Study on Diameter Structure and Growth Dynamics of Mixed Quercus mongolica Broad-leaved Natural Stands After Tending Thinning

We established 3 different thinning regimes (light, 12%; medium, 30%; heavy, 41%) and a no-thinning(control) treatment in 30 years old mixed Quercus mongolica broad-leaved natural stands. The effects of thinning on dynamic changes of diameter structure and stand growth were analyzed based on stand inventory for 23 years after thinning. The results showed that medium thinning and light thinning could promote the increase of stand diameter and volume. Compared with the control, the DBH of medium and light thinning plots increased by 1 cm and 1.7 cm, respectively, and the volume increased by 8.5% and 16.9%, respectively, after thinning for 23 years. The annual growth rate of volume of the thinning plots was significantly higher than that of the control in the first 9 years after thinning, while, it was lower or similar to that of the control during the 11^th to the 23^rd years after thinning.The mean death rate of the intensively thinning plots was 19.83% of that of the control in the first 11 years after thinning, and the mean death rate of the medium and light thinning plots were 23.93% and 67.87% of that of the control, respectively. While in the 11−23 years after thinning, the mean death rates of the thinning plots were slightly higher than that of the control. Thinning shifted the Weibull distribution curve rightwards, and the movement amount increased with thinning intensity. Location parameter 'a' was greatly affected by thinning and forest age. Therefore, location parameter 'a' could not be set as a fixed value when Weibull parameter was used to fit the stand diameter structure. In a word, medium and light thinning could promote the growth of stand DBH and volume, reduce the death rate of trees, but this effect was not always sustained, and the effect gradually decreases about 10 years after cutting. Therefore, in order to improve the stand productivity, it was suggested to set the planting thinning interval as about 10 years. This study can provide references for scientific and reasonable determination of thinning intensity and thinning cycle.