Objective To provide the reference basis for decision-making for the close-to-natural management of plantations, the changes of forest quality and the development of tree species were analyzed after the close-to-natural transformation of Pinus massoniana.

Method In 2007, 20 experimental plots for close-to-natural transformation of Pinus massoniana single species plantation were set up in the Tropical Forestry Experimental Center of Chinese Academy of Forestry in Pingxiang County, Guangxi , including16 operational plots and 4 control plots. In 2007, the first comprehensive survey was done to obtain basic data, and the implementation of selective cutting for target tree management was carried out in the winter of the same year. In the spring of 2008, trees of six broad-leaved species were planted in the stand. In 2016, the second felling for target tree tending was implemented. After 2007, the re- survey of the sample plot was carried out every 2 years, and a total of 7-phase monitoring data were obtained. Statistical calculation was made for the important value index (IVI), average diameter at breast height and pure growth of standing volume based on the monitoring data, and the effect of close-to-natural transformation of Pinus massoniana single species plantation towards multi-species mixed forest was analyzed and evaluated based on tree species composition, stand structure, stand growth, soil and microorganisms development in forest.

Result After 13 years of close-to-natural transformation of Pinus massoniana pure plantation, the stand gradually transited to the structure of uneven-aged multi-species mixed forest. The IVI of the 6 tree species enriching planted under the forest were in the top 10 of IVI series, and the natural regeneration of Styrax tonkinensis and Vernicia fordii species appeared in the stand. The distribution of diameter at breast height of Pinus massoniana transformed forest was enlarged from 5 cm to 55 cm, showing the rapid development trend of mixed uneven-aged forest under the promotion of management measures. However, the untreated control stands still showed diameter class distribution of single species even-aged stand. The growth of the transformed stand was significantly higher than that of the control stand, and the growth of the stand was mainly concentrated in the trees with larger diameter, which improved the economic value of forest woods. In addition, the soil microbial content and soil physical and chemical properties of the transformed stand were significantly improved than those of the control stand.

Conclusion Adjusting tree species composition is the most important management measure in Close-to-natural transformation of forest. Enlarging the distribution range of diameter class structure is the basic target of close-to-natural transformation of conifer even-aged plantation. Close-to-natural transformation increases significantly the forest growth, promotes the development of soil micro-organisms and improves the quality of forest ecosystem.