Objective This study aimed to investigate the effects of neighboring tree diversity and forest competition intensity on the tree growth in natural spruce-fir forests, providing a scientific basis for the structure management of spruce-fir forests.

Method Data from two fixed-sample surveys of a natural spruce-fir forest in Changbai Mountain of northeastern China in 2015 and 2018 were used to analyze the relationship between neighboring tree diversity, competition, and tree growth by a structural equation model.

Result (1) The neighboring tree diversity indexes for species, diameter classes, and tree height of the natural spruce-fir forests in the study area were all around 1.04. It indicated an overall even distribution of these three indexes, high degree of species mixture, and complex stand structure. (2) The total impact coefficients for species, diameter-classe, tree-height neighboring tree diversity indexes and the competition index on volume increment in the model were −0.001, 0.166, 0.073 and −0.489, respectively. It indicated that competition was the key factor affecting tree growth. (3) The increases in diameter-class and tree-height diversities both had positive effects on tree growth, with a direct effect of diameter-class diversity, and an indirect effect of tree-height diversity. The species diversity had a directly negative effect and an indirectly positive effect, exhibiting an overall negative effect. The increase of species diversity reduced competition intensity of trees in a stand, while the increase of tree-height diversity might lead to a differentiation of stand structure, which further promoted the growth of individual trees in a stand. (4) Growth pressure of trees in the research area may mainly come from trees of the same diameter-class. Small-diameter trees had poorer growth conditions and greater competition pressure, while large-diameter and medium-diameter trees were opposite.

Conclusion Selective cutting of the trees within the same or similar diameter-classes, and improving the level of diameter-classe and tree-height diversity within a stand, can reduce the competition level, promote individual tree growth, and enhance the productivity of spruce-fir stands.