Abstract:

Cinnamomum carnphora is an important timber tree and greening tree species in South China. At present, Cinnamomum carnphora plantations generally have problems such as uneven and slow growth, and low timber outturn rate. Improving the forest stand structure of camphor plantations and the forest stand productivity, and cultivating large-diameter timber have become the main tasks of camphor plantation cultivation and transformation. Nine fixed observation plots with 16-year-old Cinnamomum carnphora artificial forest were established, each had the size of 20m×30m. According to the total number of standing trees in the plots, three groups of thinning experiments were carried out: intensive thinning (thinning 40%), light thinning (thinning 20%) and control group (no thinning). At the same time, analytic trees were harvested to analyze the growth process of camphor plantation. The dynamic changes of the diameter structure of camphor forest stand before and after thinning were discussed. (1) For the plots with light thinning and intensive thinning, after two years of thinning, the annual growth of DBH (breast-height diameter) of single plant was 1.18cm and 1.42cm, significantly higher than that of the control group and before thinning; there was no significant difference in the tree height growth after thinning, and the annual increment was about 0.5m; and the crown width growth showed significant differences, the average crown width of the intensity thinning plots was 3.8m, 31% higher than that of the control group. (2) By analyzing the stand DBH structure of the three groups of plots, it was found that the growth potential of the pressed trees in the plots was obviously insufficient, and about 25% of the lower stands (DBH less than 12cm) had no increase in DBH; however, the number of trees with DBH above 20 cm increased significantly in intensive thinning plots, reaching 420 trees/ hm² and accounting for 47.7% in each plot, while the number in light thinning and control group was 285 and 165 trees/ hm², respectively, accounting for 26.3% and 11.9% in each plot. (3) The changes of stand volume in the three groups of plots were compared and analyzed, and the results showed that the annual growth of stand volume in the plots with light thinning and intensive thinning reached 21.91m³/hm² and 25.50m³/hm², respectively, which was significantly higher than that in the plot without thinning (18.34m³/hm²). The overall growth of mid-aged camphor forest stands can be achieved by 40% intensive thinning，and the growth rate of DBH and stand volume are significantly higher than those of the no thinning stand. The growth potential of pressed wood in the no thinning stand is insufficient, which will not only stop its own growth but also affect the growth of the whole stand. Thinning reduces the competition among trees to a certain extent, increases the illumination in the forest, improves the sanitary conditions of the forest, provide more nutrients and growth space for the reserved trees, thus promoting the growth of the forest stand. However, the recovery time of the growth potential and thinning frequency need to be further studied.

Key words: Cinnamomum carnphora plantation, thinning, growth regularity, forest cultivation