Long period exposure to serious cadmium pollution benefits an invasive plant (Alternanthera philoxeroides) competing with its native congener (Alternanthera sessilis)

Abstract

Many aggressive plants possess high tolerance to heavy metals, but little is known about their invasiveness at heavy metal polluted sites. We performed a greenhouse experiment to examine the impacts of Cd (0, 10, 30, 60, and 100 mg kg−1) and inter-specific competition on the reproductive capability of an invasive plant, Alternanthera philoxeroides, and its native congener, Alternanthera sessilis. We also examined the population dynamics of both native and invasive species in a simulated field experiment. Compared with A. philoxeroides, native A. sessilis was a stronger competitor as measured by vegetative growth, sexual reproduction, and dominance status in a mixed culture. However, A. philoxeroides showed great plasticity in root mass ratio that was positively affected by inter-specific competition and high Cd levels. Such high root allocation might allow for delayed growth of A. philoxeroides rhizomes as the relative cover of A. philoxeroides to A. sessilis in the field experiment gradually increased and > 1 after nine months culture, especially at high Cd treatment. Our results suggest that the invasiveness of A. philoxeroides is highly context- and time-dependent. In severely polluted environments, clonal propagation of A. sessilis is likely inhibited by the synergistic negative effects of inter-specific competition and heavy metal pollution, and a long time co-existence of these two competing species would facilitate the colonization of invasive plant.