Response of marginal height costs and marginal height benefits to competition

Abstract

Height growth in trees is expensive in terms of the amount of stem tissue required to maintain productive tissue in sunlight. However, shifting allocation from stem support to leader growth and foliage production may allow trees to minimize shading effects on photosynthesis, especially for shorter trees within the population. This hypothesis was evaluated with data collected in competition studies for red alder (Alnus rubra Bong.), Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco), and red pine (Pinus resinosa Ait.). Stem allocation per unit height growth was defined as a marginal height cost, and foliage added per unit height growth was defined as marginal height benefit. A symmetry index was calculated as a simple function of the cumulative relative costs and benefits of the shorter half of trees within plots. Analyses of variance indicated that marginal height costs and benefits significantly increased with age at rates unique to the different levels of competition. It also indicated that marginal height costs were related to the square of initial height at levels also unique to the competition treatments. The effects of competition on the symmetry indexes for marginal height costs and marginal height benefits significantly interacted with age for red alder and Douglas-fir, but not for red pine; for red pine, the indexes only varied with age. The slope between the two indexes was nearly one for all three species, indicating that marginal height costs and benefits change congruently with competition. The results did not support the hypothesis that shorter trees in competing populations change stem allocation to favor foliage growth. The correspondence between the symmetry indexes for marginal height cost and marginal height benefit was apparently due to coordinated development where population effects on how much and where leaf area is added with height growth determine how much stems need to grow to counter the changes in mechanical forces created by the change in crown dimensions. The results provide additional evidence that tree form is the result of coordinated development, not allometry between trees foliage and structural components.