| Pinus tabuliformis plantations play an important role on the local water and soil conservation and ecological protection of Loess Plateau. This research takes the middle-aged P. tabuliformis plantations in Huanglong Mountains as the research object. Through the typical sample plot survey method, thinning effects of plantations under light thinning(CD=0.8), medium thinning(CD=0.7), heavy thinning(CD=0.6) and CK(CD=0.9) were conducted comparative analysis. The effects of the thinning intensity on tree growth and stem form quality of P. tabuliformis were analyzed, the effects of the thinning intensity on fine root growth, morphological characteristics and spatial distribution pattern of P. tabuliformis were analyzed; the effects of the thinning intensity on the total litter storages and water-holding capacity of forest litter were analyzed. The main results were as follows:1. Thinning improved the growth characteristics and stem form quality of P. tabuliformis, but excessive thinning intensity was not conducive to the formation of good stem form quality. As thinning intensity increased, the DBH of trees increased obviously and there was a substantial increase when the reserve canopy density was 0.6. The average volume of individual tree was increased obviously as reserve canopy density decreased. Meanwhile, stand volume was reduced evidently and tended towards stability. Thinning intensity had no significant effect on tree height. As thinning intensity increased, trees stem taper, the ratio of diameter to height, stem straightness, number of branches and average base diameter of branches were all increased. Forking ratio decreased at first and then rose again. Height under living branch lowered significantly with the increase of thinning intensity.2. Trees form quality reach to the best when the reserve canopy density valued 0.7. By using the analytic hierarchy process(AHP), trees form quality of the middle-aged P. tabuliformis plantation under four different thinning intensities were evaluated. The results indicated that, by comprehensive analysis of all the three criteria(tree growth, stem form and branches) as well as ten indicators(DBH, tree height, diameter height ratio, taperingness, stem straightness, forking ratio, height under living branch, number of branches, max-branch base diameter and average base diameter of branches), the trees form quality can be best evaluated. Among all of these factors, stem straightness, forking ratio and DBH were the most important ones that influencing the trees form quality with a total weight of 0.7382. So these three indicators were major determinants of trees form quality. The comprehensive scores of the trees form quality evaluations fluctuated as the canopy density decreased and topped at 90.28 when the reserve canopy density valued 0.7.3. With the increase of thinning intensity, the fine root biomass increased at first before they are reduced. Under heavy thinning intensity fine root biomass in 0~20cm soil layer decreased significantly(p<0.05). The proportions of fine root biomass in 20~40cm soil layers and 40~60 cm soil layers increased with the thinning intensity increases. The differences of root length density and root surface area density between different thinning intensities and different soil layers were significant(p<0.05), besides, the regulations of their changes were the same to the fine root biomass. With the increase of thinning intensity, the specific root length and the specific surface area of fine roots had all increased, and their difference became significant under heavy thinning(p<0.05). Light thinning and medium thinning had significant effect on small diameter fine roots(0~1.0 mm), but had no significant effect on the big ones(1.0~2.0 mm)(p<0.05).Heavy thinning had significant effect on all of them(0~20 cm)(p<0.05).4. With the increase of thinning intensity, the total litter storages reduced gradually and ranged from 13.67 t?ha-1 to 20.50 t?ha-1. The order of the proportion of the decomposed litter in different thinning type was heavy thinning(65.85%)>medium thinning(60.44%)>light thinning(53.33%)>CK(47.96%); With the increase of immersion time, the water capacity and the water absorption rate of undecomposed litter and decomposed litter had a same change rule in different thinning types. The water capacity of litter obviously related to immersion time, which shows a logarithmic relationship(R2>0.85). The water absorption rate of litter obviously related to immersion time too, which shows a power function relationship(R2>0.95). The order of the water-holding capacity(Effective retain deepness) of forest litters in different thinning type was medium thinning(3.20 mm)> CK(2.91 mm)>light thinning(2.49 mm)>heavy thinning(2.26mm). |
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