Background Vegetation in riparian zone is an important buffer to sustain the health of a river system. It is a typical ecotone characterized as edge effect, and plays a significant role in the conservation of river system health. Therefore, it is vital to know how the vegetation in floodplain distributes and what the driven-forces of the distribution are.

Methods Taking the Lingshan River, in Longyou County, Zhejiang Province, as a typical example of small and medium rivers in hilly area, the characteristics of the vegetation distribution and its driven-forces were studied. After investigating the vegetation species in the quadrat sampling, the data was calculated and analyzed using the Clustering Analysis (CA) in SPSS software, and the Redundancy Analysis (RDA) in Canoco5 software and through which the composition and cluster numbers of vegetation species, the cosine of the angle at the origin of the two vectors, the contribution rate of driven-forces were calculated. On the basis of GPS site survey and the maps downloaded from Google Earth, the elevation and shape coefficient were analyzed by Digital Elevation Model (DEM) in Surfer software. Then combined with the regression analysis, the relationship between the vegetation and driven-forces were obtained.

Results In the floodplain of Lingshan River, there were 62 vegetation species (61 kinds of Angiospermaes and one kind of Pteridophyta) and the vegetation community structure was mainly divided into 9 classes. The major three driven-forces of the vegetation distribution were elevation factor (ButtElev), shape coefficient (Shapindx) and hydrological characteristics (HydrChar), and their contribution rates were 37.50%, 27.50%, 16.82%, respectively. Particularly, under the influence of ButtElev, from outer to inner edge of floodplain, the moisture capacity varied from strong to weak, the diversity varied from low to high, and the biomass varied from small to large. There was at least 60 percent of the guarantee rate making the vegetation diversity between 1.03 and 1.96 when the SDI coefficient was between 2.0 and 4.0, and the Pe/Pa was between 0.12 and 0.3. The hydrological characteristics affected the morphological plasticity of vegetation and the habitat conditions of the floodplain. The sand content in the outer zone of the floodplain decreased with the increase of the distance from the water, and the anti-scour ability and moisture capacity of the vegetation decreased from strong to weak.

Conclusions The elevation factor, shape coefficient and hydrological characteristics are the key driven-forces of vegetation distribution in small and medium rivers in hilly area. When we design ecological restoration projects, the three factors should be focused on, and the suitable measures should be employed in order to control and optimize the three factors.