Objective  In this paper, the forest space utilization model was established to provide the basis for the comparison of productivity among different tree species in the natural mixed forest, and provide reference for the judgement of tree maturity.

Method  The combination of tree growth and canopy size was used to define the forest space utilization. Based on the 2 268 individuals of 20 standard sites, which located in spruce-fir and broadleaved mixed forests in Changbai Mountain area of northeastern China, a mixed-effects model of forest space utilization was established to fit the spatial utilization of different species.

Result  (1) The correlation coefficient between DBH and the ratio of volume growth to the crown projection area was the highest, so it is suitable to be the calculation index. (2) Through the stepwise regression, the tree DBH, DBH square (represent area), tree height, crown width, plot accumulation, proportion of coniferous trees, slope gradient and competition index were selected as the independent variables of forest space utilization. (3) Mixed effects model was developed, including the DBH square and intercept random effect parameters by species groups and the exponential function heteroscedasticity structure. The results showed that the mixed effects model was better than the general linear model both in the modeling data and the test data. (4) Using the mixed effect model to predict the research area data, the DBH of the highest spatial utilization of each tree species was obtained: spruce (Picea koraiensis) was about 40 cm; fir (Abies holophylla), Korean pine (Pinus koraiensis), larch (Larix olgensis), and medium-speed broadleaf trees were about 37 cm; slow broadleaf trees were about 32 cm.

Conclusion  The mature DBH obtained from the forest utilization model is based on the productivity of the unit nutrient space, so that the comparison between different tree species is more reasonable. The calculation results are in accordance with the general forest rule, so it can be used as a reference to judge the tree maturity, optimize the stand structure in the coniferous and broadleaved mixed forest in that area.