The Research On The Degradation Mechanism Of Riparian In Plain River Network Area

Riparian zone is the ecotone between terrestrial and aquatic ecosystem. Riparian zone plays a key role in river basin ecosystem, and has important ecological, social, economic values. As a typical open system, however, riparian zone ecosystem can be easily degraded and result in severe consequences to the human society under the exposures of overload natural and manmade disturbance.To research the degradation mechanism of riparian zone of the plain river network area, a case study in Qingpu District, Shanghai was conducted in this research. The following works were done:1. Identifying the structures and functions of the riparian zone of plain river network area from both natural and social features; 2. Analyzing the detailed affections resulted by natural disturbance such as alien plants invasion and the manmade disturbance such as land use type varying and river water body pollution. A total of 48 riparian sites,12 water quality monitoring sites together with 11 alien plants invasion sites were selected respectively according to different works. 5 times of investigation and sampling work were done during spring,2009 to spring, 2010. The result shows:1. The riparian zone of plain river network area has its unique structure and function(1) The structure of riparian zone of plain river network area can be roughly divided into 6 different types. They are:riparian zone in agricultural land, industrial land, transportation land, residential land, green space and deserted land. Significant differences in structure characteristics can be easily found between different types of riparian zone;(2) The function of riparian zone of plain river network area are mainly decided by its location and structure characteristics, including terrain structures such as width, slope, and vegetation structure such as coverage, continuity. The riparian zone of plain river network area owns both ecological and social function. Ecological function can be divided into:buffer function, biological habitat function, soil and water conservation function; Social function can be divided into:anti-flooding function, landscape function, entertainment and leisure function, economic function.2. Land use type differing affected the riparian habitat and plant community composition.(1) At present the quality of riparian habitat in Qingpu can be ranked in the order of: wild land> green space> agricultural land> transportation land> residential land> industrial land. The main reason of this result was the changes of bank type, the width and gradient of the riparian zone, as well as the original vegetation as the land use type differing;(2) A total of 118 plants from 57 families were found in Qingpu riparian zone. The highest value of average richness appeared in green space, while the lowest in industrial land. Plant community compositions in six land use types were significantly different from each other in respect of growth form and origin. Industrial land suffered intensive disturbance and the percentage of the alien invasive plants was as high as 27.22%; The average richness of grass species in agricultural land was 11.75, which was the highest value in six land use type; the percentage of artificial plants in transportation and residential land were as high as 37.77% and 49.97% respectively; in green space, the growth form was in a relatively balanced state, and the percentage of artificial plants was 64.05%; wild land lacked trees and bushes, the percentage of the natural distributing plants was as high as 69.94%.3. Water pollution of river affected riparian zone(1) The current situation of river water quality in Qingpu was not satisfying. The quality of water from all sampling sites was under the level of V, which was mainly due to the TP TN exceeding. Certain differences were found between rivers of agricultural area and rivers of residential area. However, all the water samples were characterized of TN and TP exceeding and CODMn coming up to the standard, which means the pollution type is non-organic;(2) The richness of higher hydrophytes was low in Qingpu riparian zone during spring. Significant linear relationship was found between water body eutrophication level and the coverage of submerged plant. As the water body eutrophication level rising, there was a trend of submerged plant coverage reduction;(3) Also, the benthic animal community in Qingpu riparian zone during spring was relatively single and mainly composed of Gastropoda and Oligochaeta, which were indicator species in polluted water.4. Alien plants invasion affected the plant communication and soil characteristics of riparian zone(1) A total of 24 different invasive alien plants from 14 families were found in Qingpu riparian zone. Comparing with the whole flora, its percentages of family and species are as high as 48.3% and 26.1%, respectively.7 plants affect the ecosystem badly, they are:Solidago Canadensis, Conyza Canadensis, Aster sublatus, Alternanthera philoxeroides, Cabomba caroliniana, Trifolium repens and Eichhornia crassipes;(2) The appearances of the invasive alien plants, T. repens and S. canadensis decreased the species diversity significantly during both spring and autumn;(3) Also, these two plants changed the soil characteristics to some extent, and the significant impacts were showed in soil moisture content and pH value.This research fills the blank of the knowledge of degradation mechanism of the riparian zone in plain river network area, and can be helpful to the evaluation, restoration and management work of riparian zone in future.