The increasing frequency and intensity of droughts threaten forests and their climate change mitigation potential. Mixed-species forests are promoted to increase forest productivity and stability compared to monospecific forests, but we still lack a mechanistic understanding of the strength, nature and drivers of tree diversity effects on productivity and stability under drought. Here, I studied the stress hotter droughts inflict on trees and examined whether diversification in tree species, structures and drought-tolerance traits is a potential solution to this threat. In study 1, I found that the hotter drought years 2018–2019, the severest droughts since records, induced unprecedented tree productivity and physiological stress responses (reduced growth and increased δ13C) in a Central European floodplain forest. Hotter droughts thus constitute a novel threat. In studies 2–4, I examined diversity-productivity and diversity-stability relationships across spatiotemporal scales in a tropical (study 2) and a subtropical (studies 3, 4) tree diversity experiment specifically designed to examine biodiversity-ecosystem functioning relationships. Tree species richness consistently increased productivity and stability, and this effect was strongest at the highest levels of diversity. Structural diversity increased productivity but was unrelated to stability, while diversity in drought-tolerance traits increased stability but not productivity. Assessing drought-tolerance traits was essential for understanding the role of tree diversity during drought. Positive diversity effects on productivity scaled up from the tree neighbourhood to the community level, but effects on stability emerged only at the community level. Community stability increased with species richness due to asynchronous species responses to dry and wet years driven by species’ drought-tolerance traits. I showed that diversity but not identity in drought-tolerance traits increases community stability. Overall, promoting structurally and functionally diverse mixed-species forests may enable high productivity and stability under intensifying climate change.:1. General introduction 1.1. Mixed-species forests 1.2. Diversity-productivity relationships 1.3. Diversity-productivity relationships during drought 1.4. Diversity-stability relationships 1.5. Diversity facets 1.6. Drought-tolerance traits 1.7. Linkages between the four studies 2. Methodological features 2.1. Study sites and approaches 2.2. Productivity, stability and physiological water stress 2.3. The quantification of diversity 2.4. Spatiotemporal analyses 3. Original contributions Study 1: Cumulative growth and stress responses to the 2018–2019 drought in a European floodplain forest Study 2: Drivers of productivity and its temporal stability in a tropical tree diversity experiment Study 3: Neighbourhood species richness and drought-tolerance traits modulate tree growth and δ13C responses to drought Study 4: Species richness stabilizes productivity via asynchrony and drought- tolerance diversity in a large-scale tree biodiversity experiment 4. General discussion 4.1. Summary of main findings 4.2. Hotter droughts and forest functioning 4.3. Diversity signals across spatial scales 4.4. Diversity signals across temporal scales 4.5. Diversity facets 4.6. Context dependency and transferability 4.7. Implications for forest management in the 21st century 5. Outlook and future research 5.1. Observation and experimentation under hotter droughts 5.2. Response variables 5.3. Diversity facets 5.4. Drought-tolerance traits 5.5. Zooming in 5.6. Zooming out 5.7. From understanding to use of BEF relationships 6. Conclusion 7. Summary 8. Zusammenfassung 9. References Acknowledgements Author contribution statements Curriculum vitae List of publications Selbstständigkeitserklärung

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