关键词: 杉木人工林, 立地指数, 间伐效果, 生物量

Abstract:

Based on the six year localized observation data obtained at Daweishan Forest Experimental Station of Central South Forestry University, the thinning effects in Chinese fir (Cunninghamia lanceolata) plantations with various sites and thinning densities were studied using biomass increment as the quantitative indexes. Results showed that regardless of site conditions, the trees grew more vigorously after thinning was conducted, the biomass increment greater than that of the check plots at both the individual and stand level. For individual plants, the biomass increment responded positively to the thinning intensity, where the heavily thinned plots demonstrated 1.63～2.00 times more increment as against the check plots. However, at the level of stand, the biomass increment declined as the thinning intensity increased. While the biomass total in the newly thinned plots and the total net biomass increment of six years' growth post to the thinning were both greater than the increment in the check plots, no significant difference was observed for both cases with light thinning. The biomass total of heavily thinned plots was by 14.23～25.28t per hm² less than that of lightly thinned plots regardless of site index (SI) class. The stand productivity of the thinned plots followed the same pattern of change as that of the stand biomass increment, namely, the productivity increased when the thinning intensity decreased during the six years post to the thinning, with the productivity by heavy thinning by 2.38～4.21t per hm² and per year lower than that by light thinning and the productivity by light thinning having no significant difference from the check plots. In a word, thinning in general lengthened the prime stage of Chinese fir growth, and the lenthening effect positively correlated to the intensity of thinning. Besides, thinning helped to raise the stemwood percentage of stand, which increased with the increase of the thinning intensity. when the best effect of thinning was analyzed, it was found that the tenth year after planting was the potimal time for the first thinning, considering three SI (Site index) classes, the total biomass increment at the individual and stand level and the overall productivity of the stand. It follows that in intensively managed plantations, short intervaled light thinning is desirable. The time of the thinning should depend on the SI class. For the stand with the SI of 18 or over, the second thinning with moderate intensity is recommended at the 16th year, the final density is about 1500 stems per hm² which is expected to produce large timber. For the SI of 14～16, the second thinning with light intensity is recommended at the 16th year also, but the final density is 1800～2200 stems per hm² which is expected to produce middlewood. And the stand with SI of 12 or less is suitable for middle and small wood production, one moderate thinning without further operations afterwards is recommended. These thinning criteria provide the scientific basis for the rational cultration and management of Chinese fir plantations in various aspects.

Key words: Chinese fir plantation, Site index, Thinning effects, Biomass increment