Ecophysiological traits determine successional strategy in montane rainforest tree species in Rwanda

Photosynthetic capacity of tree leaves is typically positively related to their nutrient content. This relationship is, however, often poorly supported for tropical trees, for which interspecific differences may be more strongly controlled by within-leaf nutrient allocation than by absolute leaf nutrient content. Tree species belonging to different successional groups differ in several ecophyisological traits related to their capacity to tolerate shade. To explore the influence of within-leaf nitrogen allocation and successional strategy regarding the photosynthetic capacity of montane rainforest tree species, we collected data on photosynthetic, chemical and morphological leaf traits of 12 tree species in Rwanda. Six species were classified as pioneers and six as climax, and all were studied in a cool, high-altitude montane rainforest (~2,500 m). Pioneer species showed a higher photosynthetic capacity and across species, interspecific variation in photosynthetic capacity was not related to total leaf nitrogen content but rather to different behaviors regarding within-leaf nitrogen allocation. The nitrogen allocated to the maximizing of photosynthetic capacities was a good predictor of photosynthetic capacities. Pioneer species showed a possible trade-off between investments into compounds related to photosynthetic capacity versus light-harvesting whereas climax species did not. We found that the carbon-gain hypothesis, defining shade-tolerance, did not apply for montane rainforest tree species and therefore more research following a trait-based approach is promising in order to better understand tropical montane carbon dynamics. Key! words:! Montane'rainforest','Successional'strategy','Photosynthesis','within'leaf' nitrogen'partitioning' ! Master thesis Camille Ziegler 2015 ! 2! Acknowledgments! I had been waiting for a great experience for my internship in both Sweden and Rwanda and I have to say I did not imagine it would be so enriching in several different aspects, thanks to many people I have worked with or encountered during this project. I first would like to thank my supervisor Johan Uddling Fredin for getting me interested in plant ecophysiology, giving me the opportunity to participate in such a project, supervising my work and reviewing this thesis in a very encouraging way. Many warm thanks and thoughts to Eric Mirindi Dusenge for being the best colleague and friend throughout the entire measurement campaign and beyond. Wishing you the best on your Canadian adventure. Thank you to all my co-workers from the University of Rwanda in Butare, especially Etienne Zibera, Pierre and Innocent who were central to this measuring campaign, taught me so much about the forest and with whom I shared great times of laughter. Thanks to Theresa Ibanez, Sofia Öberg and Sofie Fegerlund for those two months spent together in Rwanda and to my friends from Butare who made this experience unique. Thanks to all members of the AiroPlant research group at the Department of Biological and Environmental Sciences at the University of Gothenburg, especially Göran Wallin for sharing his experience about Rwanda, the project and for his thoughtful advice. I want to thank Brigitte Nyirambangutse and most of all Thomas Berg Hasper for taking their precious time to explain things to me, Mats Räntfors for being so kind and running the nitrogen analysis and of course Ellen Pagel, Felicien Uwizeye Karakezi, Theresa Ibanez, and Yann Cochet for the time spent sharing our workroom. I want to thank the Department of Biology of the University of Rwanda (UR) in Butare for letting us use the laboratory, always in a friendly environment. I also want to thank the Rwandan Agricultural Board (RAB) for welcoming us and the Rwandan Development Board (RDB) for authorizing the data collection in Nyungwe National Park. Last but not least, I would like to thank my family and friends and all those who encouraged and followed me, even from a great distance.