Native plant regeneration is virtually absent in lowland, exotic-dominated forests on the island of Hawai'i

Humans have rapidly accelerated the rate of biological invasions, and the global proliferation of invasive species can result in decreased biodiversity and altered ecosystem functioning. When invasions are intense and invaders persist over time, ecosystems can be transformed and become entirely dominated by exotic species, and these exotic-dominated ecosystems are widespread and likely to increase in abundance. Despite a clear threat to biodiversity, some have suggested that exotic-dominated ecosystems may provide refugia for native species and promote their regeneration. Thus, elucidating the role of native species recruitment and regeneration in exotic-dominated ecosystems is critical for restoration and conservation strategies. We surveyed 41 sites in Hawai’i to determine whether native species occur in the understories of forests with exotic-dominated canopies, and compared native abundance with disturbance history and other site factors including canopy openness, lava age, and proximity to native propagule sources. Native trees less than 10 cm in diameter were totally absent in 25 of the 41 sites and rare in the others. In addition, natives were never the dominant understory species; in fact, they accounted for less than 10% of understory basal area at all but 5 sites. Disturbance history significantly affected native understory basal area and density (one-way ANOVA; P = .002 and .038, respectively). Natives were absent in the understories of all but one of the 17 highly disturbed sites, but were present in seven of the eight undisturbed sites. Even in these totally disturbance-free sites, however, natives accounted for only 14% of understory basal area and 8% of understory stem density. Overall, we found little evidence that native species will become a significant component of exotic-dominated forests on Hawai’i.