| Beta diversity is a measure of species replacement along environmental gradients. beta diversity reflected difference of floristic composition among different community,different community or different environmental gradient different site have less mutual species,βdiversity is larger.βdiversity is a key problem to have relationship with controlling factor of diversity of community. All viable seeds present on or in the soil or associated litter constituted the soil seed bank.researching on seed bank is the important to understand deeply structure and function of marsh. Making an intensive study of soil seed bank in marsh, grasping the characteristic and discipline of soil seed bank in marsh,have important significance.Four typical vegetation succession series were selected to evaluate the beta diversity among dishing depression and shoreline in a marsh in the Sanjiang Plain. Four typical vegetation succession series are L-M-A,P-M-A, P-J-A,R-A- D,(L:Ass.Carex lasiocarpa ,M- Ass .C. meyeriana , A: Ass.Calamagrostis angustifolia, P- Ass.C. pseudo-curaica, J- Ass.C. jaluensis, R- Ass.Phragmites australis, D - Ass. Populus davidiana).Vegetation data were sampled within continuous quadrats along transects from the centers of the dishing depression or riverbed to the uplands or shores. Using the technique of germination , investigated floristic composition and seed density of soil seed bank in different vegetation succession series, analyzed the relationship between the composition of soil seed bank and plant community in three of the vegetation succession seres,and discussed the succession mechanism of marsh vegetation. The results and conclusions were as follows: (1) There were 30 species in sere 1 (L-M-P), 25 species in sere 2 (P-M-A),9 species in sere 3( P-J-A),and 43 species in sere 4( R-A- D).The species number differences of different succession series were relationship with different Model of distribution. (2) Theβdiversity indices Jaccard Index (CJ), Sorenson index (CS), Bray-Curtis index (CN) and Morisita-Horn Index (CMH) were measured, and the results indicated that from the"starting community"to the"end"where had been the most and least abundant moisture in the series, respectively, the similarity coefficient between each community and the starting community had a gradual decreased trend along decreasing moisture gradient and hypsography heighten. Furthermore, Cody index indicated that the species turnover speed had a general increasing trend from lowland to upland among the communities in every series. In the process of succession, the general trends were that the number of plant species was increased, the shared species were reduced gradually and theβdiversity index was increased. (3) Germinationing soil seed density in different sere of 0–10 cm depth soil seed bank,is between 433.3 grain /m2 and 2066.6grain/m2 , in 10–20 cm depth soil seed bank germinationing soil seed density is between 33.0 grain /m2 and 1375.0 grain /m2.The number of species in different sere soil seed bank had a gradual increased trend along decreasing moisture gradient and hypsography heighten. the number of species and seed density of 0–10 cm depth soil seed bank was larger than the 10–20 cm depth soil seed bank.(4)The species similarity of soil seed bank had a decresed trend along decreasing moisture gradient and hypsography heighten. comparing the Coefficient of Similarity of species in ground plant and soil seed bank showed that Similarity of species in plant community and in corresponding soil seed bank was decresing with decreasing moisture gradient and hypsography heighten.(5)The result of floristic composition of ground plant, composition of soil seed bank , the relationship between floristic composition of ground plant and soil seed bank,showed that succession mechanism supported the"initial floristics composition hypothesis"in Sanjiang Plain marsh . (6) Calamagrostis angustifolia is main species in Sanjiang Plain. The similarity between Ass. Calamagrostis angustifolia(CA) and the its neighboring communities, i.e. similarity of different pair of communities, were as follows: Ass. CA– Ass. Phragmites australis > Ass. CA– Ass. Carex jaluensis > Ass. CA– Ass. Carex pseudo-curaica > Ass. CA– Carex meyeriana > Ass. CA– Ass. Populus davidiana. In the background of the habitat mesophytization and xerophilization, C. angustifolia communities was likely to invade deeply into the neighboring moisture communities and its dominance would be strengthened in the Sanjiang Plain. reclaiming wetland is forbidden, starting to"returning land for farming to wetland"policy as soon as possible ,may be the necessary choice to restore and protect Sanjiang Plain Marsh.
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