Vegetation recovery following pig removal in 'Ola'a-Koa Rainforest Unit, Hawaii Volcanoes National Park

Abstract

A total of 16, 10 x 10 m plots were established in pig disturbed areas to monitor the vegetation changes following removal of pigs from 'Ola'a-Koa Rain Forest Unit in Hawai'i Volcanoes National Park. Native understory cover increased 48% from 1991 to 1998, largely in the first two years following pig removal from the area. Understory ferns made the largest gains followed by tree ferns and native woody species. A flush of seedlings and saplings of subcanopy native trees observed after pig removal may potentially increases tree canopy cover in the future. Alien understory vegetation increased 190% throughout the seven year study. Almost all alien plant cover increases resulted from the expansion in cover of three very aggressive species, kahili ginger (Hedychium gardnerianum), Himalayan raspberry (Rubus ellipiticus), and palm grass (Setaria palmaefolia). These species are able to successfully establish independent of pig activity and are spreading in spite of pig control, not because of it. For other alien species, the greater competition and shade environment created from increased numbers of native trees, tree ferns, and understory plants, along with the lack of physical disturbance by pigs, appear to inhibit their establishment and spread. The presence of Banana poka (Passiflora mollissima) was reduced from 81 % to 40% in the plots. Several herbaceous weeds, Cardamine flexuosa, Hypericum mutilatum, Ludwigia palustris, that were initially present in a few plots were no longer present by the end of the study period. This study indicates the value of pig control as a first important step in the restoration of native Hawaiian rain forest. Monitoring in future years is needed to more accurately predict trends in native understory and sub-canopy tree species. Follow up alien plant control of the more disruptive species should begin as soon as possible after pig removal while these species densities are still low.