Experiment 3: Observation of biological characteristics
In the ES (April 20), LS (May 20), EG (June 20), LG (July 20), FR (August 20), and MR (September 20) periods in 2017, the densities, coverage, and plant heights of the two Ambrosia species and companion species were measured in the four habitats. Each observation plot was 5 m × 5 m. The plant heights were measured for 30 plants ofA. artemisiifolia and A. trifida , and 30 plants of companion species in each plot. If there were fewer than 30Ambrosia or companion species plants, we measure all of them. Three plots from each habitat were taken in three individual sites, for a total of 3 (repetition) × 4 (habitat) sampling units in 12 plots being set up.
Six A. artemisiifolia and A. trifida plants were randomly selected from each sample plot, and all seeds counted on these plants were removed in September 2017. If some seeds had fallen, we estimated the number based on the locations of the seeds. A total of 100 seeds from each plant were randomly selected, air-dried, and weighed with 0.0001 g precision on an electronic balance (BDS, China). Twenty seeds were randomly selected from each plant, and the lengths and widths of these seeds were measured with Vernier calipers (BDS, China) to calculate the average seed size using Eqs. (3) and (4), with three repetitions for each:
Seed size = seed length × seed width. (3)
Seed yield m−2 = average number of seeds per plant × the number of plants m−2. (4)