Effecs Of Thinning Treatments On Ecological Factors And Nitriogen Cycling Of The Poplar Plantations

In order to improve the quality of the poplar plantations quality and discusse large-diametertimber cultivation patterns, the6-year-old poplar plantations located in the Pukou beach ofYangtze River were chosed to study thinning effects. Five thinning treatments (contrast,30%intensity of the lower thinning,40%intensity of the lower thinning,50%intensity of the lowerthinning,50%intensity of the mechanical thinning) were designed in this experiment, and treegrowth, micro-climate, soil available nitrogen supply, nitrogen absorption in poplar andunderstory vegetation were investigated for the different thinning treatments. Results from thepresent study would provide a theoretical basis for the cultivation of poplar large-diameter timber.The main findings are as follows:1. For the6-year-old poplar plantations with initial spacing of3m×3m, thinning treatmentshad significant effects on DBH increment,but thinning treatments had significant effcets on stockincrement and the individual tree volume increment after one year of thinning, while leaf area pertree and leaf area index of the stands were significantly affected by the thinning treatments.2. In the treatment of50%intensity of the mechanical thinning, an interlaces thinning wasconducted. Therefore, the light intensity in the plantation was greatly improved, leading to ansignificant difference in the micro climate conditions with other thinning treatments. However,there were obvious differences in ecological factors among the other treatments except for thetreatment of50%intensity of the mechanical thinning.3. The soil available nitrogen had obvious changes in August after thinning, and the nitrogenconversion in the50%intensity of the lower thinning and the mechanical thinning was the largest,and their SDON (soil dissolved organic nitriogen) and SMBN (soil microbial biomass nitriogen)were also the highest.4. After one year of thinning, the nitrogen reserves in poplar plantation of the contrast (nothinning) was the largest, while the minimum was observed in the plantation of mechanicalthinning. The largest nitrogen reserves in the understory vegetation was observed in the treatmentwith the50%mechanical thinning, while the40%intensity of lower thinng was the least.However, there were no significant difference in the nitrogen reserves of poplar plantation amongdifferent thinning intensities tested.5. The analysis result of soil available nitrogen supply and poplar plantation nitrogen uptakeshowed that the understory vegetation could fix nitrogen very well after thinning.