Experimental design
We conducted the field experiment in 2007 at the Pasture Ecological Research Station of Northeast Normal University, Changling, Jilin province, China (44°45’ N, 123°45’ E). The original soil of experimental field (aeolian sandy soil, pH = 8.3) at the station had been used annually for many years, thereby low in nutrients availability (organic C 3.1 mg kg–1, available N 21.0 mg kg–1, and available P 1.1 mg kg–1) during the growth season of 2007 (Zhao et al., 2010). Seeds of A. theophrasti were collected from local wild populations near the research station in late August 2006 and dry stored at -4oC. We applied a randomized block design, with germination timing (GT) as the main factor, and block as the sub-factor. The whole plot was divided into twelve 2 × 3 m sub-plots, which were randomly assigned with four GT treatments and three blocks. Seeds ofA. theophrasti were sown at an inter-planting distance of 10 cm on June 7, June 27, July 17 and August 7, as four GT treatments of spring (GT1), late spring (GT2), summer (GT3) and late summer (GT4). The treatments of germination timing accorded with the time range of germination of A. theophrasti in its nature habitats in northeast of China. Most seeds emerged four days after sowing. Seedlings were thinned at four-leaf stage and plots were hand-weeded when necessary and watered regularly.