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.