Seed germination
Seed germination was analyzed in the laboratory. In October 2016, the seeds of A. artemisiifolia and A. trifida were collected from four habitats in the Yili Valley and combined. The amount of seeds between habitats was set to be equal, and the seeds were initially stored in the dark at 0–5°C in a cold storage room with 40% relative humidity (Bae, Benoit, & Watson, 2016). In June 2017, 50 g heat-dried in situ soil samples were weighed, and each sample was placed in a Petri dish. Next, 2.5, 5, 7.5, 10, and 12.5 g of distilled water was added to each Petri dish, resulting in the soil moisture contents in the various Petri dishes of 5%, 10%, 15%, 20%, and 25%, respectively. For each sample, 20 fully developed undamaged same-sized seeds of A. artemisiifolia or A. trifida were uniformly spread on the soil surface in Petri dishes. Each group of seeds was evenly placed in Petri dishes. The seeds were treated in a climatic chamber (GTOP-150Y, China) for 60 d at 20–10°C, 12h/12h light/darkness, and 3000 lx light intensity, after which the germination rate was calculated by counting the number of germinated seeds. Seeds with the seed radicle at least 0.2 mm long were considered to have germinated. Seed germination was checked every day. When no seeds germinated in a single Petri dish for five consecutive days, it was regarded as the end of germination.