| Thinning is a key measure for sustainable forest management.Scientific and rational tending thinning can promote tree growth,improve forest structure,improve soil nutrient status,study the effects of thinning on the growth and structure of Chinese fir(Cunninghamia lanceolata)plantation and soil under the forest,and provide reference for the scientific management of Chinese fir plantation.In this study,11-year-old Chinese fir plantation in Yanghe Village,Yanghe Town,Lujiang County,Anhui Province was used as a research object.Through the thinning experiment,the growth and stand structure and forest stand of Chinese fir plantation were studied in the year of thinning and 2 and 5 years after thinning.The characteristics of nutrient changes in the soil are summarized as follows.1.Variation characteristics of stand growthThe annual and treatment methods had significant effects on the average DBH,average tree height and basal area of Chinese fir plantation(p<0.05)..The average DBH,tree height and basal area of the stands under control and thinning in the three years of thinning(2012),2 years(2014)and 5 years(2017)after thinning shows an increasing trend with the test year The differences in DBH,tree height and basal area of the stand under the thinning treatment reached significant levels(p<0.05),and the mean value of DBH was thinning(16.2 cm)>Control(13.0 cm).Compared with the control,they increased by 21.3%,22.2%and 29.4%respectively.The average tree height was thinning(12.9 m)>control(10.7 m),which increased by 16.9%,18.3%and 26%,respectively.Basal area was thinning(38.5 m2/hm2)>control(36.6 m12/hm2),the growth rates compared with the control were-0.06%,0.07%and 13.0%,respectively.The changes in basal area between control and thinning in 2012 and 2014 were small and the difference was not significant.The promotion effect of thinning on the growth of DBH,tree height and basal area in Chinese fir forest was not obvious in the first two years,but it became more and more obvious with the extension of the year after thinning.2.Variation characteristics of stand volumeThere were significant differences in stand volume of Chinese fir plantations under different treatments(p<0.05).The average individual tree volume and stand volume of Chinese fir plantation under control and thinning treatment showed an increasing trend with the increase of years in the three years(2012),2 years(2014)and 5 years(2011)after thinning.The average individual tree volume was thinnilg(0.15 m3)>control(0.09 m3),which increased by 54.6%,56.8%and 82.4%,the stand volume was thinning(244.7 m3/hm2)>control(204.4 m3/hm2),with growth rates of 10.7%and 12.8%,31.0%,respectively.Anong them,individual tree volume under the control was significantly different between 2012 and 2017 and betweem 2014 and 2017(p<0.05)5 while individual tree volume after thinning and the stand volume under two treatments was significantly different among these three years(p<0.05).The interannual and hysteresis effects of thinning on the growth rate of individual tree volume were observed.3.Variation characteristics of the forest diameter structureThe forest DBH structure of the thinning plot and the control plot was basically normal distribution in each year.Under the two treatments,the number of small and larger-DBH individuals account for a small proportion,while the mid-DBH individuals were the most.In control plots,the DBHs of stands in the year of thinning(2012),2 years after thinning(2014)and 5 years after thinming(2017)were mainly distributed in the DBH classes from 8 cm to 16 cm,accounting for 90.3%,87.0%and 75.2%of the total individuals,respectively.In thinning plots,the main distribution of DBH in the three years shifted to the right relative to the control plot,mainly concentrated in DBH classes from 12 cm to 20 cm,accounting for 93.6%,94.6%and 78.5%of the total individuals,respectively.The percentage of individuals with DBH less than 14 cm in the control stand in three different years was significantly higher than those in the thinning stands,while the percentage of individuals with DBH of 14-20 cm and above 20 cm was lower in control than in thinning stands,indicating thinning could promote the growth of trees to a greater level.4.Characteristics of forest biomass carbon storageThe forest biomass carbon storage of Chinese fir plantations were different in different years.In the thinning year(2012),2 years(2014)and 5 years after thinning(2017),the forest biomass and carbon storage of the Chinese fir plantation were higher than the control.The average biomass ranked as thinning(120.4 t/hm2)>control(104.3 t/hm2),and the carbon storage showed thinning(61.0 t/hm2)>control(52.8 t/hm2).The biomass and carbon storage of Chinese fir stands under the two treatments showed the same pattern in different years.5.Characteristics of soil nutrient changesThe soil layer and the year had significant effects on the contents of NH4+-N and NO3--N in Chinese fir plantation(p<0.05).Soil NH4+-N and NO3'-N in the control in 2012 decreased with soil depth,and the difference between topsoil and low soil layer(30-50 cm)was significant(p<0.05).The average NH4+-N in the treatments showed thinning(1.638 mg/kg)>control(1.522 mg/kg),and NO3--N showed thinning(2.435 mg/kg)>control(1.677 mg/kg).The soil NH4+-N in the Chinese fir forest after thinning showed a trend of deereasing first and then increasing,but the difference between the soil layers was not significant.The effects of thinning on the NH4+-N and NO3--N in the topsoil were significantly different(p<0.05).The variation of NH4+-N content in the thinning stands was similar to that of the control stands,and both increased with the increase of the year.After 2 years of thinning(2014),the content of NH4+-N in all soil layers was higher than those of the control,and the increase rate of different soil layers was 38.88%,37.08%and 63.22%,respectively.The increase rate of NO3--N in different soil layer was 123.35%,230.32%and 255.56%.Soil pH value was not significantly different between different soil layers and years,but thinning reduced soil pH and increased soil acidity.Correlation analysis showed that soil NH4+-N,NO3--N and pH showed a nonlinear relationship.Compared with NO3--N,NH4+-N had a greater influence on soil pH in Chinese fir plantation,which causes soil more acid. |
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