Basal Area Increment and Growth Efficiency as Functions of Canopy Dynamics and Stem Mechanics

Crown and canopy structurecorrelate with growth efficiency and also determine stem size and taper as described by the uniform stress principle of stem formation. A regression model was derived from this principle that expresses basal area increment in terms of the amount and vertical distribution of leaf area and change in these variables during a growth period. This model was fit t o data collected from five studies analyzing various effects of cultural practices, atmospheric deposition, and stand density on the growth of loblolly pine (Pinus taeda), slash pine (P. eiliottit7, or lodgepole pine (P. contorts). For both southern pines, the model was fit to individual-tree data and plot totals measured on 1to 4-yr-old trees and from 12t o 20-yr-old trees. The model was also fit t o data measured on approximately 90-yrold lodgepole pine trees in northern Utah. The model explained an average of 85% of the variation in individual-tree basal area increment and an average of 86% of variation in gross basal area increment per hectare. By comparison, leaf area alone explained an average of 54% of the variation in the increment of individual-tree stem volume and an average of 50% of the gross increment in stem volume per hectare.Thevalue of the exponent in the model implicates stem mechanics in stem growth, and fitted exponents were all within 7% of the exponent derived from the uniform stress model. lsolines of basal area increment and basal area growth efficiency based on fitted equations indicate that increments in the product of leaf area and height to center of leaf area must increase yearly for either basal area increment or growth efficiency to be constant with age. FOR. SCI. 50(1):106-116.