Intraspecific changes in forest canopy allometries during self-thinning

Abstract

1. Leaves are the key active sites for energy and matter transfer in plants, and therefore the total one-sided area of leaves per square metre of ground (leaf area index, LAI) is a critical variable used for scaling up from plant to ecosystem level properties. 2. A commonly held belief is that LAI remains constant during the self-thinning phase of stand development, and allometric relationships between tree density, mean tree size and leaf area (or leaf mass) are frequently used to provide a mechanistic explanation for the observed thinning relationships. 3. We investigated variations in leaf allometry, stand LAI and other canopy properties along a developmental sequence of naturally occurring mono-specific self-thinning stands of mountain beech (Nothofagus solandri var. cliffortioides) in New Zealand. 4. Stand age had a consistent significant effect on both the slope and the intercept of both the leaf mass and the leaf area allometric relationships (increasing slope, decreasing intercept). This was due to trees of a given basal area having more leaf area (or leaf mass) when growing in a neighbourhood of smaller trees, than equivalently sized individuals surrounded by a lower density of trees larger than itself. 5. A peaked (non-constant) relationship was observed between LAI and stand age during self-thinning, with the mean LAI values peaking in intermediately aged large sapling stands (7.08) compared with the small sapling stands (5.42) and older pole stands (5.70). Stand level leaf mass per unit area (LMA) varied with canopy height and stand age, being highest at the top of canopies and in older stands. 6. We propose that the observed increase in canopy depth during the early stages of stand development (but after canopy closure has occurred) could be driving the increase in stand LAI due to a more even distribution of light within the canopy. 7. Our findings suggest that the commonly held assumption of constant LAI should be abandoned in favour of the notion of leaf area being at a dynamic maximum during self-thinning, with the maximum possible leaf area being influenced by age-related changes in canopy structure that occur during stand development. © 2008 The Authors.