Experimental design
The experiment was conducted in 2007 at the Pasture Ecological Research Station of Northeast Normal University, Changling, Jilin province, China (44°45’ N, 123°45’ E). Seeds were collected from local wild populations near the research station in late August 2006 and dry stored at -4oC. We used a split plot design, with soil conditions as the main factor, density and block as a sub-factor. Two large plots were assigned as two (Infertile and Fertile) soil conditions, each was divided into nine 2 × 3 m sub-plots and randomly arranged with three treatments of densities and blocks. The treatment of infertile soil conditions as a plot was established using the original soil of experimental field at the station that had been used annually for many years (aeolian sandy soil), and the treatment of fertile soil conditions was set up by covering the other large plot with 5-10 cm virgin soil transported from a nearby meadow with no cultivation history (meadow soil), with contrasting nutrient contents of the two soil conditions (Wang et al. 2017). Seeds of A. theophrastiwere sown on June 7, 2007, with three inter-planting distances of 30, 20 and 10 cm, to reach target plant densities of 13.4, 36 and 121 plants per m-2, assigned as Low, Medium and High density treatments respectively. Most seeds emerged 4 d after sowing. Seedlings were thinned to the target densities at four-leaf stage. Plots were hand-weeded when necessary and watered regularly.