Stem taper equations for three major conifer species of Northeast China

Dahurian larch (Larix gmelinii Rupr.), Korean spruce (Picea koraiensis Nakai), and Manchurian fir (Abies nephrolepis Maxim) are valuable tree species of northeast China. Management tools are required for these species to support the industrial and ecological development of forest ecosystems of China. This study aimed to develop taper equations for these species. Five well-known taper equations from two groups (segmented and variable form taper models) were evaluated using the fixed- and mixed-effect modeling. The models were independently fitted utilizing diameter and height datasets comprising 473 trees. A third-order continuous autoregressive error structure (CAR (3)) was used to account for autocorrelation. The model of Clark et al. (1991. Stem profile equations for southern tree species. Asheville, NC: USDA Forest Service. Research Paper, SE-282. doi:10.2737/SE-RP-282) was superior to other models for estimating diameter, merchantable volume, and stem volume when upper stem diameter at 5.3 m was available or predicted. After model comparison, the best Clark et al. (1991) model was refitted as a nonlinear mixed-effects model. This study also demonstrated that the fixed-effect model of Clark et al. (1991) was more accurate than Kozak (2004) mixed model when the later was calibrated for upper stem diameter at 5.3 m with validation data.