ObjectiveCrop tree release (CTR) based on single tree is an important way to realize close-to-nature forest management. In order to reveal influences of CTR on the vertical structure of natural secondary forests, natural secondary Quercus mongolica forest in the northern Changbai Mountains was used as research objects, to investigate the response of forest stand structure and single-wood growth to the different intensity tending, which can provide basic support for developing scientific and rational management.

Method16 sample plots of 0.09 ha (30 m×30 m) were set up in the Jilin Wangqing Forestry Bureau in August 2013, to carry out crop tree release and thinning test. According to the thinning intensity, 4 treatments were set as: weak (5%), mild (10%), moderate (20%) and control, each treatment had 4 replicates. All sample plots were resurveyed in 2016. The stands were divided into upper, middle and lower storeys according to the canopy competition height (CCH) in stand. The effects of different tending thinning intensity on each forest storey height, tree species composition, competition and growth were analyzed.

Result(1) the CCH of upper, middle and lower storeys was 15.27−16.12 m, 8.76−9.65 m and 2.95−3.37 m, respectively. Thinning increased the height of upper and middle forest storeys, upper forest storey height was increased from 15.42 to 17.21 by moderate thinning, which was significantly higher than other treatments, while the height of the lower forest storey remained basically unchanged. (2) Thinning adjusted the proportion of tree species in the upper and middle forest storeys. the proportion of Betula platyphylla was decreased while that of Quercus mongolica was increased in upper forest storeys, and the proportion of Quercus mongolica increased with the increase of thinning intensity. The proportion of other wood decreased while that of Pinus koraiensis increased. (3) The average DBH of upper storeys was improved under moderate thinning, with an increase of 0.57 cm, but decreased under other treatments. The average DBH of middle storey was only improved under mild thinning. Changes on average DBH of lower storey was opposite to that of upper forest storey. The proportion of upper and middle forest storeys of forest stands increased. The proportion of forest storeys in the weak, mild and moderate thinned sample plot increased by 1.92%, 11.52% and 13.15%, respectively. (4) Thinning significantly reduced the competition index of forest trees in each forest storey. The lower forest storey had the most positive response to crop tree tending and thinning. The forest competition index of lower storeys by weak thinning decreased from 0.634 to 0.455, which was the most significant. (5) DBH periodic growth rate and volume periodic growth rate of forest storey decreased with the increase of the height of forest storey. The DBH periodic growth rates of the upper, middle and lower forest storey were 1.22%−1.96%, 1.94%−2.59%, 4.02%−8.17%, respectively. The volume growth rate were 1.74%−4.10%, 3.50%−5.14%, 10.12%−18.97%, respectively.

ConclusionCTR can significantly affect the growth and structure of different forest storeys of secondary Quercus mongolica forest. On the whole, moderate thinning (about 20% of thinning intensity) is suitable crop tree thinning intensity for secondary Quercus mongolica forest in study area. The specific tending methods and follow-up time are still based on long-term observations of forest stand dynamics.