Abstract
The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre-invasion and post-invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post-invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre-invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader-induced soil chemical/microbial changes in nutrient availability are discussed.