The influence of initial and post-thinning stand density on Douglas-fir branch diameter at two sites in New Zealand

Abstract

Branches support the foliage needed for tree growth, but if the branch diameter is too large it may constitute a defect when the tree is assessed for timber quality and when the tree is processed into boards. Consequently branch diameter is an important consideration when developing a silvicultural regime. The objectives of this study were: (a) to develop site-specific models to predict branch diameter variation with position on the stem; and (b) to use the models to evaluate the influence of initial and post-thinning stand density on branch diameter in unpruned stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) at two sites in New Zealand. Branch diameters were measured using a photogrammetric technique, TreeD, on pre-selected sample trees from unpruned treatments in two silviculture experiments, one in the North Island and one in the South Island of New Zealand. The data were used to develop site-specific models to predict branch diameter along the stem. The models were then used to interpret response to a thinning at a stand age of approximately 10 years when the base of the green crown was still close to ground-level, and to make comparisons with a branch diameter limit of 40 mm. The models developed indicated that previously formed branches were influenced by the increased growing space created by thinning. At all three post-thinning stand densities (250, 500 and 750 stems ha−1), the diameter of some branches on the mean tree within a stand were likely to exceed 40 mm. In unthinned treatments, the model indicated that an initial stand density of at least 1333 stems ha−1 would be needed to keep branch diameters on the mean tree below 40 mm along the whole stem at age 29 years. This study indicates the importance of considering initial stand density, post-thinning stand density and timing of thinning when designing a silviculture regime that aims to control branch diameter.