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Individual trees with a high age are a characteristic feature of old-growth forests. However, for plants, and here especially for trees, the terms senescence, age and even death are difficult to define. This is partly because humans tend to quantify tree age according to astronomical cycles, more specifically using annual tree rings, while for trees age is a relative quantity related to the pace of ontogenetic deve-lopment. The slower the biological processes unfold, the older individual trees become. This is why the oldest living trees with an age close to 5,000 years are found on extreme sites. Senescence is based on the principle of ‘programmed cell death’, the timing of which may vary between individual cells, tissues types, organs and whole plants. For example, an earlywood tracheid dies after 20 days before it becomes functional as water-conducting tissue, while a neighboring parenchyma cell may survive for 50 years before serving the purpose of mechanical stabilisation as a dead cell. The individual ramets (shoots) of a clonal tree die after 50 years, while the genet may survive for over 10,000 years. In this chapter, we critically discuss different concepts of longevity and review what is known about the variation in and limits to the longevity of woody plants.
