Diameter-based biomass regression models ignore large sapwood-related variation in Sitka spruce

Abstract

Precise estimates of biomass are needed in productivity and nutrient cycling studies, and for improved estimates of potential productivity. Improvements in prediction of foliage and branch biomass were sought by comparing multiple regression models using stem diameter, sapwood radial thickness, and tree height as independent variables in stands of Sitka spruce (Piceasitchensis (Bong.) Carr.) in southeast Alaska. Five sites were sampled by stratifying trees into four diameter and three sapwood-thickness classes. Within stands, sample trees with thick sapwood consistently had 2–3 times more foliage and branch biomass than paired trees with thin sapwood but nearly equal diameter. Inclusion of both diameter and sapwood thickness in equations increased precision of foliage and branch biomass, leaf area, and net primary productivity by 15–31% and reduced standard error by 35–48% when compared with equations containing only diameter as an independent variable. Height growth over the last 30 years of intermediate and codominant trees with thick sapwood was 12–27% greater than that of paired trees with thin sapwood but nearly equal diameter at breast height. The addition of total tree height to multiple regression models, however, had little effect on their precision. Stem biomass equations were not improved by including tree height or sapwood thickness. The use of a diameter – sapwood thickness sampling matrix for construction of biomass equations may reduce the sample size needed and result in equations with wider application.