Assessing alternative allometric algorithms for estimating leaf area of Douglas-fir trees and stands

Allometric approaches based only on tree diameter at breast height (DBH) have been used to estimate leaf biomass and leaf area index (LAI) in coniferous forests. This approach has been suggested to sometimes produce unrealistically high LAI estimates, notably in old-growth stands in the Pacific Northwest (PNW). Leaf area to sapwood area relationships offer an alternative basis for estimating LAI and a number of empirical and modeling approaches have been used for generating tree-level sapwood area and leaf area estimates. To evaluate effects of the alternative assumptions among these approaches, we compared five algorithms for estimating leaf area per tree and stand-level LAI in stands dominated by Douglas-fir (Pseudotsuga menziesii var menziesii). Initially, 383 trees representing the major species and the complete range of tree sizes in the study area were examined for species, DBH, crown ratio, and sapwood area. There were significant species-specific relationships between sapwood and DBH for these trees. Neither crown ratio nor the ratio of observed tree height to expected tree height were effective in predicting the sign and magnitude of the residuals in the sapwood area/DBH relationships. Nine stands (three young, three mature, and three old-growth) in and near the H.J. Andrews Experimental Forest in the western central Oregon Cascades were then studied with species, DBH, and crown ratio recorded for all trees in a set of subplots. At the stand level, the LAT algorithms based on sapwood area at breast height produced significantly lower LAI estimates than the DBH-alone algorithm in mature and old-growth stands. The algorithm relying on estimated sapwood area at the base of the crown further reduced LAI estimates relative to those based on sapwood area at breast height. Although the differences between the DBH-based and sapwood area-based approaches tended to be greatest in the older stands, estimated LAI was higher in the older stands than in the younger stands for all algorithms. Comparative studies using allometric, litterfall, and optical approaches to LAI estimation are needed to resolve issues such as trends in LAI with succession and maximum possible LAI in Douglas-fir forests. (C) 2000 Elsevier Science B.V. All rights reserved.