Early succession in pig-disturbed mountain parkland: Hawai'i Volcanoes National Park

Abstract

Prior to their eradication in 1987, feral pigs (Sus scrofa) were a common source of ground disturbance in the mountain parkland ecosystem of Hawai'i Volcanoes National Park, especially in grasslands. In 1985-1986, 27, 2x5m plots were established in 9 open, grassy sites recently damaged by pigs. No pig digging occurred after the plots were initially established. The purpose of monitoring vegetation changes was to evaluate the efficacy of pig control in promoting community recovery and to detect any changes that might need further management intervention. Percent cover of plants was determined by point-intercept methods, and woody plants were counted in height classes in 1985, 1986, 1987, 1989, and 1992. The 27 plots were stratified into 5 plant communities based on a Braun-Blanquet analysis of surrounding vegetation, and successional changes were graphed. Deschampsia nubigena, a native bunchgrass, consistently increased in cover in most communities where it was originally a dominant or codominant species in surrounding vegetation. It did not increase in cover where it was initially a minor component. These trends suggest that Deschampsia will continue to increase in importance. Velvet grass (Holcus lanatus), the main competitor with Deschampsia in mid and upper elevation sites, recovered rapidly at first but generally declined from its maximum cover after 2-4 years. However, Holcus cover may be stable at upper elevation sites where it was dominant prior to disturbance. In lower and mid elevation sites, Holcus species may continue to decline in importance, while other alien grass species, e.g., sweet vernal grass (Anthoxanthum ordoratum) and beardgrass (Schizachyrium condensatum), show signs of becoming more abundant. There was some recruitment of native shrubs in the disturbed areas, especially at lower elevations. Koa (Acacia koa), a root-sprouting, clonal tree in the study area, invaded many sites. The continued expansion of koa colonies may, at least locally, be more ecologically important than competition between Deschampsia and alien grasses.