Experimental introduction of the alien plant Hieracium lepidulum reveals no significant impact on montane plant communities in New Zealand

Aim There is debate over whether alien plants necessarily alter the communities they invade or can coexist with native species without discernable impacts. We followed the fate of montane plant communities in response to the experimental sowing of the alien weed Hieracium lepidulum, looking for changes in plant community composition and structure over 6 years. Location Craigieburn Range, New Zealand. Methods We used a replicated randomised block design, with 30 x 30 cm plots (n = 756) subdivided into 5 x 5 cm cells to examine and compare the effects of H. lepidulum at 0.09 m2 (plot) and 0.0025 m2 (cell) scales. Plots were sown with between 0 and 15,625 H. lepidulum seeds in 2003, forming gradients of invader density and cover. Measurements comprised community richness, evenness and diversity along with H. lepidulum density and cover at both scales. The relationships between the invader and local community attributes were modelled using hierarchical mixed-effect models. Results Plant communities differed in the extent to which they became invaded, with H. lepidulum cover in the plots ranging from 0% to 52%, with a mean of only 1.89%. Plot species richness increased from 2003 to 2009, with a component of this increase (+0.002 species per year) associated with increasing H. lepidulum density. Other relationships between the plant community and H. lepidulum were generally non-significant. Main conclusions In these montane plant communities, it appears H. lepidulum coexists with the native community with no measurable negative effects after 6 years on species richness, evenness or diversity, even where density and cover of the invader are highest. We suggest H. lepidulum has persisted preferentially at those sites with abiotic conditions sufficient to support a species-rich assemblage.