- Wilson, Duncan Steil
The Graduate School, University of Maine
A theory is developed that explains plant self-thinning, which is based on certain allometric relationships within plants. The theory is based on two premises. First, there is a finite and measurable amount of available growing space on a site, which is partitioned among individual plants in varying amounts. The growing space is directly transferable between plants through competition. Second, biomass or volume increment for an individual plant is directly related to increases in the amount of occupied growing space. The amount of potential growing space for a site is linearly related to the site maximum leaf area. Similarly, the amount of growing space occupied by an individual plant is linearly related to its leaf area. The relationship between leaf area increment and biomass increment, however, is shown to have a non-linear form. The theory is formalized through a series of equations for red spruce and balsam fir in Maine, and parameter estimates are presented for these species.
A number of results follow that relate to self-thinning patterns: (A) The self-thinning intercept is directly related to the potential leaf area on a site. Thus, the stickability of a site can be manipulated through treatments that affect the potential amount of leaf area, such as fertilization. The intercept is not influenced by stand structural characteristics. (B) The self-thinning slope is species specific, and unrelated to site quality or stand structural characteristics. These results are shown to account for much of the experimental data from past self-thinning studies.
Implications of the theory were extended to account for patterns of conifer growth efficiency found in the literature. Growth efficiency is defined as the ratio of stem volume increment to leaf area. Previous explanations of growth efficiency patterns were based on the notion that trees have set carbon allocation priorities for the various structures within a tree, and that stem wood has a low priority. The theory's predictions are shown consistent with common patterns of growth efficiency, and presents an alternate explanation; thus the theory challenges the notion that trees have carbon allocation priorities, and presents an equally valid model based on plant allometry.