Landslides, Alien Species, and the Diversity of a Hawaiian Montane Mesic Ecosystem1

Abstract

In the Ninole ridges of Hawai'i, we evaluated changes in plant species diversity and composition and the effect of alien species on the growth and recruitment of native species after disturbance by landsliding. We chose nine landslides representing three age categories (young, 4–17 yr; intermediate, 18–42 yr; and old, ca 130 yr) plus three undisturbed forest sites (325–525 yr) to sample and manipulate the vegetation; the undisturbed forest developed on tephra-derived soils underlain by basalt. The ordination of sites and species using detrended correspondence analysis (DCA) showed that species establishing on landslides were different than those found in the undisturbed forest; moreover, a large fraction of the species establishing on young landslides were represented by aliens, mostly grasses and orchids. The manipulation of alien species (clipping of aboveground parts and removal of above- and belowground parts, including soil) on young landslides (<17 yr) significantly affected seedling growth of the native tree species Metrosideros polymorpha and the recruitment of native species. In addition, the manipulation of grasses and orchids favored the recruitment of a second group of alien species that were uncommon in, or absent from, our study sites. The removal of tephra-derived soils by landslides, in combination with an increased presence of alien species, may greatly alter successional trajectories in the Ninole ridges of Hawai'i.