Abstract: Overgrazing has resulted in degradation of grassland ecosystems, with the consequences of decreased dominance of foraging grasses and increased dominance of some poisonous species. As an indicator of grassland degradation, the impacts of poisonous species on adjacent plant species and soil environment are one of the important scientific issues in ecological research. To better explore the impacts of poisonous species in the restoration process of degraded grassland ecosystem, we selected the fenced and restored Leymus chinensis grasslands in Hailaer naturereserve as the research object. According to branch number of S. chamaejasme (a poisonous plant) per unit area, plant communities were classified into three types: community with high-density, low-density, and without S. chamaejasme, to investigate community species composition and soil physicochemical characteristics. The results showed that: (1) High density of S. chamaejasme altered plant community composition at small spatial scale by increasing Shannon index and Patrick index, which indicated the significant effects of S. chamaejasme on species richness. (2) With increasing S. chamaejasme density, the association properties of some Gramineaespecies and S. chamaejasme were changed. The association properties of Poa annua,Stipa baikalensis and S. chamaejasmewere changed from positive to negative, while that of Koeleria litvinowiiand S. chamaejasme was changed from no association to negative. The associations amongLeymus chinensis, Cleistogenes squarrosa, Achnatherum sibiricumand S. chamaejasme were unchanged. (3) S. chamaejasme could alter the relative importance of plant families in the community. Increased S. chamaejasme dominance enhanced the importance value of Leguminosae and Rosaceae, but decreased that of Compositae.There was no obvious change of Gramineae in importance value. (4) The presence of S. chamaejasme affected soil total nitrogen and pH. The community with low density S. chamaejasme had significantly higher soil total nitrogen content than the other two groups. It also had significantly higher soil pH than the community without S. chamaejasme. There were no significant differences in total carbon, available nitrogen and phosphorus in soils among all the three types of plant communities. The smallscale change of soil physicochemical properties might be responsible for promoting vegetation restoration in degraded grasslands.  

Key words: Stellera chamaejasme, diversity index, interspecific association, soil nutrient.