“Drought tolerance of Douglas-fir, Norway spruce and Silver fir in the Black Forest region – a dendrochronological analysis” Thesis submitted in partial fulfilment of the requirements of the degree Doctor rer. nat. of the Faculty of Environment and Natural Resources, Albert-Ludwigs-Universität Freiburg im Breisgau, Germany by Valentina Vitali Freiburg im Breisgau, Germany July, 2017 Dean: Prof. Dr. Tim Freytag First examiner: Prof. Dr. Jürgen Bauhus Second examiner: Prof. Dr. Ulrich Kohnle Third examiner: Prof. Dr. Ulf Büntgen Date of thesis defence: November 2017 Statement of originality I hereby declare that this thesis has never been submitted, in the same or similar form, to another examination commission in Germany or in another country for a degree. This thesis contains no material previously published or written by another person except where due acknowledgment is made in proper manner. Valentina Vitali Freiburg, 14th July 2017 III Statement of contributions and manuscripts This doctoral research was carried out between December 2015 and July 2017. Most of the work was performed by myself, from the joint development of the research concept and its implementation, data collection, preparation and analysis, to the writing of the manuscripts, introduction and final Chapters. Three manuscripts were prepared for peer-reviewed publication in scientific journals as follows: Vitali V., Büntgen U., Bauhus J. “Silver fir and Douglas-fir are more tolerant to extreme droughts than Norway spruce in south-western Germany”, Global Change Biology, 10.1111/gcb.13774. Vitali V., Büntgen U., Bauhus J. “Effects of past and future seasonal trends in temperature and precipitation on conifer species growth in the Black Forest, Germany” Dendrochronologia, 10.1016/j.dendro.2018.01.001. Vitali V., Forrester, D.I., Bauhus J. “Know your neighbours – Drought response of Norway spruce, Silver fir, and Douglas-fir in mixed forests depends on species identity and diversity of tree neighbourhoods”, Ecosystems, 10.1111/gcb.13774. Prof. Dr. Jürgen Bauhus and Dr. Rüdiger Unseld developed the project proposal and initial design of the study. Prof. Bauhus supervised the projected and actively participated throughout the whole process. Data collection was carried out under the KLIMOPASS project, financially supported through a KLIMOPASS grant (no. 4500354096/23) provided by the Baden-Württemberg Ministry of Environment, Climate and Energy to Jürgen Bauhus. Prof. Dr. Ulf Büntgen helped with the handling of the dendroecological data and development of manuscripts. Dr. David Forrester helped with data analysis and manuscript development. ForstBW and the competent forestry districts and foresters supported the site selection and early stages of data collection. Thomas Weich supported the selection of field sites, communication with forest districts and field work. Audrey Louy, Clara Arranz and Raphaële Piché helped in the field data collection and laboratory analysis. We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP and the climate data projections, and we thank the climate modelling groups for producing and making available their model output. Lastly the main results from the manuscripts were presented by myself at international conferences. IV I dedicate this thesis to my parents Dedico questa tesi ai miei genitori V Acknowledgments This dissertation would not have been possible without the support and encouragement of numerous people. Special thanks go to my supervisor Prof. Dr. Jürgen Bauhus, and Prof. Dr. Ulf Büntgen for their support and advice, helpful stimulating discussions, and guidance to make the best work I could do. In addition I thank … all the people that contributed to the project “Klimopass” that allowed the collection and analysis of such an amount of data; and all the people in the Waldbau department who have provided guidance and support in these 3 years. Especially to Dr. David Forrester who helped me to understand the interaction in mixed-species forests. … Prof. Dr. Klaus Puettmann for introducing me to my supervisor, and making this whole Ph.D. possible, and the bottle of schnapps that brought us together in the first place. … Dr. Angela Luciana De Avila; Dr. Julia Sohn, Dr. Claudia Hermes and Yuki Henselek for sharing their knowledge, PhD student experience and litres of coffee with me; without them I would have been lost. … Audrey Louy, my French girl that made the fieldwork and my time in Freiburg just so much more awesome; and Raphaële Piché, my French Canadian girl that showed me what being though really means. Without them I could not have made it, and they thought me how to be a good leader and a good friend at the same time; and showed me how a team really works. … Dr. Steffan Klesse, my dear friend, always ready to answer any questions from dendroecology to the mysteries of life or of the dancing floor. Also from the other side of the world the “sparkle effect” keeps on brightening my days. Richard Peters, for teaching me everything I know on detrending. Anna Stritih and Franziska Mörgeli, for being there when I needed a break. And all the people I have met at my time at WSL that make me understand what science is all about. … Adriana Ballón Ossio, for her wisdom and positivity that kept me focused during the long period of writing, and helped me up in the moments of discomfort. Freiburg would have not being the same without. … Julia Böning, Alejandra Saramiento, Lucia Gaitan and Maria Montes for being my friends no matter where we are in the world, ready to listen or party depending on the mood. I am lucky to have met four of the best people in the world. … Ekaterina Morozova and Laura Salvucci, my anchors at home, always ready to welcome me back; that support made my life of expatriate much more bearable. … Dr. Jannik Donner, for being my most trusted proof-reader, and official translator of anything that is German; being so patient with me and my moments of drama. Your effort and tribulations will not be forgotten. … my parents, for making me the strong independent adult I am today. In particular to my dad, Vitali Maurizio, for driving me across Europe and back repeatedly, just to see me and bringing me real food; and my mum, Bedetta Cristina that was always worried I was not eating enough. … my grandmas, Nonna Elena that thought me that doing something with a smile always pays off, or at least you look pretty doing it; and Nonna Graziella, that thought me that we can achieve anything and that we have stubbornness running in our blood to ensure it. VI Table of contents Zusammenfassung XII Introduction 1 1.1. Sustainable forest management in light of climate change 2 1.2. Adaptation challenges in the Black Forest 4 1.3. Research motivation and questions 8 1.4. Study framework 8 1.5. Thesis Outline 14 Silver fir and Douglas-fir are more tolerant to extreme droughts than Norway spruce in south-western Germany 17 2.1. Abstract 18 The effects of past and projected seasonal climate change on growth of native and exotic conifers in central Europe 20 VII 3.1. Abstract 21 Know your neighbours – Drought response of Norway spruce, Silver fir, and Douglas-fir in mixed forests depends on species identity and diversity of tree neighbourhoods 23 4.1. Abstract 24 Discussion 26 5.1. Summary of results 27 5.2. Study strengths and limitations 29 5.3. Challenges to changes in the Black Forest species composition 39 Synthesis: Towards resilient and sustainable management under climate change in the Black Forest 44 6.1. Managing the Black Forest as a resilient system 45 References 48 List of Figures 66 List of Tables 72 VIII Summary Background Climate change is affecting forest growth and vitality globally, driving shifts in tree species distributions, which could result in major changes of important ecosystem services. One of the main forest functions in the region of the Black Forest is timber production. A large proportion of that timber is obtained from Norway spruce (Picea abies) monocultures. However, it has been shown that spruce is highly drought-sensitive. Its growth potential under future climatic conditions, which are characterized by an increasing frequency and intensity of summer droughts, has been questioned. In the long term, it will be inevitable to replace large proportions of current spruce forests with other tree species. Here I assessed the suitability of two potential substitute species, which are economically interesting and presumably more drought-tolerant, than spruce: Silver fir (Abies alba) and Douglas-fir (Pseudotsuga menziesii). Since mixed-species forests are assumed to increase ecological stability of forests in relation to stress and disturbances and thus to reduce future risks, the drought-tolerance of these three conifer species was assessed in mixed stands. Aim The aim of this thesis was to provide an assessment of the growth potential these three species under future climatic conditions in the Black Forest. A direct comparison of their drought responses and an understanding of their interactions in mixed-species stands is still lacking. The responses to past extreme drought events were measured along altitudinal gradients through a dendrochronological analysis. Climate correlation analyses were used to quantify the potential effect of shifts in seasonality of precipitation and temperature on radial growth of these species for the past 60 years and under future climate scenarios. I also quantified the net effects of mixing these three species on radial growth in “normal” and in drought years. Research questions 1) Are Silver Fir and Douglas-fir more resistant and resilient to extreme drought events than Norway spruce across altitudinal gradients in the Black Forest? 2) How will these tree species react to future shifts in seasonality of precipitation and temperatures connected to climate change? 3) Does the mixing of the tree species influence their growth and resistance and resilience to drought? IX Study site and sampling design A total of 270 trees per species were sampled in 18 managed and mature mixed-species stands at the western slopes of the southern, central and northern Black Forest in southwest Germany.