| Chinese fir(Cunninghamia lanceolata)is a fast-growing species of timber tree widely cultivated in southern China.High-intensity 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 implement in a young(nine-year-old)Chinese fir plantation of the Tashi State-Owned Forest Farm(Huarong County,Hunan Province,China).Four different intensities of thinning treatments were set up,which were control(T1:0%,2500 stems ha-1),light thinning(T2:20%,2000 stems ha-1),moderate thinning(T3:30%,1750 stems ha-1),and heavy thinning(T4:40%,1500 stems ha-1).During the investigation process,the characterastics of tree growth,understory vegetation,biomass and the soil physiochemical properties were investigated and analyzed after 6 years of thinning.In order to provide a theoretical basis and technical support for the multi-objective and sustainable management of plantations,this study explores the suitable thinning intensities for Chinese fir plantations from the perspective of raising economic and ecological benefits.The results showed that:1.Tending thinning can promote the growth of DBH,tree height and individual volume and stand volume of Chinese fir.Heavy thinning significantly increased the average annual increment of DBH and individual volume,which was benefical to the cultivation of large-diameter Chinese fir.The stand volume has the maximum value under moderate thinning treatment,and the effect of thinning on stand volume should be balanced between individual volume and retained tree.2.Tending thinning promoted the development of understory vegetation,and significantly increased the number and diversity of understory vegetation species.The effects of thinning intensity on the species richness and diversity index of understory vegetation were generally shown as T4>T3>T2>T1,and the effects on Pielou evenness index were different.3.The effects of tending thinning on aboveground biomass of tree layer(trunk,bark,branch,leaf),increased first and then decreased with the increase of thinning intensity,and T3 and T4 treatments were significantly higher than T1(P<0.05),and T3 treatment had the maximum value(150.22 t·hm-2).The biomass of root was significantly increased with the increasing of thinning intensity,and the difference among all treatments was significant(P<0.05).The biomass allocation ratio of different thinning treatments was trunk>branch>root>leaf>bark.Thinning increased the biomass of understory shrub layer,and the effect of intensity thinning was the most significant,while the litter biomass decreased with the increase of thinning intensity.The effect of thinning on the total ecosystem biomass was the maximum under T3 treatment(153.85 t·hm-2),followed by T4 treatment(152.09 t·hm-2).The biomass allocation ratio of forest ecosystem was ranked as tree layer>litter layer>shrub layer>herb layer,and moderate thinning retained relatively higher biomass.4.Tending thinning reduced soil bulk density and increased soil porosity and water content,and the improvement effect was greater in the topsoil layer(0-20 cm)than in the deep soil layer(20-40 cm).Thinning could significantly increased the contents of organic matter and total nitrogen in soil surface(P<0.05),but had no significant effect on the contents of total phosphorus and total potassium in each soil layer(P>0.05).5.The results of correlation analysis among the factors related to the effect of tending and thinning of Chinese fir plantation showed that the species diversity index of understory vegetation was significantly positively correlated with soil water content,porosity and organic matter,and negatively correlated with bulk density and significantly correlated with the biomass of tree and shrub layer.Soil water content and porosity were significantly correlated with tree layer biomass,organic matter and shrub layer biomass.In summary,soil porosity,bulk density and organic matter are important environmental factors affecting the growth and biomass of vegetation in this region.6.Principal component analysis showed that individual volume,soil porosity and average DBH played a leading role in the effects of tending thinning on vegetation-biomass-soil.The comprehensive score from high to low were T4>T3>T2>T1.Based on the experimental results,tending thinning can promote the growth of Chinese fir,facilitate the cultivation of large diameter wood,and improve understory vegetation diversity and biomass accumulation.According to different cultivation objectives,different thinning intensities were adopted.For the cultivate large diameter timber to maximize economic benefits,heavy thinning(approximately 1500 stems ha-1)could be adopted.For the operation of carbon sink forest and ecological forest,moderate thinning(approximately 1750 stems ha-1)could be adopted. |
