Objective  Based on the data of 3 164 Larix olgensis sample trees and 3 574 Fraxinus mandshurica sample trees from 24 sample plots in mixed plantations from Maoershan Mountain Forest Farm and Yimianpo Forest Farm in Shangzhi City, Heilongjiang Province of northeastern China, the crown width models for Larix olgensis and Fraxinus mandshurica were developed.

  Method  By analyzing the variation rules of crown width of Larix olgensis and Fraxinus mandshurica in different mixed stands and the relationship between crown width with forest competition factors, the best model was selected from the basic models of six commonly used linear and nonlinear crown width models. The mixed proportion S_i and the tree position P in the mixed strip were taken as dummy variables, and other tree variables and stand variables were added. The crown width models of Larix olgensis and Fraxinus mandshurica were developed, and the fitting effects of the models were evaluated.

  Result  The crown width of Larix olgensis and Fraxinus mandshurica was different in different mixed proportion S_i and different position P in the mixed strip. The results showed that the ratio of tree DBH to dominant tree diameter (DDH) and the ratio of tree height to dominant tree height (HDH) were positively correlated with crown width, and the basal area of larger trees (BAL) was negatively correlated with crown width. Distance independent competition factors can reflect the competitive pressure of trees and have effect on crown width. The results showed that the crown ratio (CR) was positively correlated with crown width of Larix olgensis, the ratio of total tree height to DBH (HD) was negatively correlated with crown width of Larix olgensis, the dominant tree height of Fraxinus mandshurica (H_0Fra) was positively correlated with crown width of Fraxinus mandshurica, the HD was negatively correlated with crown width of Fraxinus mandshurica. For the crown width models of Larix olgensis and Fraxinus mandshurica with the dummy variable P and S_i, the R_a² was 0.564 2 and 0.545 9, and for the crown width models of Larix olgensis and Fraxinus mandshurica with the tree variables (CR and HD) and stand variable (H_0Fra), the R_a² was 0.6745 and 0.5896.

  Conclusion  The crown width models of Larix olgensis and Fraxinus mandshurica, including the dummy variable S_i and P, tree variables (CR and HD) and stand variable (H_0Fra), have good fitting effects and prediction accuracy. Therefore, the crown width models established in this study can well predict the crown width of Larix olgensis and Fraxinus mandshurica in mixed plantations, and provide a basis for further study on the crown structure in mixed plantations.