Abstract
Tree mortality is a fundamental process governing forest dynamics, but understanding tree mortality patterns is challenging because large, long-term datasets are required. Describing size-specific mortality patterns can be especially difficult, due to few trees in larger size classes. We used permanent plot data from Nothofagus solandri var. cliffortioides (mountain beech) forest on the eastern slopes of the Southern Alps, New Zealand, where the fates of trees on 250 plots of 0.04 ha were followed, to examine: (1) patterns of size-specific mortality over three consecutive periods spanning 30 years, each characterised by different disturbance, and (2) the strength and direction of neighbourhood crowding effects on size-specific mortality rates. We found that the size-specific mortality function was U-shaped over the 30-year period as well as within two shorter periods characterised by small-scale pinhole beetle and windthrow disturbance. During a third period, characterised by earthquake disturbance, tree mortality was less size dependent. Small trees (<20 cm in diameter) were more likely to die, in all three periods, if surrounded by a high basal area of larger neighbours, suggesting that size-asymmetric competition for light was a major cause of mortality. In contrast, large trees (≥20 cm in diameter) were more likely to die in the first period if they had few neighbours, indicating that positive crowding effects were sometimes important for survival of large trees. Overall our results suggest that temporal variability in size-specific mortality patterns, and positive interactions between large trees, may sometimes need to be incorporated into models of forest dynamics.
Highlights
Size-specific mortality rates of trees have a fundamental influence on the structure [1,2,3] and composition [4,5] of forests, influence geographical range limits [6], determine forest carbon storage capacity [7], and can be sensitive to climatic change [8,9]
Such a pattern is thought to be largely a consequence of asymmetric competition for light causing relatively high mortality of small trees and exogenous disturbance often causing relatively high mortality of large trees, while trees of intermediate size are less affected by either process [4,13]
Because sizeasymmetric competition is less important for taller trees, which are on average less shaded by neighbours, mortality rates should progressively decline with tree size if size-asymmetric competition is the dominant cause of tree mortality [24]
Summary
Size-specific mortality rates of trees have a fundamental influence on the structure [1,2,3] and composition [4,5] of forests, influence geographical range limits [6], determine forest carbon storage capacity [7], and can be sensitive to climatic change [8,9]. A U-shaped size-specific tree mortality pattern is sometimes observed when measurements are made over large areas or long time frames [10,11,12]. Such a pattern is thought to be largely a consequence of asymmetric competition for light causing relatively high mortality of small trees and exogenous disturbance often causing relatively high mortality of large trees, while trees of intermediate size are less affected by either process [4,13]. Because sizeasymmetric competition is less important for taller trees, which are on average less shaded by neighbours, mortality rates should progressively decline with tree size if size-asymmetric competition is the dominant cause of tree mortality [24]
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