Effect Of Thinning On Growth And Diversity Of Understory Vegetation And Enzyme Activity In Soils Under Poplar Plantations

Poplar is an industrial tree species in China. In order to establish suitable silvilcuturepatterns of large-diameter timber and maintain long-term productivity of poplar plantations, it isvital to understand the impact of thinning on poplar plantation ecosystem. Therefore, a researchtrial of thinning methods and intensities was designed and established in early2012on an8-year-old poplar plantation. Microclimate, plant diversity, soil characteristics, and plantationproductivity following different thinning patterns were investigated. Key findings aresummarized as follows:1) Thinning significantly affected poplar growth. A singnificant variation in DBH growthafter one year of thinning was observed among four thinning treatments, in the order of50%mechanical thinning>50%lower thinning>30%lower thinning> control. As for individual treevolume and aboveground biomass growth following one year of thinning, the ranking was50%lower thinning>30%lower thinning>50%mechanical thinning> control. As a whole, thinningsignificantly improved forest DBH growth and thus improved individual tree volume growth.However, a long-term study is needed to determine effects of thinning on plantation productivity.2) Thinning showed some effects on microclimate of the plantations. Among the four testedthinning treatments, ranking of photosynthetic active radiation during a year was50%intensityof the mechanical thinning>50%intensity of the lower thinning>30%intensity of the lowerthinning> control. In all treatments, the highest photosynthetic active radiation of ten dayoccured in mid-March and mid-Augest, while photosynthetic active radiation was highest inspring and lowest in winter. There were little significant differences in air temperature among thefour thinning treatments. Seasonal dynamics of air temperature showed a normal distributionpattern, which was summer> autumn> spring> winter. Little significant difference in relativehumidity was observed among the four treatments. The highest was observed in summer and thelowest in spring.3) Soil temperatures were somewhat affected by thinning. Generally, soil temperatures inplots with50%intensity of the mechanical thinning and50%intensity of the lower thinning weregreater than those with30%intensity of lower thinning and control. Seasonal ranking of soiltemperature was summer> autumn> spring> winter. Similar to soil temperature, soil moisturein plots with50%intensity of the mechanical thinning and50%intensity of lower thinning weregreater than other treatments evaluated, with highest values observed between July and Augest.Overall, soil moisture at soil depth of15cm was higer than that of5cm.4) Understory vegetation biomass was affected significantly by thinning, while plantdiversity was not significantly different among the four treatments. In all seasons, treatments in the50%of mechanical thinning and the50%of lower thinning had richer understory vegetationthan the30%of lower thinning and control. Seasonal variation of vegetation biomass was in theorder of autumn> summer> spring> winter. The highest understory vegetation biomass wasdetected in the50%mechanical thinning in autumn (786.11kg·hm-2), and the lowest was in thecontrol in winter, which was only303.72kg·hm-2. In all tested treatments, the50%mechanicalthinning led to consistently the highest diversity index of understory vegetation, with the greatestimportance value of Compositae and Gramineae.5) Significant variations in soil microbial biomass were found among the four thinningtreatments. During spring and autumn, soils under high thinning intensities showed highermicrobial biomass than those under low thinning intensity or the untreated control. Moreover,both soil microbial biomass carbon (MBC) and nitrogen (MBN) of0~10cm soil depth weresignificantly higher than those of10~20cm soil depth.6) Thinning significantly affected soil enzyme activities. Generally, catalase activities in the50%mechanical thinning and50%lower thinning were significantly higher than the other twothinning treatments. Seasonally, catalase was most active in summer, followed by autumn, springand winter. Further, catalase activity decreased with increasing soil depth. Saccharase activityshowed significant variations among the treatments only in summer and autumn. The generaltrend for saccharase activity was50%mechanical thinning>50%lower thinning>30%lowerthinning> control. The trend was more evident in summer. Saccharase activity of0~10cm soildepth in50%mechanical thinning was significantly higher than those in other treatments. In10~20cm soil depth, saccharase activities in50%lower thinning and50%mechanical thinningwere significantly higher than those in30%lower thinning and control treatments.