Regulation Of Thinning On The Tree Growth In Chinese Fir Plantations

High-density monocultures and untimely management practices have identically resulted in the lack of stability,losses in essential soil nutrients and long-term productivity in plantations of China.To solve the above issues,a thinning experiment was implemented which included five different thinning intensities（0%,20%,25%,33%,and 50%）in Chinese fir plantations planted in 2005,located in Jiangle County,Fujian Province.During the investigation process,the characterastics of understory vegetation,the decomposition of litters,the soil peoperties,the community of arbuscular mycorrhizal fungi（AMF）,and the tree growth were investigated,in order to analyze the differences on all these stand factors among different thinning intensities.At the same time,the intrinsic relationship among all these factors was revealed to detect the regulation of thinning on the tree growth,and the thinning growth model to establish based on this regulation.The results showed that:（1）Thinning measurement would improve the development of understory vegetation and the formation of community structure.The species richness,species diversity,total coverage,average height,and biomass of the understory vegetation were observed to be apparently increased with increasing thinning intensity,especially the high intensity thinning（about 50%）.Moreover,a stable vegetation community could be formed under 50%thinning.（2）Thinning intensity had significant effects on the decomposition of litters.Decomposition of litters under 50%thinning was significantly higher than 25%.（3）Thinning intensity had significant effects on the contents of soil organic matter,available phosphorus and available potassium,with 50%thinning significantly higher than unthinning.（4）Thinning intensity had no significant effects on the colonization,diversity,and community compositions of AMF,while the altitudinal slope position had significant effects on colonization,diversity,and community composition of AMF.Colonization by AMF on the lower slope position was significantly higher than on the upper slope position,while AMF diversity on the upper slope position was higher than on the middle and lower slope positions.（5）Thinning intensity had significant effects on the increment of stand average DBH and individual volume,but had no significant effect on the growth of stand basal area and stand volume.（6）Thinning affected on the tree growth indirectly through other factors.Sunlight could affect the tree growth indirectly by affecting other stand factors,such as the development of understory vegetation,increased soil nutrient content,and AMF colonization.However,the decomposition rate of litters was negatively correlated with tree growth.（7）After adding the environmental factors of stand and meteorological factors in individual DBH growth model,R²_adj was increased from 0.32 to 0.52,and the interpretation of variables was increased from 33.51%to 52.88%.（8）The DBH increment first decreased and then increased with the increase of DBH at the beginning.DBH increment decreased with the increase of the number of trees per hectare and the sum of basal area larger than the subject tree.Soil nutrient content was the most suitable for Chinese fir DBH growth at the range of 70–140 g kg^-1.DBH increment increased with the increase of soil magnesium content,but had a saturation point(75 mg kg^-1).The DBH growth increased with the increase of undergrowth vegetation coverage and soil microbial biomass carbon.DBH growth first increased and then decreased with the increase of average temperature in the growing season,and reached its maximum at 24.4℃.The annual maximum precipitation had a negative effect on the DBH growth of Chinese fir.These findings suggested that high-intensity thinning（about 50%）should be carried out in Chinese fir plantation with high planted density(3000±300 N hm^-2),which could significantly improve the development of the understory vegetation and community structure,promote nutrient regression,increase soil nutrient contents,and ultimately promote the tree growth.After long-term management under such high-intensity thinning,the productivity of the stands would be improved significantly,which had great significance of maintaining the stability of plantation and improving the productivity.