Re-utilizing forest genetic trials to inform reforestation during a time of rapid climate change

European forest ecosystems are changing drastically. The accelerating speed of current climate warming, further paired with drought events, storms and insect calamities, already results in reduced fitness, changes in forest health, species composition and productivity. Forest management is committed to develop suitable adaptation strategies to maintain the multitude of ecosystem services. In this context, the choice of seed origin for forest conversion and reforestation represents a key question to their success. Provenance trials, that test a wide range of provenances planted across multiple test sites, provide the urgently required evidence of population specific response to variable environmental conditions and may be used to derive recommendations under future conditions. This thesis concentrates on two economically and ecologically important native tree species, European beech and Norway spruce, as well as one of the most common introduced broadleaf species, Northern red oak. It provides a comprehensive analysis of their adaptive population differentiation and ability of phenotypically plastic response. For beech, 85 range-wide provenances were observed for 25 years in western, northern and eastern Germany. Considerable provenance differentiation exists, explaining 21% (stem form) to 45% (basal area) of the variance at individual sites and 20 to 39% across sites, while interactions between provenance and environment were absent. At the landscape level, spatially neighboring provenances showed similar trait expressions. These patterns were similar for height and basal area, but different for stem form. They could be directly linked to geographical variables using a multivariate regression tree analysis, which captured 58% of the phenotypic variation, whereby eleven ‘ecotypes’ characterized by local adaptation were delineated. A selection by two multi-trait indices gave highly concordant results and further underlined the trade-off between growth and stem form. Overall, the intermediate elevations of the central highlands in western Germany host highly suitable provenances. Lower elevation provenances from the southeast adapted to continental climate with harsh and cold winters, but warm and dry summers profit from the transfer to the favorable growing conditions in Germany and represent potential candidates for assisted migration. For red oak, each six provenances sourced from German stands and from North America were examined at age 33 on sites in northern, eastern and central Germany. In contrast to beech, provenance by environment interactions were significant and the thorough investigation of their nature provided highly consistent results. German provenances were mostly superior. This was particularly prevalent at the Atlantic site in Schleswig-Holstein, where the most outstanding provenance yields a basal area sum per hectare of +130% compared to the mean across six American provenances. Four of six German provenances were categorized as productive generalists, being suitable for planting across environments, while the other two are productive specialists for humid sites. Provenances from the native range had a lower performance, despite for the driest environment in eastern Germany, where one Canadian provenance sourced north of the Great Lakes emerged as a suitable specialist. Provenances from higher elevations in Georgia and Tennessee grew poorly and had low survival rates, especially in Schleswig-Holstein. Growth performance and survival of spruce was investigated based on the most extensive provenance trial series worldwide, established to test 1,100 provenances in 13 European countries. The analysis of 33 German sites of 100 provenances each, summarized to ten aggregated provenance groups, revealed relatively consistent ranks between groups up to age 51. Provenances from the Hercyno-Carpathian domain had superior short-term and positive long-term growth, while those towards the northern and southern range limits performed poorly. To develop a universal response function, the data set was extended to a total of 97 sites with 100 provenances each. Increasing summer heat paired with decreasing water availability was identified as the main climatic variable that restricts growth, while a prolonged frost-free period enables a longer period of active growth and therefore increasing growth potential within the restricted, remaining area. Clear signals of local adaptation to climatic clines spanning the entire range are barely detectable, as they are disguised by a latitudinal cline. This cline strongly reflects population differentiation for the Baltic domain, but fails to capture the high phenotypic variation associated to the geographic heterogeneity in the Central European Mountain ranges paired with the species history of postglacial migration. Finally, growth predictions of the trait-based model were compared to the predictions of a classic occurrence-based species distribution model. Both models showed a substantial retreat towards the northern latitudes and higher elevations (-55% and -43% by the 2080s). However, thanks to the species’ particularly high phenotypic plasticity the decline is delayed. Model-based prescriptions for assisted migration may not decrease the predicted range decline for Norway spruce, but may help to capitalize on potential opportunities associated with warming climate in the remaining area.:1 General introduction 1.1 European forests under climate pressure 1.2 The capacity of natural populations to cope with changing conditions and its limits 1.3 Strategies of adaptive forest management 1.4 Provenance trials to study population response 1.5 The theory of local optimality and its significance for seed deployment 1.6 Thesis outline and objectives 1.7 Species of interest and experimental data basis 1.8 The analytical tool set 1.9 Author contributions 2 Ecotypic variation in multiple traits of European beech: selection of suitable provenances based on performance and stability 2.1 Abstract 2.2 Introduction 2.3 Material and Methods 2.3.1 Experimental design 2.3.2 Measurement of traits 2.3.3 Quantification of provenance and site effects 2.3.4 Spatial interpolation of trait variation 2.3.5 Multi-variate grouping of provenances 2.3.6 Calculation of multi-trait indices for provenance selection 2.4 Results 2.4.1 Differentiation among provenances at individual sites 2.4.2 Provenance by environment interactions 2.4.3 Spatial patterns in provenance performance 2.4.4 Selection of suitable provenances 2.5 Discussion 2.5.1 Absence of provenance by environment interaction 2.5.2 Ecotypic variation across species range 2.5.3 Trade-off between growth and stem form 2.5.4 Concluding remarks 2.6 Supplementary information 3 Provenances from introduced stands of Northern Red Oak (Quercus rubra L.) outperform those from the natural distribution 3.1 Abstract 3.2 Introduction 3.3 Material and methods 3.3.1 Provenances and experimental design 3.3.2 Measurements 3.3.3 Data analysis 3.4 Results 3.4.1 Percent gain in productivity 3.4.2 Provenance-environment interaction 3.4.3 Generalists vs. specialists 3.4.4 Differences in stem form 3.5 Discussion 3.5.1 Superiority of German provenances 3.5.2 Suitable P×E approaches for provenance trials 3.5.3 Use of P×E to improve provenance recommendations 3.6 Conclusion 3.7 Supplementary information 4 The IUFRO Norway spruce provenance test from 1964/68: a first conclusive evaluation of the German trial sites based on provenance groups 4.1 Abstract 4.2 Zusammenfassung 4.3 Einleitung 4.4 Material und Methoden 4.5 Ergebnisse 4.6 Diskussion 4.7 Schlussfolgerung und Ausblick 5 High phenotypic plasticity, but low signals of local adaptation to climate in a large-scale transplant experiment of Picea abies (L.) Karst. in Europe 81 5.1 Abstract 5.2 Introduction 5.3 Material and Methods 5.3.1 Common gardens 5.3.2 National forest inventories 5.3.3 Climatic and geographic data 5.3.4 OccurrenceSDM 5.3.5 TraitSDM 5.3.6 Species range change 5.4 Results 5.4.1 Habitat projections by OccurrenceSDM 5.4.2 Growth projections by TraitSDM 5.4.3 Comparison of future species range 5.5 Discussion 5.5.1 Phenotypic plasticity largely determines height growth 5.5.2 OccurrenceSDM underestimates species range 5.5.3 Limitations of the TraitSDM 5.5.4 Implications for forest management 5.6 Supplementary information 6 General discussion and synthesis 6.1 Geographic variation within species 6.2 Plasticity as a main driver of juvenile growth 6.3 Absence vs. presence of provenance by environment interactions 6.4 Evidence-based seed transfer 6.5 Implications for assisted migration 6.6 Outlook Cumulative Bibliography

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