Effects of spatial structure on DBH increment of natural spruce-fir forest

In order to explore the effects of the spatial structure of natural spruce-fir forest in Changbai Mountain of northeastern China on the individual DBH increment, two typical plots of natural spruce-fir forest were set in Jin'gouling Forest Farm in Wangqing County of Jilin Province of northeastern China to calculate the mingling and the neighborhood pattern at the stand and individual levels and discuss the distribution of spatial structure. Then, the effects of different spatial structure of spruce and fir on the DBH increment were analyzed combined with the mean values of the mean DBH increment of spruce and fir in the latest 5 years. The results showed that the average neighborhood patterns of both plots were about 0.5, indicating a random distribution. The average mingling of both plots were 0.65 and 0.73, respectively, showing intense mixed state. The largest tree density appeared when neighborhood pattern and mingling were 0.5 and 0.75, respectively. In terms of stand, the average DBH increment of spruce and fir in plot 2 was slightly higher than plot 1. The random distribution of fir and spruce had the largest DBH increment. With the increase of mingling of spruce and fir, the DBH increment of spruce and fir showed an increasing trend, suggesting that the increase of mingling can promote the growth of spruce and fir. In addition, the promotion was stronger for fir, showing that its growth is more sensitive to the intensity of the mixed.