Modelling specific gravity and diameter inside bark of western hemlock and Sitka spruce growing in southeast Alaska

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Sitka spruce (Picea sitchensis (Bong.)) are two commercially important species in Alaska, with harvests beginning to focus on naturally regenerated young-growth. We developed within-tree models of ring specific gravity (SG) and diameter inside bark (DIB) for young-growth western hemlock and Sitka spruce. Eight even-aged stands (age <75 years) in southeast Alaska were felled and disks collected from multiple height levels; 128 trees and 451 disks were collected for western hemlock, and 217 trees and 952 disks were collected for Sitka spruce. Radial strips were prepared and scanned using X-ray densitometry. We fitted nonlinear mixed-effects models to the data, with cambial age, height within tree, and dominance class used as explanatory variables. The R 2 values (fixed effects only) for the SG models were 0.48 and 0.42 for western hemlock and Sitka spruce, respectively. The corresponding fit indices (R 2 ) for the DIB models were 0.86 and 0.85 for western hemlock and Sitka spruce, respectively. Tree maps depicting the within-tree variation in SG showed more variability in Sitka spruce than in western hemlock. The wood and growth properties of young-growth trees in Alaska will continue to become more important as the US Forest Service transitions away from harvesting old-growth trees.