Effects Of Wedelia Biflora Expansion On Litter Decomposition Of Dominant Species In The Representative Vegetation On The Paracel Islands

Wedelia biflora(Asteraceae)is a climbing herb with certain ornamental value.It is mainly distributed in grasslands,forest edges and coastal sand lands,and is common on the Paracel Islands of China.In recent years,the number and distribution area of W.biflora shows an increasing trend on the Yongxing Island of the Paracel Islands.W.biflora can climb up to the top of the Scaevola sericea plants,and invade to the forest gaps of partially degraded Ceodes grandis forest or Guettarda speciosa forest.This seriously affects the growth and regeneration of native vegetation on the Paracel Islands.The expansion of W.biflora also leads to the input of its litter into the native plant community.In this study,we set up treatments with and without the addition of W.biflora litter to the dominant species of the representative plant communities in Yongxing Island.In order to assess the effect of W.biflora expansion on the litter decomposition of dominant species(C.grandis,G.speciose,S.sericea)in the representative vegetation,we determined litter decomposition indices including substrate quality,the remaining rates of litter mass,nutrient contents,and enzyme activity.We hope to explore the mechanism of species interaction during the litter decomposition process,and provide a theoretical basis for the control and management of W.biflora.on the tropical coral islands.The main results are as follows:(1)The half-life and turnover period of understory litter decomposition in representative plant communities in the Paracel Islands are 0.37-0.74 year and 1.6-3.20 year,respectively.(2)Among all treatments,the W.biflora treatment(Ta)has the best initial substrate quality,including the highest nitrogen content,the lowest ratio of C/N and lignin/N,a high level of the litter substrate quality in the decomposition process,and fast litter decomposition rate and nutrients release rate.(3)Compared with the C.grandis + G.speciose(Tb)litter,addition of W.biflora litter can improve the initial substrate quality of litter treatment of C.grandis + G.speciosa + W.biflora(Tb+a),and promote litter decomposition and release of C,N and P elements.Comparing the observed value and expected value of the remaining rates of litter mass and nutrient contents of Tb+a,we found that addition W.biflora litter can promote the decomposition and release rate of C,N and P elements of Tb+a treatment in the later period.(4)Compared with the C.grandis + G.speciose + S.sericea(Tc),addition of W.biflora litter can improve the litter initial substrate quality of C.grandis + G.speciose + S.sericea + W.biflora treatment(Tc+a),but has no significant effect on the litter decomposition rate and the release of C,N and P elements.Comparing the observed value and expected value of the remaining rates of litter mass and nutrient contents of Tc+a,we found that addition W.biflora litter inhibited the decomposition rate of the Tc+a treatment in the later period,but have no significant effect on the release of C,N and P elements of Tc+a treatment.(5)The enzyme activities of four hydrolases of ?-1,4-glucosidase(?G),?-D-1,4-cellobiosidase(CBH),?-1,4-N-acetyl-glucosaminidase(NAG)and alkaline phosphatas(ALP)showed an increasing trend during the litter decomposition process.The enzyme activities of two hydrolytic enzymes of Polyphenol oxidase(PPO)and peroxidase(POD)showed a decreasing trend during litter decomposition process.Overall,no significant difference was detected to the activities of the same enzyme in different litter combination treatments.The expansion of W.biflora promoted the mass decomposition of the dominant species litter in the native plant communities on the Paracel Islands,and accelerates the release rate of nutrients into the soil.However,due to the special soil structure,high temperature and humidity,and strong light conditions on the coral islands,faster litter decomposes can result in decreased soil water holding capacity.and makes it difficult to the plant growth,and decomposers reproduction,and threaten safety and health of the ground-underground ecosystem.On the other hand,after long-term evolution and adaptation,the nutrient release and absorption of the plants or decomposers are in a balanced status on the coral islands.However,the input of W.biflora litter will accelerate the release of soil nutrients in the community,leading to leaching of the excessive release of nutrients,and eventually accelerate the degradation of the original vegetation.