The expansion of koa forest after cattle and goat removal, Hawai'i Volcanoes National Park

Abstract

Cattle (Bos taurus) were the most destructive agents of Hawaiian forests in the nineteenth century. Cattle continuously grazed in koa (Acacia koa) communities on Mauna Loa in Hawai'i Volcanoes National Park from the early 1800s to 1948. Park managers documented the deleterious impact of cattle grazing on koa regeneration in 1941 by establishing exclosures and vegetation plots. Prolific regeneration occurred with cattle exclusion. We relocated their exclosures in 1992-1994, re-photographed the stands from the same photopoints, counted koa and other trees by height and/or diameter classes, and determined cover of understory vegetation. We also located the exclosures on aerial photographs from 1954, 1965, 1977, 1986, and 1992. We found that koa has nearly disappeared from the adjacent ranch land where cattle grazing continued, whereas koa stands have expanded greatly by root suckering to form large stands of continuous forest in the Park. Koa stands expanded little in the 20 yrs following termination of grazing in 1948, probably because of the presence of stray cattle and building populations of feral goats (Capra hircus). By the mid-1970s stray cattle and feral goats were controlled and expansion of koa stands greatly accelerated. Koa forest canopy may become more open in the future. Dense mats of alien vegetation have developed in the koa understory and may be inhibiting koa regeneration, and a lethal vascular wilt disease has spread. We need to understand the current dynamics of koa forest that have developed in the absence of cattle and goats better before additional management of koa forests is undertaken.