The relationships between invasive plant species and environmental and anthropogenic factors were analyzed in five nature reserves in northern China. The distribution of invasive plants was analyzed using a Canonical Correspondence Analysis. Stepwise backward multiple regression was then done to identify the most important environmental and anthropogenic factors determining the richness and distribution of alien plants in the reserves. Total invasive plant richness varied was 25 species, 12 species, 13 species, five species and five species across the nature reserves. Invasive species richness decreased in response to environmental factors such as maximum temperature and minimum precipitation and increased in line with human activities, the total area of farmland and volume of freight. Tropical plant species declined from 31% to zero while the species originally from North America gradually increased. In respect of family composition of invasive species, most species belonged to the composites, whose proportion increased from 15%-40% from north to south across the five nature reserves while species from other families varied greatly along this gradient. Life-forms of invasive plant species varied between the reserves, but annual and perennial herbs were generally dominant. Pathways of such species were highly related to human activities and utilizations.
The objective of this paper is to test three main hypotheses in grassland succession by using 17-year observational data： （i） species diversity increases during early pioneering stages; （ii） the similarity rate of experimental plant communities increases along with the succession process （dissimilarity rate among plots decrease with succession age）;and （i i） plant communities in different ifeld plots converge towards a quite similar composition during the initial years of succession. Results draw from quantitative statistics which including： the general development of plant communities, Shannon’s H and E, Bray-Curtis dissimilarity and community turnover, indicate that after first 5 years of succession, species diversity rapidly becomes alike among all the plots. The average species number in al plots increased constantly from 8 to 25. Meanwhile, the total species number for al plots increased constantly from 23 to 55, accompanied by a steady decrease in Standard Deviation （S.D.） from 4.6 to 0.3. Additional y, dissimilarity of al stands in species composition decreased, indicating a clear trend towards a rapid convergence in species richness and composition. The most important effect of cessation of weeding is that the artiifcial diversity gradient from 1 to 16 species gets diminished by immigration in species poor plots.
