Branch characteristics of widely spaced Douglas-fir in south-western Germany: Comparisons of modelling approaches and geographic regions

Models of Douglas-fir branch and whorl characteristics were developed from contrasting spacing experiments in southwest Germany. The dataset was based on 100 young (20–30 years old), unpruned and partially pruned trees from a 100, 200, and 1200 stems ha−1 spacing experiment on Douglas-fir that was replicated 3 times across the region. The material was used to predict (1) the number of branches whorl−1, (2) branch angle, (3) status (living/dead) of the branches within the living crown, (4) maximum branch diameter whorl−1, and (5) relative diameter of branches within a whorl. For each of these models (except branch status), both a linear and nonlinear, generalised hierarchical mixed effects equation was developed. The comparison of the linear and nonlinear approaches showed that both had a relatively similar level of bias, but the nonlinear equations generally performed better (reduction in mean absolute error of 1.1–69.5%). Overall, individual branch and tree properties were sufficient to give logical and precise predictions of the branch characteristics for the models across the range of sampled stand densities. In addition, the models showed a similar behaviour compared to models on Douglas-fir crown structure from the Pacific Northwest, USA. This suggests that the allometric relationship between tree size and branch characteristics for a given species may be relatively consistent across regions, even ones with highly contrasting growing conditions like in this study. The models performed well across a range of stand conditions and now will be further integrated into an individual tree growth and yield simulations system.