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Sheng Weitong(Research Institute of Forestry, CAF Beijing 100091)  
Since 1987 to 1999, a study on the density management of Chinese fir plantations of 9 year old was made continuously in the Experimental Center for Subtropical Forestry under the Chinese Academy of Forestry (CAF) located in Fenyi County, Jiangxi Province. Based on the establishment of 5 fixed sample plots in the stands with different retained density after thinning, the undergrowth vegetation had been observed for 13 years, the soil fertility in the stands had been tested for 9 years, the soil erosion as well as the amount of litters and their decomposition rates had been monitored for 8 years in the stands. The results showed that in all thinned stands with different thinning intensity, the undergrowth vegetation had developed very well, with the number of undergrowth plant species being increased to 30~36 species, the coverage of undergrowth vegetation being increased to 80%~90% , and the biomass of undergrowth vegetation being increased to 5 t·hm -2 in 13 years, while in the dense stand without thinning the undergrowth vegetation developed poorly and 8~9 years later to reaching the same level than that in the thinned stands. The reasonable stand density management could also increase the amount of micro organisms by 2.42 times in the soil, which resulted in faster decomposition of litters in the stands with lower density and well developed undergrowth vegetation. Generally, when the Chinese fir plantation grows into the middle aged, the content of organic mater starts decreasing more or less in the soil, particularly in the soil layer at depth of 16~30 cm underground , the content of available nitrogen and phosphorous decrease significantly. So, in the dense and fast growing stands, the higher the stand density is , the more the available nitrogen and phosphorous decrease significantly in the soil. Therefore, the reasonable decreasing of stand density by means of thinning can conserve the soil fertility, so that the long term productivity of forest land can be maintained.
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