Grading and distribution patterns of invasive plants in riparian area of Hai Basin.
Investigating distribution patterns of invasive plants in riparian areas and evaluating their invasive grades are greatly useful to understand the degree of degradation of river ecosystems. The riparian vegetation from 402 sampling sites in nine river systems of Hai Basin was investigated. Invasive species were confirmed according to the Checklist of Chinese Invasive Plants. Invasive grades were evaluated via the species appearance frequency and the percentage of sites where the species were dominant. The relationship between plant invasion and environmental factors was analyzed as well. The results show that 48 invasive species were registered, of which 72.9% are annual plants. Asteraceae, Amaranthaceae and Gramineae were the families with most invasive species. Amaranthus retroflexusn, Abutilon theophrasti, Chenopodium serotinum and Eclipta prostrata were classified as the invasion grade I, appearing in more than 25% of sampling sites. 10 species including Bidens pilosa, Bidens frondosa, Conyza canadensis and Pharbitis purpurea, were classified into grade II, and 32 as grade III. The average number of invasive species in each river system was 23. Ziya River system had the highest number at 33 species, while Zhangwei River system had the least at 15 species. The altitude and land use type outside dykes were the two major factors affecting the species distribution, and also indirectly reflecting the pressure from populations.