Can Non-Native Species Explain Patterns of Convergence and Deviation in Regenerating Coastal Dune Forest?

The successful restoration of disturbed habitat is influenced by many factors; not least is the introduction of non-native species into the regional species pool. Such species may preclude native colonization and deflect regeneration trajectories away from restoration targets. Successful restoration (commonly measured against reference sites) may therefore be an unobtainable goal. We aimed to identify whether non-native species divert regenerating trajectories of coastal dune forest. Using measures of ecological distance we first determined if successional trajectories of the herbaceous plant community in regenerating coastal dune forest sites were convergent. We then determined if multiple regenerating coastal dune forest sites became more similar to an undisturbed reference site as they aged and which species (both natives and non-natives) contributed the most to dissimilarity between the reference site and regenerating sites. The species composition in regenerating coastal dune forest plots became increasingly convergent as the time since disturbance increased. However, species composition appeared to deviate from that within an undisturbed reference site. Contrary to our expectations, non-native species did not contribute the most to dissimilarity, and thus not to the recorded deviation. The deviation from the reference forest is attributable to the higher abundance of 1) a native forest specialist in the reference site, and 2) the higher abundances of native species adapted to open canopy in the regenerating sites. This deviation of the species composition in regenerating sites from that in the undisturbed reference site may therefore be indicative of successional changes and is not attributable to the presence of non-native species.