Invasive alien clonal plants are competitively superior over co-occurring native clonal plants

Abstract

Alien plant invasions are a major component of global change, and can severely threaten local floras and global biodiversity. A striking pattern of invasive floras is that many of the world’s worst invasive plants are clonal species. However, it is still unclear whether invasive clonal plant species are in general competitively superior over native clonal plant species and whether this pattern can be affected by environmental conditions. To test whether nitrogen (N) availability and suppression of enemies can alter the competitive outcome between invasive alien and resident native clonal plants, we selected five pairs of invasive alien and co-occurring native clonal plant species in China. We grew them both in monocultures (i.e. with intraspecific competition) and in two-species mixtures (i.e. with interspecific competition) with or without N addition, and with or without suppression of enemies (using insecticide and fungicide application). Overall, invasive alien clonal plants produced significantly more biomass than native clonal plants, and took more advantage of N addition. Invasive clonal plants tended to grow better with interspecific than with intraspecific competition, whereas the reverse was true for native clonal plants. This species-origin-by-competition interaction, however, was weaker with than without N addition. Without enemy suppression, native clonal plants experienced more leaf damage than invasive aliens when they grew in interspecific competition with each other, but only in the absence of N addition. Nevertheless, enemy suppression benefited growth of invasive alien and native clonal plants to similar degrees. Our results show that already under strongly N-limited conditions, invasive clonal plants in China are competitively superior over natives, and that this competitive superiority of invasive clonal plants becomes even stronger after N addition. This implies that the ongoing eutrophication of many habitats is likely to further stimulate invasions by clonal plants.