Characteristics Of Riparian Vegetation And Its Correlation With Environmental Factors In Danjiangkou Reservoir

Riparian zone is the transition area between aquatic and terrestrial ecosystems located along rivers, streams as well as lakes, and it is the critical zone for the interaction between aquatic and terrestrial ecosystems. Although riparian zone is a small portion of the global ecosystem, it plays an important role in ecological system. Riparian vegetation performs important ecological functions in protecting water ecological security, stabilizing the bank, maintaining habitats connectivity and enhancing local biological diversity, which is the last defense against water pollution. However, riparian vegetation has been degraded severely throughout the world because of various factors. Danjiangkou Reservoir (32°36’-33°48’N,110°59’-111°49’E), located in Central China, is the water source for the Middle-Route of the South-to-North Water Transfer Project. Danjiangkou Reservoir has operated for almost40years since the dam was completed in1973. Through the natural ecological succession of40years, the riparian vegetation of Danjiangkou Reservoir is present distinctive characteristics in the area between the lowest water level of140m and the highest water level of157m, and the area above the highest water level which may be affected by elevated water level or flooding occasionally, which is distinctive form terrestrial and aquatic vegetation. The water transfer project will operate in2014, the normal water level of Danjiangkou Reservoir will be up to170m. Most of the existing riparian vegetation will been submerged, and the new riparian zone will been shaped at the higher elevation. How to protect and restore the riparian vegetation and giving full play to its ecological functions is significant to the protection of water quality and the normal operation of the Middle-Route of the South-to-North Water Transfer Project. Exploring the characteristics of the riparian vegetation and its relation to environment factors has important significance for conservation and restoration of riparian vegetation. In this paper, the characteristics of the riparian vegetation and its relation to environmental factors were analyzed based on field investigation and laboratory experiment. The main results are as follows:(1) There were285species in the riparian zone of Danjiangkou Reservoir, which belonged to208genera and72families. Among them there were3species,3genera and3families of pteridophyte,4species,3genera and2families of gymnospermae,278species,202genera and67families of angiosperm. Species composition of the riparian vegetation displayed two obvious trends in families, on the one hand the big world families, such as Leguminosae, Gramineae and Compositae took large proportion, on the other hand small families and regional single-species families also owned a certain proportion. It indicated that the characteristic in species composition of this area is few large families and some small families. In addition, the dominant genera were evident. And small genera and regional single-species genera also owned a large proportion. This indicated that the environmental conditions of the riparian zone were complex, and high environmental heterogeneity provided habitats for most species. The floristic analysis showed that tropic and temperate types were dominant, this indicated that the climate feature of Danjiangkou Reservoir was the transition from North Temperate Zone to subtropical zone. There were35trees,49shrubs,7woody vines,3herbaceous pteridophytes,73annual or biennial herbs and118perennial herbs. Herb species were the dominant groups, which were the major constructive species of the riparian vegetation.(2) Seven plant community types were clearly identified in the riparian zone of Danjiangkou Reservoir. There were Polygonum aviculare community, Abutilon theophrasti community, Oenanthe dielsii var. stenophylla community, Cynodon dactylon community, Populus adenopoda community, Pyrus betulifolia community and Platycladus orientalis community. The riparian plant communities were dominated by herbaceous plant communities, with a small number of woody plant communities. It indicated that the severe environmental conditions of the riparian zone resulted in the simplification of vegetation structure. Seven plant community types distributed along the gradients of elevation and flooding influence, while there were some overlaps among different plant community types. Plant communities commonly displayed a distribution pattern with annual herbaceous species dominating the lower wet zones, perennial herbaceous species occupying the moderate elevation, larger shrubs appeared at the mid-up land, and trees occurring in the high zones where less prone to inundation. The total macroscopic feature of the riparian vegetation is herbaceous plant communities based with a few woody communities appearing at the upper land.(3) Height above water and hydrologic factors (flood duration and flood frequency) were significantly associated with the variation in species composition within the riparian vegetation. The soil property also had a strong influence on the characteristics of the riparian vegetation, while the topographic factor, slope and aspect, had a little influence on the riparian vegetation. There were respectively13environmental variables significantly explaining the variation of species composition, flood duration had the strongest explanatory power, followed by height above water, flood frequency, soil organic matter content, soil pH, soil thickness, soil available phosphorus content, soil total nitrogen content, soil available nitrogen content, slope, aspect, soil total phosphorus content and soil thickness, whose the explanatory power decreased successively. With regard to topographic, soil and hydrological factors, the explanatory power of soil factor was more than the explanatory power of topographic factor, and the explanatory power of hydrological factor was least. There were interactive effects among environmental factors, the interactive effect among topographic, soil and hydrological factors was largest, followed by the interactive effect between topographic and hydrological factors, the interactive effect between soil and hydrological factors. The interactive effect between topographic and soil factors was smallest. The collected environmental factors accounted for21.99%of the variation of the riparian vegetation, the unexplained portion accounted for78.01%.(4) Based on Principal Component Analysis and multiple regression, the regression models was obtained between plant diversity of the riparian vegetation and the environmental factors. The results indicated that there were greater correlation between the species richness and aspect, soil thickness, slope, height above water and soil available nitrogen content. And there were greater correlation between the species diversity and soil thickness, aspect, soil organic matter content, soil available nitrogen content and flood frequency. Based on Pearson correlation coefficient, the species richness correlated significantly with height above water, soil texture, flood duration, aspect and soil thickness, and to the significant correlate to the species diversity were soil thickness, soil organic matter content, flood frequency and soil available nitrogen content. Based on the analyses of the explanatory power of environmental variables respectively, the distribution discipline of species richness along the gradient of height above water was quadratic parabola, it along the gradient of soil texture was cubic parabola, it along the gradient of flood duration per year was cubic parabola, and it along the gradient of soil thickness was power function curve; The relationships between the species diversity and soil thickness was power function curve, the species diversity and soil organic matter content showed a cubic parabola relationship, and the distribution discipline of species diversity along the flood frequency was exponential curve.(5) There were44species in the soil seed bank of the riparian vegetation in Danjiangkou Reservoir, which belonged to38genera and20families. Herbs species were the dominant groups, and only two woody species appeared, Sophora davidii and Discocleidion rufescens. The average seed density of the soil seed bank was8950seeds/m2, and annual herbs had the greatest seed density. Comparing the quantitative characteristics of the soil seed banks from the different flood disturbance zone, it showed that the species richness and seed density of the soil seed bank were increased with the increase of flood disturbance. This result indicated that the vegetation degradation causing by flooding disturbance couldn’t have a significant impact on the seed density of the soil seed bank in the riparian zone, and the restoration of seriously disturbed riparian vegetation was not seed-limited. The soil seed bank and aboveground vegetation were obviously different in the riparian zone, and the flood disturbance had obvious significance to the similarity between the soil seed bank and aboveground vegetation. The discrepancy between the soil seed bank and aboveground vegetation was greatest at the moderate inundated zone, and smallest at the permanent inundated zone. Low similarity between the soil seed bank and the aboveground vegetation in riparian land indicated that the soil seed bank played a minor role in contributing to the regeneration of vegetation, and the managers could not just rely on soil seed bank for vegetation restoration. However, the soil seed bank was closely connected with the aboveground vegetation and was an important source for vegetation regeneration and restoration. The relationships between the soil seed bank and the environmental factors were analyzed. The seed density of the soil seed bank was mainly influenced by the cover of the above-ground vegetation, soil available nitrogen content and soil total phosphorus content. Based on Pearson correlation coefficient, the seed density of the soil seed bank correlated significantly with soil organic matter content, the cover of the aboveground vegetation, soil available nitrogen content, flood disturbance and saturated moisture content of soil. The seed density of the soil seed bank revealed a linear increasing trend along the increase of saturated moisture content of soil, and the seed density of the soil seed bank and flood disturbance showed an exponential curve relationship.