Growth and Climate Sensitivity of Northern Red Oak (Quercus rubra L.) Provenances in Central Europe

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

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