| It is an important management measure for plantation to thinning, and has an important effect on plantation development. The reasonable thinning is very significant in theory and practice to improve the nutrition space of canopy and conditions of water and nutrition under ground, to guarantee individual and community development and to improve forest community biomass and productivity.The study of thinning effects will be helpful not only to build and perfect the model system of planted forest growth and yield, but also to take a reasonable thinning technique. The optimal forest management will be realized.Based on the Chinese fir invested by World Bank, this paper discussed the influence of thinning on the stand-diameter structure, growth and the plants diversity under forest crown. At the same time, it also analyzed the effects for different thinning intensities on forest growth.1,The research of diameter structure for artificial Chinese fir stands. The three-parameter Weibull function was used to simulate stand diameter distribution.The relationship between Weibull function and the characteristics of the stand was analyzed. Maximum likelihood estimation method was used to calculate parameters of Weibull distribution (b, c).The x~2 test showed: its credibility degree was up to 88.9% for the stands thinned. It proved three-parameter Weibull function to be effective to describe the diameter distribution of the stands thinned. It can be understood that the effect of different thinning intensity on the diameter distribution from the change of the Weibull distribution parameters before and after thinning. The results showed: the minimum diameter of stands thinned were the larger than the control area, its change regulation is intension > moderate > weak > the control area at the minimum diameter of stands. It was indicated that the speed of stand diameter increased the fastest for intensity thinning; simultaneously, it was a positive impact on the stand diameter distribution for thinning operations in the Wood, and can raise the overall growth potential.2,It studied the effects of different thinning intensities on average stand diameter, height, volume and unit area volume. The results showed:(1) Under the 1% significant level, there was significant impact on the stand average diameter about different thinning intensities. The overall trend is that the base diameter grows fast and fast with the thinning intensity increased. The LSD method was used to compare intensity,moderate, weak and the controlled area, it showed intensity thinning was more significant difference than moderate, weak and the controlled area. The regressive relationship between stand base diameter and thinning intensity was studied, the results showed Richards curve equation was the best:y=2.1675/((1-1.0125exp (-0. 013398x))^(1/-2.6798)) R=0.9563(2) There was no significant difference on tree height of different thinning intensities, which was similar with previous studies results.(3) With 1% significant level, there was significant impact on the stand individual volume about different thinning intensities. The individual volume was gradually increasing with the thinning intensity increased. The LSD method was used to compare the difference of the volume growth among the intensity, moderate, weak thinning and the controlled area. The results showed that the influence on individual volume growth at different thinning intensity was the strength > moderate > weak; Meanwhile, the models between individual volume and the thinning intensity, it indicated Logistic-curve model was the best one, the formula is:y=2. 2259/((1-0.997130exp(0.117383x))^(1/3. 3107)) R=0.9374(4) There was no significant effect on the unit area stand volume at the different thinning intensity, which was consistent with the results studied by other. The total harvest was close to each other for the stand of different thinning intensity, and was in conformity with "the principle of constant yield".3,The relationship between the thinning intensities and the plant diversity under the crown of Chinese fir plantation were studied. The results showed: the relationship between Shannon-wiener index, as well as the species evenness (P=0.05), and thinning intensity (P=0.01), was remarkable positive correlation. But the relationship between the dominant species and the intensity of thinning was remarkable negative correlation (p = 0.01).The results studied also showed that the dominant species of plants under Chinese fir plantation were mainly Boehmeria grandifolia,Viola diffusa,Loropetalum chinense,Cyrtomium fortunei,Dryoathyrium okuboanum and so on. It showed us that it is difficult to natural regeneration by thinning. So we should adopt the artificial regeneration measures such as replanting, sowing. |
