One of the great biological principles is that the development, adaptation, and survival of living organisms are the results of closing down options. The genome represents the impractical unedited totality of what the organism is capable of. Successful organisms do more than possess, express, and pass on the right genes -they refrain from expressing inappropriate potential. Music provides an analogy. Western music uses the 12 notes of the chromatic scale. Imagine sitting at the keyboard of an

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... gan. Simultaneously hold down the 12 keys corresponding to the chromatic scale. Cease playing 3 minutes later. Within that cacophonous 3 minute block of sound are all possible 3-minute musical works. But "Tea for two" (Tatum 1933) is 3 minutes of musical genius, and why? Overwhelmingly, because of the notes that were not played. So it is with living organisms -the genome is the chromatic scale, the surviving organism is the harmonised musical line. The selectivity that orchestrates expression of genomic potential comprises cellular processes that repress and destroy. Might it not be that ageing is the long-term revelation of these negative, but nonetheless essential, forces that animate the machinery of living matter?" Howard Thomas (2003) Do green plants age, and if so, how? I wish to thank Jürg Stöcklin for the opportunity to accomplish a challenging PhD in such an interesting field of research. Your supervision was full of enthusiasm and encouragement, and you provided a work environment that motivated and inspired me during the past years. Thank you a lot also for the rapid and excellent proof reading. Further, I am grateful to Pierre Taberlet for his scientific advice, for motivating discussions during EcoChange meetings and for co-examining my thesis. Great thanks to Niek Scheepens not only for sharing his vast knowledge with me, but also for his valuable proof reading, statistical advice, assistance during field work, and finally for cheering me up and being a good friend. I am grateful to Georg Armbruster for sharing his knowledge on genetics, for his assistance with laboratory work and for being a highly regarded colleague. I wish to express my thanks to Christian Körner, who was always open for inspiring discussions and who gave me valuable advice. I was very happy to discover the beauty of the Arctic and the Carpathians together with various colleagues from the EcoChange project and I especially wish to thank Inger Alsos, Terry Callaghan and Mihai Puşcaş for field assistance and hosting me in Longyearbyen, Tromsø, Abisko and Cluj, respectively. The enormous amount of fieldwork during this PhD could only be realized with the help from my colleagues and friends Willem van Esch, Niek Scheepens, Judith Trunschke, Daniel Scherrer, Mihai Puşcaş, Christoph Gloor and my parents. Thanks a lot! Many thanks to Ludovic Gielly, Delphine Rioux, Benedicte Poncet and Aria Minder for their assistance with the AFLPs and for teaching me laboratory skills. I would like to acknowledge