The endemic tree Corynocarpus laevigatus (karaka) as a weedy invader in forest remnants of southern North Island, New Zealand

Native species can display invasiveness even in their natural systems. Corynocarpus laevigatus (Corynocarpaceae), karaka, is a tree species apparently native to the north of the North Island, New Zealand, with a history of human cultivation further south, where it appears to be outcompeting other native tree species. Invasiveness was assessed by comparing vegetation in paired plots with and without karaka at 14 sites in forest remnants in the lower half of the North Island. Potential to spread was estimated via dispersal and recruitment patterns around parent trees. Karaka has several weedy characteristics. Most seed falls near the parent, but recruitment apparently improves outside the shady canopy, resulting in concentric invasion rings. Compared with nearby non-karaka plots, karaka plots have fewer non-karaka trees, lower abundances of seedlings of other woody species, and slightly higher numbers of exotic species. Karaka appears to be homogenising vegetation, through reduction in herb cover and increase in shrub cover and probably eventual replacement of canopy trees. With its range anthropically extended, management of karaka's invasiveness appears necessary in lower North Island, South Island, and the Kermadec and Chatham Islands, involving elimination or control, depending on local cultural values.