Effects of interspecific alien versus intraspecific native competition on growth of native woody plants

Abstract

The success of invasive plants and their impacts on community structure are commonly explained by referring to their supposed higher competitive ability. However, invasive plants do not consistently outperform native species; and the role of competition may also depend on the stage of the invasion process. This study investigated competitive effects of woody alien plant species at an early stage of invasion (Archontophoenix cunninghamiana and Psidium guajava) on closely related New Zealand native species of similar life form (Rhopalostylis sapida and Lophomyrtus bullata, respectively) at several points over time in a shadehouse experiment. Effects of interspecific competition from the paired alien species seedlings on a native seedling were compared to the effects from intraspecific competition of other seedlings of the native species over a 65 week period. Mortality was low throughout the experiment. The native species were affected by density-dependent competition generally, resulting in decreased performance with increasing neighbour density or biomass. The alien palm, A. cunninghamiana, however, had stronger competitive effects on the native palm, R. sapida, than intraspecific competition among R. sapida individuals. Also, P. guajava showed some stronger competitive effects on L. bullata than intraspecific competition among L. bullata individuals. These alien species displayed some traits often associated with invasive plants: A. cunninghamiana had higher RGR, height, biomass production and SLA than R. sapida, while P. guajava had higher SLA than L. bullata. Competitive ability of these newly naturalised species may be an important factor influencing their establishment success and subsequent invasion potential.