Effects Of Thinning Treatments On Plantation Productivity And Nutrient Cycling Of Poplar Plantation

In order to improve the quality of the poplar plantations and study the cultivation pattern ofbig-diameter wood, the6-year-old poplar plantations were selected to study thinning effects. Fourthinning treatments (contrast,30%intensity of the lower thinning,50%intensity of the lowerthinning and50%intensity of the mechanical thinning) were designed in this experiment. Theeffects of thinning treatments on poplar plantation microclimate, forest structure, plant diversity,nutrient cycling and poplar plantation productivity were investigated. The main findings are asfollows:1. Poplar plantation microclimate was significantly affected by different thinning treatments. Insummer, Forest photosynthetic active radiation of50%intensity of the mechanical thinningwas apparently higher than the other three thinning treatments, while in other seasons, thisdifference was not significant. Forest air temperatures of different thinning treatments in thesame season, except for summer, were not significantly different. In summer, the airtemperature of mechanical thinning was significantly higher than the other three treatments,and the average temperature of mechanical thinning reach to21.25. The forest air moisturesof different treatments were not significantly different in the same season. For all types ofthinning treatments, the soil temperature difference and moisture difference only existed in0-10cm soil layer.2. The canopy densities of contrast thinning treatment was significantly higher than30%intensity of the lower thinning, the canopy of which was then significantly higher than50%intensity of the lower thinning and50%intensity of the mechanical thinning; In summer,forest leaf area index corresponded to change of canopy densities.3. One year later, Understory vegetation plant diversity and importance value of different plantsshowed that for the same season and different thinning treatments, mechanical thinning hadthe highest Shannon-Wiener biodiversity, while contrast treatment had the highest dominanceindex, which showed that dominant species was most concentrated in contrast treatment, leastin mechanical treatment. And in the same season, biomass of50%intensity of the lowerthinning and50%intensity of the mechanical thinning was significantly larger than that ofthe other two treatments.4. Under different thinning treatments, the contents of available nitrogen were significantlydifferent. The contents of inorganic nitrogen?DON (soil dissolved organic nitrogen) andMBN (soil microbial biomass nitrogen) of50%intensity of the lower thinning and50%intensity of the mechanical thinning were significantly higher than that of the other twotreatments. The surface gathering of the available nitrogen was found for all the treatments,which meant that the contents of available nitrogen increased along with the depth of soil,from the bottom soil to the surface soil.5. In summer and autumn, the net soil mineralized nitrogen contents of50%intensity of the lower thinning and50%intensity of the mechanical thinning were significantly higher thanthat of contrast and30%intensity of the lower thinning. Compared to the contrast thinning,low intensity of thinning didn't promote the increase of net soil mineralized nitrogen. In theother two seasons, the net soil mineralized nitrogen contents of different thinning treatmentshad no significant difference. Under the same season and the same thinning treatment,commonly the net soil mineralized nitrogen contents of0-5cm layer were significantly higherthan that of5-10cm layer and10-20cm layer, the difference of which two was not significant.6. Under different thinning treatments, the soil respiration had significant difference only inspring. It is a long and slow process for thinning to affect microbial biomass carbon. Duringthe six months after thinning, between different thinning treatments did the thinningtreatments only had influence on0-10cm layer. The contents of microbial biomass carbon of50%intensity of the mechanical thinning were significantly higher than that of the other threethinning treatments.7. In growing season (spring and autumn), compared to contrast and low intensity of thinning,high intensity of thinning(50%) and mechanical thinning were more efficient in improvingenzyme activity of0-10cm soil layer. Commonly, the higher the intensity of thinning themore active those soil enzymes would be. Then when it came to the same thinning treatments,the enzymes activity, especially protease, had no significant difference between different soillayers. Low intensity of thinning promoted the activity of urease in growing season. Andprotease activity (summer) and catalase activity (autumn) of low intensity of thinning (30%)had no significant difference with contrast group.8. One year after the thinning individual plant DBH increment, quantity increment, in thetreatments with50%intensity of the lower thinning and50%intensity of the mechanicalthinning were all significantly higher than that of contrast and30%intensity of the lowerthinning.9. The research on nitrogen nutrient cycling of plantation after thinning showed that underdifferent thinning treatments, annual forest nitrogen supply was always larger than forestnitrogen demand, which showed that this experimental forest could meet the need of forestgrowth at the present stage. And the annual nitrogen supply of mechanical thinning was thebiggest while the annual demand of it was the least; the annual nitrogen demand of30%intensity thinning was the biggest while the annual nitrogen supply of contrast was the least.