenzial hat als oft angenommen.

Changement climatique, sécheresse et dépérissement du sapin suscitent le débat scientifique sur les perspectives d’avenir de cette essence. Des résultats récents suggèrent que son potentiel est meilleur qu’on ne le pense.

The rapid development of ancient DNA (aDNA) analysis in the last decades has induced a paradigm shift in ecology and evolution. Driven by a combination of breakthroughs in DNA isolation techniques, high-throughput sequencing and bioinformatics, ancient genome-scale data for a rapidly growing variety of taxa is now available, allowing researchers to directly observe demographic and evolutionary processes over time. However, the vast majority of palaeogenomic studies still focuses on human or animal remains. In this article, we make the case for a vast untapped resource of ancient plant material that is ideally suited for palaeogenomic analyses: Plant remains such as needles, leaves, wood, seeds or fruits that are deposited in natural archives, such as lake sediments, permafrost or even ice caves. Such plant remains are commonly found in large numbers and in stratigraphic sequence through time and have so far been used primarily to reconstruct past local species presences and abundances. However, they are also unique repositories of genetic information with the potential to revolutionize the fields of ecology and evolution by directly studying microevolutionary processes over time. Here, we give an overview of the current state-of-the-art, address important challenges, and highlight new research avenues to inspire future research.

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 F_V/F_M, PI_abs, and PI_tot. 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.

Heterogeneous environments, such as mountainous landscapes, create spatially varying selection pressure that potentially affects several traits simultaneously across different life stages, yet little is known about the general patterns and drivers of adaptation in such complex settings. We studied silver fir (Abies alba Mill.) populations across Switzerland and characterized its mountainous landscape using downscaled historical climate data. We sampled 387 trees from 19 populations and genotyped them at 374 single-nucleotide polymorphisms (SNPs) to estimate their demographic distances. Seedling morphology, growth and phenology traits were recorded in a common garden, and a proxy for water use efficiency was estimated for adult trees. We tested whether populations have more strongly diverged at quantitative traits than expected based on genetic drift alone in a multi-trait framework, and identified potential environmental drivers of selection. We found two main responses to selection: (i) populations from warmer and more thermally stable locations have evolved towards a taller stature, and (ii) the growth timing of populations evolved towards two extreme strategies, 'start early and grow slowly' or 'start late and grow fast', driven by precipitation seasonality. Populations following the 'start early and grow slowly' strategy had higher water use efficiency and came from inner Alpine valleys characterized by pronounced summer droughts. Our results suggest that contrasting adaptive life-history strategies exist in silver fir across different life stages (seedling to adult), and that some of the characterized populations may provide suitable seed sources for tree growth under future climatic conditions.

Silver fir (Abies alba Mill.) is a keystone conifer of European montane forest ecosystems that has experienced large fluctuations in population size during during the Quaternary and, more recently, due to land-use change. To forecast the species' future distribution and survival, it is important to investigate the genetic basis of adaptation to environmental change, notably to extreme events. For this purpose, we here provide a first draft genome assembly and annotation of the silver fir genome, established through a community-based initiative. DNA obtained from haploid megagametophyte and diploid needle tissue was used to construct and sequence Illumina paired-end and mate-pair libraries, respectively, to high depth. The assembled A. alba genome sequence accounted for over 37 million scaffolds corresponding to 18.16 Gb, with a scaffold N50 of 14,051 bp. Despite the fragmented nature of the assembly, a total of 50,757 full-length genes were functionally annotated in the nuclear genome. The chloroplast genome was also assembled into a single scaffold (120,908 bp) that shows a high collinearity with both the A. koreana and A. sibirica complete chloroplast genomes. This first genome assembly of silver fir is an important genomic resource that is now publicly available in support of a new generation of research. By genome-enabling this important conifer, this resource will open the gate for new research and more precise genetic monitoring of European silver fir forests.

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 or simple sequence repeat (SSR) markers and 70 single nucleotide polymorphisms (SNPs) in 24 candidate genes potentially involved in stress response and phenology. Both SSR and SNP markers revealed high genetic diversity in the studied populations and low but statistically significant population differentiation. The SNPs were searched for F-ST outliers using three different methods implemented in LOSITAN, Arlequin, and BayeScan, respectively. Additionally, associations of the SNPs with environmental variables were tested by two methods implemented in Bayenv2 and Samada, respectively. There were 14 (20%) SNPs in 12 (50%) candidate genes in the saplings and 9 (12.8%) SNPs in 7 (29.2%) 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.

Silver fir is after Norway spruce and European beech the third most common tree species in Switzerland, accounting for 15 % of total growing stock. The results of a genecological study with 90 Swiss silver fir populations showed that phenotypic differentiation among populations is rather low, but phenotypic variation within populations large. Silver fir can, therefore, be described as an "adaptive generalist", in contrast to the "adaptive specialist" Norway spruce. Consequently, the species has a low risk of insufficient genetic adaptation to a warmer and drier climate. This is supported by the fact that radial stem growth of silver fir during drought periods decreases only slightly, and recovers relatively quickly. More than 6000 years ago, when annual precipitation was similar to today (> 700 mm) and temperatures were 5 to 8 degrees warmer, silver fir was widespread in mixed forests in southern Switzerland and Italy. For these reasons, silver fir is a promising tree species for the future. However, its high sensitivity to forest fires and browsing by ungulates as well as its susceptibility to bark beetles in extreme drought represent risks that need serious consideration. The outlined potentials and risks of silver fir suggest that it should be promoted as a mixed tree species in climate change adapted forest management. Its high shade tolerance makes it perfectly suited to such conditions.

Geomorphology has increasingly considered the role of biotic factors as controls upon geomorphic processes across a wide range of spatial and temporal scales. Where timescales are long (centennial and longer), it has been possible to quantify relationships between geomorphic processes and vegetation using, for example, the pollen record. However, where the biotic agents are fauna, longer term reconstruction of the impacts of biological activity upon geomorphic processes is more challenging. Here, we review the prospect of using environmental DNA as a molecular proxy to decipher the presence and nature of faunal influences on geomorphic processes in both present and ancient deposits. When used appropriately, this method has the potential to improve our understanding of biotic drivers of geomorphic processes, notably fauna, over long timescales and so to reconstruct how such drivers might explain the landscape as we see it today.

• Ancient DNA from historical and subfossil wood has a great potential to provide new insights into the history of tree populations. However, its extraction and analysis have not become routine, mainly because contamination of the wood with modern plant material can complicate the verification of genetic information.
• Here, we used sapwood tissue from 22 subfossil pines that were growing c. 13 000 yr BP in Zurich, Switzerland. We developed and evaluated protocols to eliminate surface contamination, and we tested ancient DNA authenticity based on plastid DNA metabarcoding and the assessment of post-mortem DNA damage.
• A novel approach using laser irradiation coupled with bleaching and surface removal was most efficient in eliminating contaminating DNA. DNA metabarcoding confirmed which ancient DNA samples repeatedly amplified pine DNA and were free of exogenous plant taxa. Pine DNA sequences of these samples showed a high degree of cytosine to thymine mismatches, typical of post-mortem damage.
• Stringent decontamination of wood surfaces combined with DNA metabarcoding and assessment of post-mortem DNA damage allowed us to authenticate ancient DNA retrieved from the oldest Late Glacial pine forest. These techniques can be applied to any subfossil wood and are likely to improve the accessibility of relict wood for genome-scale ancient DNA studies.

Understanding the genecology of forest trees is critical for gene conservation, for predicting the effects of climate change and climate change adaptation, and for successful reforestation. Although common genecological patterns have emerged, species-specific details are also important. Which species are most vulnerable to climate change? Which are the most important adaptive traits and environmental drivers of natural selection? Even though species have been classified as adaptive specialists vs. adaptive generalists, large-scale studies comparing different species in the same experiment are rare. We studied the genecology of Norway spruce (Picea abies) and silver fir (Abies alba), two co-occurring but ecologically distinct European conifers in Central Europe. For each species, we collected seed from more than 90 populations across Switzerland, established a seedling common-garden test, and developed genecological models that associate population variation in seedling growth and phenology to climate, soil properties, and site water balance. Population differentiation and associations between seedling traits and environmental variables were much stronger for Norway spruce than for silver fir, and stronger for seedling height growth than for bud phenology. In Norway spruce, height growth and second flushing were strongly associated with temperature and elevation, with seedlings from the lowlands being taller and more prone to second flush than seedlings from the Alps. In silver fir, height growth was more weakly associated with temperature and elevation, but also associated with water availability. Soil characteristics explained little population variation in both species. We conclude that Norway spruce has become an adaptive specialist because trade-offs between rapid juvenile growth and frost avoidance have subjected it to strong diversifying natural selection based on temperature. In contrast, because silver fir has a more conservative growth habit, it has evolved to become an adaptive generalist. This study demonstrates that co-occurring tree species can develop very different adaptive strategies under identical environmental conditions, and suggests that Norway spruce might be more vulnerable to future maladaptation due to rapid climate change than silver fir.

Tree growth and species distributions are expected to be altered by climate change. European beech (Fagus sylvatica), one of the major tree species in Central Europe, is considered to be particularly threatened by the expected changes in local water regimes. Basic knowledge on the species' genetic variation, environmental adaptation, and phenotypic plasticity is required to assess its potential for climate change adaptation, but sufficient information is lacking. Here, we describe a seedling common garden study at two field sites incorporating 77 natural populations of European beech from an environmentally heterogeneous mountain region in the center of its distribution. We aimed to identify patterns of genetic variation and phenotypic plasticity in growth and phenology, and to associate these with seed source environments. Population differentiation was greater for phenology than for growth (Q_st = 0.18–0.32 vs. 0.00–0.16), but within-population genetic variation was large for all seedling traits. The phenotype-environment associations indicated adaptive divergence in phenology and growth with respect to temperature and water availability, but not to soil characteristics, latitude, longitude, or topography of the seed source locations. Phenotypic plasticity was detected in growth and leaf duration, the magnitude of which differed among populations with different seed source temperatures. We conclude that seedling phenology is key to temperature and drought adaptation in European beech. Changes in local temperature and water regimes might result in local phenological maladaptation of European beech populations, although within-population genetic variation, gene flow, and phenotypic plasticity might mitigate the negative effects of climate change.

Tree populations usually show adaptations to their local environments as a result ofnatural selection. As climates change, populations can become locally maladapted and decline in fitness. Evaluating the expected degree of genetic maladaptation due to climate change will allow forest managers to assess forest vulnerability, and develop strategies to preserve forest health and productivity. We studied potential genetic maladaptation to future climates in three major European tree species, Norway spruce (Picea abies), silver fir (Abies alba), and European beech (Fagus sylvatica). A common garden experiment was conducted to evaluate the quantitative genetic variation in growth and phenology of seedlings from 77 to 92 native populations of each species from across Switzerland. We used multivariate genecological models to associate population variation with past seed source climates, and to estimate relative risk of maladaptation to current and future climates based on key phenotypic traits and three regional climate projections within the A1B scenario. Current risks from climate change were similar to average risks from current seed transfer practices. For all three climate models, future risks increased in spruce and beech until the end of the century, but remained low in fir. Largest average risks associated with climate projections for the period 2061–2090 were found for spruce seedling height (0.64), and for beech bud break and leaf senescence (0.52 and 0.46). Futurerisks for spruce were high across Switzerland. However, areas of high risk were also found in drought-prone regions for beech and in the southern Alps for fir. Genetic maladaptation to future climates is likely to become a problem for spruce and beech by the end of this century, but probably not for fir. Consequently, forest management strategies should be adjusted in the study area for spruce and beech to maintain productive and healthy forests in the future.

Pflanzungen haben im Schweizer Wald durch die Fokussierung auf Naturverjüngung und die hohen Arbeitskosten in den letzten Jahren stark abgenommen. Immer weniger Fachleute beschäftigen sich mit forstlichem Vermehrungsgut (FoV), weshalb auch das Wissen zum Umgang damit zurückgegangen ist. Im Hinblick auf den Klimawandel bieten sich aber bei der Produktion und Verwendung von FoV mehrere Möglichkeiten, aktiv auf die zukünftige genetische Zusammensetzung der Baumbestände Einfluss zu nehmen. Durch Strategien wie «unterstützten Genfluss» können Waldbewirtschafter versuchen, möglichen negativen Auswirkungen der Klimaveränderungen entgegenzuwirken. Das Ziel dieser Pilotstudie war es, den Status quo beim Umgang mit FoV in der Schweiz und im nahen Ausland zu erfassen. Zudem sollten aufgrund des heutigen Kenntnisstandes die Anforderungen an einen sachgerechten Umgang mit FoV in einem sich wandelnden Klima beschrieben werden. Daraus wurden schliesslich unter Berücksichtigung der verschiedenen Akteure Handlungsbedarf und potenzielle Handlungsoptionen abgeleitet.

In Mitteleuropa gilt die Tanne im Hinblick auf den Klimawandel als potenzieller Ersatz der Fichte. An der Tannentagung der ANW und des VFS am 14. Oktober 2016 in Maulbronn beleuchteten die Autoren den Waldbau mit Tanne in der Schweiz. Sie gingen dabei insbesondere der Frage nach, welche Potenziale und Risiken die Tanne in Zukunft mit sich bringt, wenn das Klima wärmer und trockener wird.