Northern red oak (Quercus rubra L.), a major commercial tree species native to North America, is an introduced tree species in Europe and is currently the most common non-native deciduous tree species in its introduced range. Although the species is widespread in the introduced range, there is a lack of knowledge on growth and climate sensitivity, especially at the provenance level. The aim of this thesis is to conduct a detailed evaluation of provenances from North America and the introduced range with respect to growth and retrospective climate sensitivity in order to identify suitable provenances for adaptive forest management in the face of more frequent extreme events in the future. The emphasis is on the differentiation between a sample of provenances, originating from Canada and the United States respectively, and from introduced stands in Germany, planted on three sites of a provenance trial in 1991 along an environmental gradient. The provenance comparison enables to address the question from which region of the natural distribution a potential seed transfer should originate in order to increase productivity and improve climate adaptation. In this context, the evaluation of adaptive strategies and phenotypic variation was pursued, focusing on the site-specific provenance response to extreme events, such as droughts and late frosts. A specific goal was to examine provenance-specific safety-efficiency trade-offs in resistance to these extreme events and absolute growth, as well as resistance and vessel size, to identify differences in adaptive strategies as response to drought and late frost. The growth analyses showed a significant increase in the performance of introduced provenances compared to those of North America, particularly at the two sites with high water availability. On the continental site, the differences in performance between provenance were less pronounced, with those from Canada in particular showing comparable growth to German provenances. Due to the strong growth differentiation, the quantification of provenance × environment interactions enabled the classification of generalists, i.e. provenances with an environment-independent growth, and specialists, i.e. provenances with an environment-dependent growth, in order to identify site-specific superior provenances. The examinations revealed most introduced provenances as good performing generalists or specialists on sites with higher water availability, whereas provenances from North America were classified as poor performing generalists, except for a Canadian provenance which showed comparable growth patterns at the continental site. The study of climate-growth relationships detected summer drought and spring temperatures as main climatic constraints for the red oak provenances. Here, provenances differed in their climate sensitivity, revealing site-specific climatic dependencies based on the prevailing environmental conditions. In contrast to tree-ring width, the study of climate variables on wood anatomical traits detected two main physiological essential periods that are decisive for wood formation. On the one hand, the climatic conditions in the previous year play a key role in the storage of reserves needed for growth in spring. On the other hand, the temperature and moisture conditions in late winter and spring determine the onset of wood formation and consequently the length of the growing season. In general, the intra-specific variation in wood anatomical traits was high, showing a high phenotypic plasticity at all sites and consequently the trait expression was mainly determined by the site-specific environmental conditions. The response in tree-ring width to extreme drought and late frost showed a high variation within sites with distinct rank changes of provenances between drought years. Further, a trade off between growth and resistance to extreme events could not be proved. On the contrary, high growth was coupled with a high frost hardiness, whereas drought hardiness showed no significant trend. Consistent with these results, no trade-off between resistance to extreme events and vessel size could be demonstrated, suggesting that the commonly reported but controversially discussed relationship between wide vessels and higher resistance to embolism does not apply to the red oak provenances studied. The findings of this thesis contribute to a broader understanding of the climate adaptation of red oak provenances for future perspectives in the introduced range. While the effect of studied summer droughts on the growth response was negligible in terms of the overall growth rates, late frost events may significantly impact red oaks. Since late frost events are suggested to increase in frequency and severity, cold adaptation is suggested to be one of the most important traits considerable for future adaptive forest management. Further, the site-specific adjustment in vessel traits to extreme climatic effects indicates a high phenotypic plasticity, which can be used to improve provenance recommendations. For this reason, provenances from introduced stands provide the best choice for forest management, whereas provenances from Canada are suggested to be promising for a continental climate, representing partly the predicted future climate. In contrast, provenances originating from the southern part of the natural distribution should not be considered in recommendations.:Acknowledgments Summary Kurzfassung List of Tables List of Figures Abbreviations 1 Introduction 1.1 Alternative tree species to mitigate climate impact 1.2 Northern red oak 1.3 Climate adaptability 1.4 Assessment of climate adaptability 1.5 General research objectives 1.6 Thesis outline 1.7 Study design and research approach 1.8 Author contributions 2 Provenances from introduced stands of Northern Red Oak (Quercus rubra L.) outperform those from the natural distribution Appendix A 3 Growth behavior of Northern Red Oak (Quercus rubra L.) in two provenance trials in Germany 4 High risk, high gain? Trade-offs between growth and resistance to extreme events differ in Northern Red Oak (Quercus rubra L.) Appendix B 5 Variation in vessel traits of Northern Red Oak (Quercus rubra L.) provenances revealed high phenotypic plasticity to prevailing environmental conditions Appendix C 6 Synthesis 6.1 Provenance differentiation 6.1.1 Local adaptation 6.1.2 Origin of introduced stands 6.1.3 Epigenetic effects during seed development 6.2 Provenance response to climate 6.2.1 Spatial and temporal variations in growth response 6.2.2 Intra-specific variation to climate 6.2.3 Response to drought and frost 6.3 Provenance choice in the face of climate change 6.3.1 The presence of provenance × environment interactions 6.3.2 Comprehensive evaluation of provenances 6.4 Concluding remarks and outlook References

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

The most common tool to predict future changes in species range are species distribution models. These models do, however, often underestimate potential future habitat, as they do not account for phenotypic plasticity and local adaptation, although being the most important processes in the response of tree populations to rapid climate change. Here, we quantify the difference in the predictions of future range for Norway spruce, by (i) deriving a classic, occurrence-based species distribution model (OccurrenceSDM), and (ii) analysing the variation in juvenile tree height and translating this to species occurrence (TraitSDM). Making use of 32 site locations of the most comprehensive European trial series that includes 1,100 provenances of Norway spruce originating from its natural and further beyond from its largely extended, artificial distribution, we fit a universal response function to quantify growth as a function of site and provenance climate. Both the OccurrenceSDM and TraitSDM show a substantial retreat towards the northern latitudes and higher elevations (−55 and −43%, respectively, by the 2080s). However, thanks to the species’ particularly high phenotypic plasticity in juvenile height growth, the decline is delayed. The TraitSDM identifies increasing summer heat paired with decreasing water availability 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. Still the model is used to provide recommendations of optimal provenance choice for future climate conditions. In essence, assisted migration may not decrease the predicted range decline of Norway spruce, but may help to capitalize on potential opportunities for increased growth associated with warmer climates.

In Mitteleuropa werden zukünftig häufigere Trocken- und Hitzeperioden mit wirtschaftlichen Einbußen in der Waldwirtschaft erwartet. Die Douglasie (Pseudotsuga menziesii (Mirb.) Franco) wird als Alternative für die wirtschaftlich bedeutsame, jedoch trockenheitsempfindliche Fichte diskutiert (Picea abies (L.) H.Karst.). Zwei Unterarten, die Küsten- (FDC) und die Inlandsdouglasie (FDI), sind im ausgedehnten natürlichen Verbreitungsgebiet in Nordamerika beheimatet, welches ein großes Potenzial für die Auswahl produktiver und trockenresistenter Herkünfte bietet. Unser Ziel war, die Trockenreaktion verschiedener Douglasienherkünfte unter Verknüpfung morphologischer und physiologischer Parameter und die der Trockenheitsresistenz bzw. -empfindlichkeit zugrundeliegenden Mechanismen zu erforschen. Ein Herkunftsversuch in Südwestdeutschland ermöglichte die Untersuchung 50-jähriger Douglasien verschiedener Herkünfte entlang eines Höhengradienten. Unter kontrollierten Bedingungen simulierten wir die Effekte einer Hitzewelle auf Jungbäume zweier Provenienzen. Wir analysierten die Kohlenstoff- und Sauerstoff-Stabilisotopenzusammensetzung, den Gaswechsel der Blätter, Veränderungen im Stoffwechsel und das Baumwachstum. Unsere Ergebnisse zeigen bei FDC aus humiden Regionen hohe Wachstumseinbußen unter Trockenheit und moderat bis stark verringerte stomatäre Leitfähigkeit, unterstützt durch Photoprotektion. FDC aus Regionen mit starker Sommertrockenheit reagierten kaum mit Stomataschluss und Wachstumseinbußen auf Trockenheit, jedoch mit starker Osmoregulation und Monoterpen-Emissionen, welche zur Trockenresistenz beitragen könnten. FDI aus einer ariden Region zeigten hohe An, geringes Wachstum und stark antioxidative und photoprotektive Mechanismen. Die Herkünfte unterscheiden sich stark in ihrer Trockenreaktion und ihren Schutzmechanismen. Der Anbau trockenresistenter Herkünfte wird an Standorten von Vorteil sein, für die eine Häufung von ariden Sommerperioden vorhergesagt wird. / In Central Europe, more frequent periods of dry and hot weather are expected in the future with economic losses in the forestry sector. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is discussed as a timber species alternative to the economically important but drought-sensitive spruce (Picea abies (L.) H. Karst.). Two subspecies, the coastal (FDC) and the interior Douglas-fir (FDI), are native to an extensive natural range in North America, offering a great potential for the selection of productive and drought tolerant provenances. Our goal was to investigate the drought response of different Douglas-fir provenances on the morphological and physiological level, as well as the mechanisms underlying drought resistance or susceptibility. A provenance trial in southwestern Germany established in 1958 allowed the study of 50-year-old Douglas-fir trees of diverse provenances along a height gradient. Under controlled conditions, we simulated the effects of a heat wave on young trees of two provenances. We analyzed carbon and oxygen stable isotopic composition, leaf gas exchange, changes in metabolism and tree growth. FDC from humid regions responded to drought with strong growth decline and a medium to strong stomatal closure, supported by enhanced instantaneous photoprotection. FDC from regions with very dry summer conditions showed a small growth decline and anisohydric regulation of stomatal conductance under drought, supported by high levels of osmotic adjustment. High monoterpene emissions might contribute to the drought resistance. FDI from an arid region showed high assimilation rates, low growth potential and a high antioxidant, photoprotective, drought and heat protective potential. The provenances differ greatly in their dry reaction and their protective mechanisms. The cultivation of drought resistant crops will be beneficial at sites predicted to accumulate arid summer periods.