Heavy metal pollution is more conducive to the independent invasion of Solidago canadensis L. than the co-invasion of two Asteraceae invasive plants

Allelopathy is one of the primary factors that affects the successful invasion of invasive plants. Two invasive plants can often co-invade the same habitat. However, increasing amounts of heavy metal contamination can alter the independent allelopathy and co-allelopathy of two invasive plants that co-invade the same habitat. However, research on the co-allelopathy of two invasive plants treated with different types of heavy metals is limited. This study aimed to analyze the independent allelopathy and co-allelopathy of the two Asteraceae invasive plants horseweed (Conyza canadensis [L.] Cronq.) and Canadian goldenrod (Solidago canadensis L.) on the seed germination and seedling growth of the horticultural Asteraceae species lettuce (Lactuca sativa L.) treated with different types of heavy metals, including the independent contamination with copper (Cu), the independent contamination of lead (Pb), and the co-contamination of Cu and Pb. The aqueous extract of S. canadensis leaves triggers stronger allelopathy than that of C. canadensis. There is an antagonistic effect for the co-allelopathy of the two Asteraceae invasive plants compared with their independent allelopathy. Both types of heavy metals can significantly intensify the independent allelopathy of S. canadensis and the co-allelopathy of two Asteraceae invasive plants but weaken the independent allelopathy of C. canadensis. Thus, the increase in heavy metal contamination can favor the independent invasion of S. canadensis and the co-invasion of two Asteraceae invasive plants by intensified allelopathy. The intensity of influence of co-allelopathy of both invasive plants was lower than that of the independent allelopathy of S. canadensis treated with heavy metals. Thus, heavy metal contamination is more conducive to the independent invasion of S. canadensis than the co-invasion of the two Asteraceae invasive plants.