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Abstract:
1. Partitioning of tree mortality into different modes of death allows the tracing and mechanistic
modelling of individual key processes of forest dynamics each varying depending on site, species
and individual risk factors. This, in turn, may improve long-term predictions of the development of
old-growth forests.
2. Six different individual tree mortality modes (uprooted and snapped (both with or without rot as
a predisposing factor), standing dead and crushed by other trees) were analysed, and statistical models
were derived for three tree species (European beech Fagus sylvatica, hornbeam Carpinus betulus
and common ash Fraxinus excelsior) based on a repeated inventory of more than 13 000 trees in a
28 ha near-natural deciduous forest in Central Germany.
3. The frequently described U-shaped curve of size-dependent mortality was observed in beech and
hornbeam (but not ash) and could be explained by the joint operation of processes related to the six
distinct mortality modes. The results for beech, the most abundant species, suggest that each mortality
mode is prevalent in different life-history stages: small trees died mostly standing or being
crushed, medium-sized trees had the highest chance of survival, and very large trees experienced
increased rates of mortality, mainly by uprooting or snapping. Reduced growth as a predictor also
played a role but only for standing dead, all other mortality modes showed no relationship to tree
growth.
4. Synthesis. Tree mortality can be partitioned into distinct processes, and species tend to differ in
their susceptibility to one or more of them. This forms a fundamental basis for the understanding of
forest dynamics in natural forests, and any mechanistic modelling of mortality in vegetation models
could be improved by correctly addressing and formulating the various mortality processes.
