Cattle grazing, forest loss, and fuel loading in a dry forest ecosystem at Pu'u Wa'aWa'a Ranch, Hawai'i

摘要

We used aerial photographs to measure long-term loss of forest cover in dry to mesic forests at Pu'u Wa'aWa'a on the island of Hawai'i. The aerial extent of dense forest decreased 62 percent between 1954 and 1994, and the area covered by grassland increased by 237 percent. We then used field data and modeling to evaluate the consequences of removing cattle to the composition, biomass, and flammability of the grass community. Field studies of exclosures demonstrated that grazing reduced biomass of kikuyu grass (Pennisetum clandestinum) from 770 to 229 g/m~2, and canopy height from 26.8 to 6.1 cm. Grazed kikuyu was unable to carry fast-spreading or intense fires (maximum flame length 70 cm, covering 1.4 ha after one hour), while ungrazed kikuyu could carry catastrophic fires (flame lengths 3.0 m, covering >75 ha after one hour). In addition, removing cattle from drier areas allowed the spread of fountain grass (Pennisetum setaceum), which created a higher risk of fire due to its architecture and low fuel moistures.