| Development of the understory vegetation is one of the important measures for the transformation of low efficiency plantation.Understory vegetation is one of the main components of the forest ecosystem,it plays an essential role in nutrient cycling,biodiversity conservation and maintaining soil fertility.Development of the understory vegetation may affect the nutrient cycling in the forest ecosystem by changing the nutrient surface enrichment,affecting the understory and soil microenvironment.Litter decomposition is one of the most important ecological processes dominating nutrient cycling in forests.However,the change of understory vegetation can increase the heterogeneity of forest and soil microenvironment,change the quality of the litter,and promote the complexity and mechanism of litter decomposition.Canopy density can affect Light intensity to develop the understory vegetation As yet,little attention has been paid to the development of understory vegetation and litter decomposition by controlling the canopy density.To understand the development of understory vegetationthe,decomposition characteristics of P.massoniana foliar litter and the degradation of its refractory compounds in Pinus massoniana plantations with a range of canopy densities(A: 0.91,B: 0.83,C: 0.74,D: 0.62 and E: 0.49),a descriptive survey of the sampling sites was undertaken.Understory vegetation,Dynamics of the soil environment and soil fauna were also measured.After that a field litter bag experiment was conducted in pure P.massoniana plantations of low mountainous region in the upper and middle Yangtze River area.Our results showed that:(1)Over the 2-year experimental period,The species number of understory vegetation increased from 13 to 31 with the decreasing canopy density form 0.6 to 0.8.The understory vegetation was mainly Phanerophytes,followed by Hemicryptophytes and Geophytes,the percentage of Hemicryptophytes and Micro-phanerophytes decreased with the decreasing canopy density.The life forms of understory vegetation were more complicated in 0.5,0.6,0.7 and were more stable.the Margalef richness index,the Simpson dominance index and the Shannon-wiener species diversity index of understory increased with the decreasing canopy density,while the Pielou evenness index was decreased in canopy density of 0.5.Canopy density of 0.6 was the best-performing canopy density for increasing the species number,diversity and life-form structure of understory vegetation.(2)Over the 2-year experimental period,The canopy density had significant impact on the C content of Quercus serrata and its content was higher in canopy density of 0.6,0.7.The canopy density had no significant impact on the N content of Quercus serrata and Dianella ensifolia,meanwhile their N content were lowest in the canopy density of 0.5.The P content of shrub and herbage plants had different trends with the decreasing canopy density,the P content of shrub plants decreased with the decreasing canopy density,the P content of herbage plants had an inverse relationship with the shrub plants.The C/N of Quercus serrata and Dianella ensifolia had significant increased in 0.5 canopy density than 0.9 canopy density.The C/P of Quercus serrata had significant increased in canopy density of 0.5.The canopy had no significant impact on N/P of Quercus serrata and Dianella ensifolia,The nitrogen phosphorus ratio average of the Dianella ensifolia was more than 18,it would restricted by the P content,the nitrogen phosphorus ratio of Q.serrata was 15.4,it would restricted by N content and P content.(3)Over the 2-year experimental period,the C content of soil was lower in the canopy density of 0.6,0.7,0.8 during the sampling time,except winter in 2014 and the spring in 2016.The C content of soil in all of the canopy was higher in spring.the sampling time had significant impact on C/N and C/P,The C/N and C/P of soil was lower in canopy density of 0.6,0.7,0.8(except winter in 2014 and spring in 2016),The C/N and C/P was highest in spring.(4)Over the 2-year decomposition period,Litter mass loss decreased significantly with the decreasing canopy density.The residual of carbon,nitrogen,phosphorus lignin and cellulose in the Pinus massoniana foliar litter is lower in 0.6 and 0.7 canopy density than others.The decomposition of carbon,nitrogen and phosphorus,the degradation of lignin,cellulose in the foliar litter of Pinus massoniana mainly occurred during the first year(0-392d).foliar litter nitrogen displayed enrich process during the second year(575-768d),whereas phosphorus displayed enrich process during the end of decomposition period at the first year(300-330)and the early and middle decomposition season at the second year(330-672).The percentage of the carbon,nitrogen and phosphorus release rate and the cellulose and lignin degradation rate during the first year compared to the entire 2-year period accounted for important proportion for foliar litter of Pinus massoniana,and the K value on them was highest in the 0.6?0.7canopy density.The C/N,C/P,N/P,Lignin/N,Lignin/P and Lignin/Cellulose for the Pinus massoniana foliar litter have an increase before decrease in the same canopy density with the different decomposition time.For the fast decomposition of N,P and Cellulose in the canopy density 0.6 and 0.7,the C/N,C/P,N/P,Lignin/N,Lignin/P and Lignin/Cellulose was higher in 0.6 and 0.7 canopy density than others.(5)Over the 2-year experimental period,the total number of individuals and the individuals of meso/micro fauna in litter layer increased with increasing canopy density,form 0.6 to 0.8.The macro and meso/micro fauna had significant difference on diversity characteristics in the soil layer.The Simpson dominance index of meso/micro fauna increased with the decreasing canopy density,while the Shannon-wiener species diversity index and the Pielou evenness index decreased with the decreasing canopy density.The Margalef richness index had significant increase form 0.5 to 0.6 canopy density.The seasonal factors has significant impact on soil fauna,and its individual density displayed higher in the spring.In winter,the soil macro fauna individual density displayed lower than other seasons,meanwhile the meso/micro fauna individual density was lower in summer and had significant increase than it in the first sample year.In 0.6 and 0.7 canopy density,there were no significant impact on diversity characteristics of soil fauna with the season changed,it reflected the best stability in the canopy density of 0.6 and 0.7.(6)The relative light intensity were correlated or significant correlated with the diversity characteristics of understory and the nutrient content of dominant species.The relative light intensity and relative humidity were correlated or significant correlated with the individual density and diversity characteristics of soil fauna.During the two years decomposition time,the relative light intensity,the nutrient of soil,the individual density and diversity characteristics of soil fauna were correlated or significant correlated with the foliar litter mass loss,nutrients release,the degradation of lignin and cellulose,and the individual density and diversity characteristics of soil fauna were the main factors correlated with the litter decomposition.Conclusively,canopy density had significant impacts on the understory vegetation and understory environmental factors such as soil properties,soil fauna community structure etc.,canopy density between 0.6 and 0.7 might be a favorable management practice promoting the development of understory and litter decomposition,conducive to the decomposition of litters,and the degradation of litters recalcitrant components in P.massoniana plantations... |
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