Spatial Distribution Pattern Of Plant Community And Its Habitat Impact Analysis In The Drawdown Zone Of Pengxi River In The Three Gorges Reservoir

Drawdown zone is a typical ecological ecotone,and it is the critical zone for the interaction between terrestrial and aquatic ecosystems.Riparian vegetation is the last ecological barrier to protect the water quality of the reservoir and prevent water pollution.It has important ecological function,such as protect the ecological security of reservoir and stabilize embankment.Due to the periodic submerged and summer rain erosion,the original habitat of the drawdown zone has been destroyed,and is forming a new habitat.Three Gorges Reservoir currently is one of the world's largest artificial reservoirs.It gradually improved the water level to 175 m from 2008,and has experienced seven complete dry and wet cycles.After seven years of natural succession,In the area between the lowest water level of 145 m and the highest water level of 175 m,and the area more than 175 m where may be affected by flooding,the Three Gorges Reservoir has formed a suitable vegetation,which is distinctly different from the terrestrial or aquatic vegetation.How to protect and restore the drawdown vegetation,to give full play to its ecological function,has a great significance to the water quality protection of the reservoir and the normal operation of the Three Gorges Project.It has great theoretical and practical significance to explore the vegetation characteristics and its relationship with habitat in the drawdown zone.Based on the vegetation ecology,quantity ecology and related theories,and based on the field investigation and laboratory analysis,our study analyzed vegetation characteristics and its relationship with habitat in the Pengxi River's drawdown zone of Three Gorges Reservoir.The results show:(1)There were 115 species in the drawdown zone of Pengxi River,which belonged to 100 genera and 46 families.Species composition of the drawdown vegetation displayed two obvious trends in families,on the one hand the big families,such as Leguminosae and Compositae took large proportion,on the other hand small families and regional single-species families also owned a certain proportion.The floristic analysis showed that tropic and temperate types were dominant,this indicated that the climate feature of Pengxi River was the transition from warm temperate zone to tropical zone.There were 22 trees,14 shrubs,35 annual herbs,3 biennial herbs and 41 perennial herbs.Herb species were the dominant groups,which were the major constructive species of the drawdown vegetation.The diversity index showed an increasing trend along the river flow direction and the altitude gradient.The whole plant community can be divided into four types: Cynodon dactylon + Paspalum thunbergii community,Conyza canadensis + Xanthium sibiricum + Bidens pilosa community,Setaria viridis + Vitex negundo community,Xanthium sibiricum + Bidens pilosa + Arthraxon hispidus community.Along the altitude gradient,the number of species and plant community types increased.Along the river flow direction,the trend of species and plant community types in drawdown zone of Pengxi River was: midstream > upstream > downstream.(2)According to CCA ordination between the plant community types and habitat variables in drawdown zone of Pengxi River,Four plant community types were gradient distributed along the first ordination axis: Cynodon dactylon + Paspalum thunbergii community,Conyza canadensis + Xanthium sibiricum + Bidens pilosa community,Xanthium sibiricum + Bidens pilosa + Arthraxon hispidus community and Setaria viridis + Vitex negundo community.There are different degrees of overlap between the various types.The position of various living forms in the ordination space reflects the ecological characteristics of the life style.Annual and perennial herbs are widely distributed along the ordination axis.Which distribute from the left side of the first ordinal axis,the area with greater influence of flooding,to the right end of the ordinal axis,the area with less influence of flooding.Shrubs and arbor species are intolerant flooding,all distributed on the right side of CCA first ordination axis.The influence of flooding disturbance on plant subsistence is confirmed.The spatial pattern that herbaceous plant community dominates constitutes the overall appearance of drawdown vegetation.The diversity of the drawdown zone is mainly affected by the flooding factors and human activities.(3)According to the multi-scale analysis of the relationship between species distribution and habitat variable in the drawdown zone of Pengxi River,under three scales,the vegetation distribution characteristics in drawdown zone of the 1 × 1 m and 1 × 5 m scale are similar,but the distribution of species in transect scale has a significant differences with the 1 × 1 m and 1 × 5 m scale.At the three scales,the main habitat factor groups and factors affecting the distribution of species in the drawdown zone were the same.Soil is the main factor group affecting the distribution of species in the drawdown zone.The factors related to water disturbance are the main factors affecting the distribution of species in the drawdown zone.According to the variance decomposition of habitat factors affecting the distribution of species in the drawdown zone,at the 1 × 1 m and 1 × 5 m scale,there is a strong interaction between the various factors.But at the transect scale,the interaction between factors is not strong,the interaction between soil and historical land use type is strong.According to the ordination results of habitat factors and species distribution,at the 1 × 1 m and 1 × 5 m scale,soil nutrient has little effect on the distribution of species in the drawdown zone.But at the transect scale,soil nutrient has an important effect on the distribution of species in the drawdown zone.In short,hydrological and water level changes are the main drivers of plant community change in the drawdown zone,and other factors are coupled with this main drivers to affect the distribution of species under this large background.