Effects of tree vigor, competition and stand conditions on branch diameter for mixed plantations of Fraxinus mandshurica Rupr. and Larix olgensis Henry in Northeast China

Recently, the connection between multispecies tree planting and the production of high-quality wood has received much attention. The branch diameter (\(BOD\)) is a crucial factor that influences tree growth and wood quality. Research on \(BOD\) in mixed plantations has lagged behind that on monocultures, where the effects of trees, stands, and competitive forces on the \(BOD\) have been extensively studied. In this paper, 105 Manshurian ash (Fraxinus mandshurica Rupr.) and 105 Changbai larch (Larix olgensis Henry) trees from different row-wise mingling patterns on plantations in northeastern China were destructively sampled to construct two-level mixed-effects \(BOD\) models. We examined how tree vigor and species mixture affect \(BOD\) at various positions within the crown. The results showed that in addition to tree vigor, competition and stand structure had impacts on mixed plantation \(BOD\). Furthermore, in mixed plantations, the \(BOD\) of Changbai larch was affected by intraspecific and interspecific competition, while that of Manshurian ash was affected only by intraspecific competition. The \(BOD\) in the upper crown was strongly influenced by the position of the branches, while the \(BOD\) in the lower crown was strongly influenced by tree vigor, competition, and stand structure. The introduction of a two-level mixed-effects model significantly improved the prediction accuracy of \(BOD\), for which the \({R}^{2}\) values were 0.85 and 0.88 for Manshurian ash and Changbai larch, respectively. The size of branches is an important factor affecting wood properties and prices, and it is a variable that is important to control. The quantitative simulation of \(BOD\) provides a theoretical basis that can inform silvicultural and management plans and that can determine the appropriate proportion of species based on actual timber quality requirements and stand development needs.