Competition is critical to the growth of Larix gmelinii and Betula platyphylla in secondary forests in Northeast China under climate change

Individual tree growth is facilitated by biotic and abiotic variables and their interactions. Understanding the relationships between biotic and abiotic variables is critical for tree growth research. By using the random forest model for variable selection, two piecewise structural equation models with random effects were constructed to explore the effects of biotic and abiotic factors on individual tree growth (5-year diameter growth increment) of the dominant tree species, Dahurian larch (Larix gmelinii) and white birch (Betula platyphylla), based on the National Forest Continuous Inventory dataset from 2010 to 2015 in the Eastern Da Xing'an Mountains, Northeast China. The results showed that (1) individual tree growth was primarily determined by fixed effects, with biotic variables dominating over abiotic variables and the sum of the basal areas of trees larger than the subject tree had the greatest influence on diameter growth among all the variables. (2) Among the variables, the total effects of structural heterogeneity on the two tree species were positive, whereas the total effects of altitude, competition, species diversity, and the ratio of the Dahurian larch basal area were negative. (3) The most significant climate variables varied between the two tree species. The average precipitation in spring restricted Dahurian larch growth, whereas the annual mean temperature difference promoted white birch growth. (4) The effects of abiotic factors on tree growth could be enhanced or weakened under the mediation of biotic factors. The results of this research enhance our understanding of the multiple ecological mechanisms of tree growth drivers and can provide a foundation for future scientific management of natural forests in Northeast China.