The Distribution Pattern And Response To Different Disturbance Of Alien Herbs In Yiluo River Watershed

Yiluo River is the largest tributary of the Middle Yellow River downstream of the Xiaolangdi Reservoir. The Yiluo river basin consists of mainly mountainous and hilly terrains. The upstream areas are rich in forest resources; the middle-stream region is dominated by hills endowed with rich deposits of minerals; the downstream region is occupied largely by an alluvial plain that is one of the major grain production bases of the Henan province. Anthropogenic disturbance gradually intensifies as one moves downstream from the source of the Yiluo river.This paper describes the first ever comprehensive investigation of alien herbaceous plants in the Yiluo river basin. We selected 51 sampling sites along the courses of the Yiluo river and its major tributaries. Each sampling site consisted of three habitat types:riparian, farmland, and artificial woodland. Within each habitat type of a sampling site, we set up a sampling plot (20m×20m), which contained 5 quadrats (1m×1m). We recorded the species and number of alien herbaceous plants in each quardrat. We also measured leaf area, plant height, stem diameter, flower/fruit number, and root length of several representative alien species.We recorded 27 alien herbaceous species, belonging to 15 families, in the Yiluo river basin: Phytolacca americana of the Phytolaccaceae family; Chenopodium ambrosioides of the Chenopodiaceae family; Alternanthera philoxerides, A. retroflexus, A. viridis, and A. spinosus of the Amaranthaceae family; Leoidium virginicum of the Cruciferae family; Medicago sativa, M. polymopha, and Meliotus albus of the Leguminosae family; Ricinus communis of the Euphorbiaceae family; Gaura parvfloa of the Onagraceae family; Hibiscus trionum and Abutilon theophrasti of the Malvaceae family; Daucus carota of the Umbelliferae family; Pharbitis nil and P. purpurea of the Convolvulaceae family; Datura stramonium of the Solanaceae fanmily;Oxalis corymbosa of the Oxalidaceae family; Veronica polita of the Scrophulariaceae family; Conyza canadensis, C. bonariensis, Erigeron annuus, Aster subulatus, and Bidens pilosa of the Compositae family; Aegilops squarrosa, and Avena fatua of the Gramineae family. Alternanthera philoxerides is an invasive species. We counted 571 alien herbaceous plants in riparian plots,452 in woodland plots, and 299 in farmland plots. The number of alien herbaceous species appeared to increase in the downstream direction.The alien species showed clear habit preferences. Amaranthus retroflexus, Abutilon theophrasti were typically found in farmlands; Bidens pilosa, Aster subulatus and Alternanthera philoxerides occurred mainly in riparian strips; Oxalis corymbos, Ipomoea purpurea, Pharbitis nil and Medicago sativa preferred woodlands; Conyza canadensis, Erigeron annuus and Gaura parvflo were equally well adapted to riparian and woodland habitats. The 5 Compositae species were the most widely distributed alien herbaceous plants found in the Yiluo river basin. Large numbers of Alternanthera philoxerides were observed in downstream areas. Leaf area, plant height, stem diameter, and root length of the representative alien species appeared to increase with disturbance intensity.