| Pinus massoniana plantation ecosystem is the ecological protective screen in upper Yangtze river low hilly land,due to long-term taking the single pure species and high afforestation density forest management pattern,the forest productivity,species diversity and woodland soil fertility declined,ecological service function dropped.Understory vegetation is one of the main components of forest ecosystem,it plays a vital role in maintaining woodland soil fertility,species diversity and stability of forest.Canopy coverage density reflected the structure and environment of forest,controlled the light,temperature and precipitation in forest,therefore,it affect the understory vegetation diversity,growth and nutrient accumulation,at present,the related research is rarely reported.We expect to find a reference canopy coverage density and stand density for inefficient P.massoniana plantation,to improve the stability and productivity of plantation.In the present study,by uniformly thinning,pruning,combining canopy analysis system(CI-110),formed five canopy density gradient plots of 11 years P.massoniana plantation about 0.9,0.8,0.7,0.6,0.5,investigated the understory vegetation species composition,diversity and season dynamics,the stoichiometry characteristics of five main understory plants,the results showed that:(1)Understory vegetation is scarce in P.massoniana plantation with 0.9 canopy coverage density.With the decrease of canopy density,the species number had a trend of increase,from 13 to 31.The understory vegetation was mainly Phanerophytes about 60%,followed by Hemicryptophytes and Geophytes,the percentage of Hemicryptophytes and Micro-phanerophytes decreased with the decrease of canopy density.Life-form was more diversified in 0.5,0.6 and 0.7 canopy density,community structure was more stable.The community similarity coefficient of adjacent two canopy densities in 0.5,0.6,0.7,0.8 canopy density were higher(more than 70%)than 0.9 canopy density,common species of understory vegetation in 0.5,0.6,0.7 canopy density had obviously increased,plant species were more stable.(2)With the decrease of canopy density,coverage,Margalef richness index,Simpson dominant index and Shannon-wiener diversity index of understory vegetation have a trend of increase,the coverage of understory vegetation in 0.7 and 0.8 canopy density were higher than 0.9 canopy density,but it had no significant difference,it had increased significantly in 0.5 and 0.6 canopy density.The richness index and diversity index of understory vegetation in 0.8 canopy density was significant higher than 0.9 canopy density,the dominant index in 0.5,0.6,0.7 and 0.8 canopy density,the richness index and diversity index in 0.5,0.6,0.7 canopy density had less obvious differences.The species of woody plants and the evenness index of understory vegetation had decreased in 0.5 canopy density.In general,0.6 canopy density was the best-performing canopy density for increasing the species number,diversity and life-form structure of understory vegetation.(3)Many of the dominant species are evergreen,some accompanying species and occasional species regenerate in spring and summer,few kind of species regenerate in autumn,therefore,it had no significant difference among different sampling time(seasons).The coverage and diversity index of understory vegetation are almost highest in autumn,lowest in spring,summer near to autumn.The species number,coverage,richness index and diversity index of understory vegetation in April 2016 were markedly increased than last year.(4)The C content of five main understory plants were lower in 0.8 and 0.9 canopy density than others,were highest in 0.6 or 0.7 canopy density,the C content in 0.5 canopy density was lower than 0.6 canopy density,only the C content of Quercus serrata in different canopy densities had significant difference.Canopy density size had less obvious impact on N content of five main understory plants,the N content of Mallotus barbatus and Dianella ensifolia had a trend of increase with the decrease of canopy density,the N content of D.ensifolia was highest in 0.6 canopy density,the N content of M.barbatus was highest in 0.5 canopy density,the N content of Q.serrata in 0.5 canopy density was significantly lower than other canopy densities,the N content of D.ensifolia in 0.5 canopy density was lower than 0.6 canopy density.The P content of woody plants and herbage had different trends with the decrease of canopy density,the P content of woody plants decreased with the decrease of canopy density,the P content of herbage had an inverse relationship with canopy density.(5)The carbon nitrogen ratio of five main understory plants in 0.5 canopy density were significantly higher than 0.9 canopy density,the carbon nitrogen ratio and carbon phosphorus ratio of Q.serrata in 0.5 canopy density were significantly higher than 0.9 canopy density;the carbon phosphorus ratio of M.barbatus in 0.8 canopy density was significantly higher than 0.9 canopy density;the carbon phosphorus ratio of Dicranopteris linearis in 0.6 canopy density was significantly lower than 0.9 canopy density.Canopy density size had no significant impact on nitrogen phosphorus ratio of four main understory plants instead of M.barbatus,the nitrogen phosphorus ratio of M.barbatus had a trend of increase with the decrease of canopy density,from 12.8 in 0.9 canopy density to 26.9 in 0.5 canopy density,from being restricted by N to being restricted by P.The nitrogen phosphorus ratio of D.linearis had a trend of decrease with the decrease of canopy density.The nitrogen phosphorus ratio average of the same plant were more than 18,restricted by P,instead of Q.serrata,the nitrogen phosphorus ratio of Q.serrata was 15.4,restricted by N and P. |
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