| Chinese fir has the characteristics of fast growth and excellent material.It is a common timber forest tree species in southern China.The afforestation area and volume account for the first place in China’s timber forest.For a long time,the afforestation density of Chinese fir in China is generally too large.After the canopy is closed,due to the poor sales of small-diameter timber and the increase in labor costs,thinning is not carried out in time,resulting in the phenomenon of over-dense and over-pure Chinese fir forest,resulting in slow growth of trees,decline of soil fertility,reduction of biodiversity,and ecological and economic benefits.It is not conducive to the sustainable management of Chinese fir plantations.At the same time,the current plantation management has changed from a single economic benefit to a multi-functional benefit.For over-dense plantations,only by tending thinning to promote tree growth and cultivate large-diameter timber can improve economic benefits.Only by tending thinning to promote the growth of undergrowth plants,improve stand structure,increase biodiversity,accelerate litter decomposition,and improve soil physical and chemical properties can improve ecological benefits and sustainable forest management.There have been many studies on the thinning of artificial forests in China,but most of the previous studies focused on young and middle forests,and there were few studies on near-mature forests.In the evaluation of the effectiveness of thinning,single management objectives were considered in the past,and multi-objective management was rarely considered.In this paper,the 21-year-old Chinese fir near-mature forest in Qingyang Lake State-owned Forest Farm of Hunan Province was used as the research object.Four treatments were set according to the thinning intensity of the number of plants,which were CK(0%,1353 plants/hm2),T1(20%,1082 plants/hm2),T2(30%,947 plants/hm2),T3(45%,750 plants/hm2),and each treatment was repeated three times.The indexes of forest growth,understory vegetation,litter interception,surface runoff and soil physical and chemical properties were observed for 4 consecutive years.The effects of thinning on each observation index of Chinese fir near-mature forest were analyzed.The economic benefits of different treatments were calculated by market substitution method.The ecological benefit scores of different treatments were calculated by analytic hierarchy process or ecological index weight,combined with standardized data.The comprehensive benefit evaluation of all indexes was carried out by principal component analysis.Finally,the evaluation scores and ranking of different thinning intensities were obtained,which provided practical and reliable theoretical basis for different Chinese fir plantation management objectives.The main conclusions are as follows:(1)After 4 years of thinning,the DBH,tree height,individual volume and annual growth of Chinese fir were T3>T2>T1>CK,and there were significantdifferences among different treatments(P<0.05).The measured value of stand volume was CK>T1>T2>T3,CK and T3 had significant difference(P<0.05),and there was no significant difference with other treatments(P>0.05).The annual growth of stand volume was T2>T1>CK>T3,CK and T3 had no significant difference(P>0.05),and there was significant difference with other treatments(P<0.05).The offset to the right of the thinning plot of the diameter distribution is greater than that of the non-thinning,and the greater the thinning intensity,the more the offset.(2)After 4 years of thinning,the number of understory vegetation species increased,the number of light-loving plants increased,and the shade-tolerant plants decreased.The Margalef index,Shannon-wiener index and Simpson index of understory vegetation were T3>T2>T1>CK,and the Pielou index was T1>T2>T3>CK.There were significant differences between thinning plots and non-thinning plots(P<0.05).The variation of each index of species diversity was T3>T2>T1>CK.The thinning plots in 2017 were significantly different from those in 2021(P<0.05),and there was no significant difference in CK in two years(P>0.05).(3)After 4 years of thinning,the interception capacity of semi-decomposed litter in Chinese fir forest land was better than that of undecomposed litter.The water holding rate and maximum water holding rate of litter were T2>T3>T1>CK,and there were significant differences between thinning plot and non-thinning plot(P<0.05).The litter accumulation was T1>CK>T2>T3,and there were significant differences among different treatments(P<0.05).The amount of litter int erception was T1>T2>CK>T3,and there were significant differences between no thinning and other treatments(P<0.05).The order of scouring amount was T3>CK>T2>T1,runoff was T3>CK>T1>T2,runoff coefficient was CK>T3>T1>T2;there was no significant difference in the indexes of surface runoff between non-thinning and severe thinning(P>0.05),but there was significant difference between mild and moderate thinning(P<0.05).(4)Thinning can reduce soil bulk density,increase soil maximum water holding capacity,total porosity;capillary porosity and forest water storage capacity,and the effect on topsoil(0cm-20cm soil layer)is greater than that on subsoil(20cm-40cm soil layer).The bulk density of topsoil was T1>CK>T2>T3,and the reductionwas T3>T2>T1>CK;the soil layer was CK>T2>T1>T3,and the surface reduction was T2>T3>T1>CK.The maximum water holding capacity of the topsoil was T3>T2>T1>CK,and the increase was T2>T3>T1>CK;the subsoil layer was T3>CK>T1>T2,and the increase was T1>T2>T3>CK.The gross surface soil layer of total porosity is T3>T1>T2>CK,and the increase is T1>T2>T3>CK;the soil layer was T3>CK>T2>T1,and the increase was T1>T3>T2>CK.The capillary porosity of each soil layer was T3>T1>T2>CK,the increase of topsoil was T2>T1>T3>CK,and the increase of subsoil was T3>T2>T1>CK.The measured value and increase of forest water storage capacity in each soil layer were T2>T1>CK>T3.There were significant differences in bulk density(P<0.05),total porosity,capillary porosity and capillary porosity between thinning plots and non-thinning plots.There was no significant difference in the maximum water holding capacity between CK and T3(P>0.05),but there was significant difference between CK and T1 or T2(P<0.05).There were significant differences in bulk density,maximum water holding capacity and capillary porosity between thinning plots and non-thinning plots(P<0.05).There was no significant difference between CK and T3 in maximum water holding capacity and forest water storage capacity(P>0.05),but there was significant difference between CK and T1 or T2(P<0.05).(5)Different thinning intensities can increase soil nutrient content,and the improvement of nutrient content in topsoil is better than that in subsoil.The content of hydrolyzed nitrogen in the topsoil was T3>T2>T1>CK,and the increase was T3>T2=T1>CK.The measured value and increase of the soil layer were T2>T3>T1>CK.The available phosphorus content in each soil layer was T2>T3>T1>CK,the increase of topsoil was T2>T3>T1>CK,and the increase of subsoil was T3>T2>T1>CK.The content of available potassium in the topsoil was T3>T2>T1>CK,and the increase was T2>T3>T1>CK;the soil layer was T3>T1>T2>CK,and the increase was T3>T2>T1>CK.The content of organic carbon in the topsoil was T3>CK>T1>T2,and the soil layer was T3>T1>CK>T2,and the increase of each soil layer was T3>T2>T1>CK.There were significant differences in the increase of soil nutrient content between thinning plots and non-thinning plots(P<0.05).There were significant differences in available potassium and organic carbon content between thinning plots and non-thinning plots(P<0.05).There were significant differences in hydrolyzed nitrogen content between CK and T1 or T2(P<0.05),but there was no significant difference between CK and T3(P>0.05).There were significant differences in available phosphorus content between CK and T3(P<0.05),but there was no significant difference between CK and T1 or T2(P>0.05).(6)The economic benefits of different treatments were calculated by market substitution method.The order of value of different treatments was T2(225055 yuan/hm2)>T1(281769 yuan/hm2)>T3(287180 yuan/hm2)>CK(279307 yuan/hm2).The ecological benefit scores of different treatments were calculated by analytic hierarchy process,and the order was T2(0.78)>T3(0.17)>T1(0.09)>CK(-1.04).Using principal component analysis to comprehensively calculate all indicators,the comprehensive benefits of different treatments were ranked as T2(1.0133)>T1(0.2179)>T3(0.1482)>CK(-1.3790).It can be seen that moderate thinning,that is,947 trees/hm2 in the 21-year-old Chinese fir near-mature forest,can obtain the best thinning effect. |
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