Seasonal Dynamics And Stabilization Mechanism Of Vegetation In The Mixed Community Of Phragmites Australis And Triarrhena Lutarioriparia In East Dongting Lake Wetlands

Phragmites australis,a perennial grass reed,is one of the constructive species in Dongting Lake.In the 1960s or 1970s,Triarrhena lutarioriparia was planted in Dongting Lake Wetland.And then,area of T.lutarioriparia has been gradually expanded,while the area of P.australis has been gradually decreased.And now,two species coexisted and formed the most dominant vegetation type in the Dongting Lake Wetland.In order to explore the formation mechanism of the mixed communities of T,lutarioriparia and P.australis,the field in situ monitoring method was used to reveal the response differences of the two species to seasonal flooding.Differences of survival,growth,reproduction and competitiveness of the two species were explored by simulation experiments on plant growth.The main results were listed as follows:(1)In East Dongting Lake Wetland,situ monitoring of seasonal natural flooding(from May to Augest)and water withdrawal(from Augest to October)were carried out for three communities(M1 represents the sample plot with 91.43%of T.lutarioriparia,M2 represents the sample plot with 68.07%of T.lutarioriparia,and M3 represents the sample plot with 54.17%of T.lutarioriparia)of T.lutarioriparia and P.australis mixed communities,one community of T,lutarioriparia and one community of P.australis.The survival rate of T.lutarioriparia and P.australis decreased obviously during the period of flooding,but the recovery rate of P.australis was higher than T.lutarioriparia after flooding.Compared with flooding before,proportion of T.lutarioriparia increased in M1,decreased in M2 and M3 after flooding.During the monitoring period,plant height and biomass of P.australis in community M1 showed a negative growth,the plant height of T.lutarioriparia showed a negative growth and biomass showed a positive growth.Biomass accumulation and high elongation of the two species were positive in the community M2 and M3.There were more tillers of T.lutarioriparia and fewer tillers of P.australis.Both before and after the flooding,the competition intensity of P.australis was strong,and intraspecific competition was stronger than interspecific,while the interspecific competition of T.lutarioriparia was stronger than intrespecific.After water withdrawal,niche separation in community M1 was weakened,niche separation in communities M2 and M3 was more obvious,and P.australis was dominant during the monitoring period.(2)Five planting modes were used for seed germination to simulate the plant growth of T.lutarioriparia and P.australis in five proportions(ST means T.lutarioriparia;P.australis=4:0,H1 means 3:1,H2 means 2:2,H3 means for 1:3 and SP means 0:4)from April to October.Survival rate of P.australis was higher than T.lutarioriparia,the survival rate of P.australis decreased slowly,but T.lutarioriparia decreased quickly.In the mixed planting community,proportion of T.lutarioriparia decreased and P.australis increased.The survival rate of T.lutarioriparia decreased with the decrease of their own proportion,while P.australis increased.With the planting times going,growth of plant height and biomass of both T.lutarioriparia and P.australis slowed down with the decrease of their own proportion,and the growth of tillers and buds increased.In the communities of H1 and H2,the competition intensity and intraspecies competition of P.australis were strong in the early stage,and the competition intensity and intraspecies competition of T.lutarioriparia were strong in the later stage.In the community of H3,the competition intensity and intraspecies competition of T.lutarioriparia were always strong.Three mixed communities were always shown niches separation.In the early stage,species with low planting proportion dominated,and in the later stage,species with high planting proportion dominated.Overall,T.lutarioriparia occupied the dominant position.(3)The experiment of seed vigor and germination of T.lutarioriparia and P.australis have shown,seed vigor and germination rate of P.australis were higher than T.lutarioriparia(P<0.05).Seed germination rate of T.lutarioriparia with or without light was not significant different,the highest rate was under varying temperature of 30/20? and 30/25?.Germination rate of P.australis under light was generally higher than without light,the highest rate with light was under 20?.Seed germinations of water level have significant difference(P<0.05).Seed germination of T.lutarioriparia at 0 cm water level was best,germination rate decreased with the increase of water depth.While the germination of P.australis at 0 cm water level was worst,germination rate increased with the increase of water depth.The Seeds of P.australis are sensitive to light and resistant to water,Seeds of T.lutarioriparia are less sensitive to light and not resistant to water,and the light energy can supplement the temperature to some extent.To sum up,time and space structure of T.lutarioriparia and P.australis can be different in natural,which leads to the separation of ecological niche and coexistence of the two species.In natural flooding,P.australis has storger competition ability,could be dominanted.However,in no natural flooding,T.lutarioriparia has storger competition ability,could be dominanted.Thus it can be inferred that,competition ability of T.lutarioriparia stronger than P.australis,may be an important reason for T.lutarioriparia habitat expansion in the short term in no flooding.While competition ability of P.australis more stronge than T.lutarioriparia during the flooding,may be the fundamental reasons for the stable coexistence of the P.australis and T.lutarioriparia.