Long-term study by UH Hilo scientists shows Hawai‘i Island forests can regenerate once cattle and pigs are fenced out
Twenty-five years ago, a UH Hilo biologist tagged 7,000 trees in a declining Hawai‘i Island rainforest. A recent survey of the site reveals conservationists’ efforts are paying off.
Researchers at the University of Hawai‘i at Hilo waited 25 years for field results from a study investigating whether Hawai‘i Island’s higher-elevation tropical forests could rejuvenate after destructive cattle and pigs were fenced out. The recent discovery of new and thriving growth of keiki ʻōhiʻa and koa in the studied area is good news about the forests’ native trees and the threatened bird species for whom the trees provide habitat.
UH Hilo scientists Patrick J. Hart, Thomas Ibanez, Shea Uehana, and Joshua Pang-Ching‘s paper, “Forest regeneration following ungulate removal in a montane Hawaiian wet forest,” was published January in the journal Restoration Ecology. Hart is a professor and Ibanez a post-doctoral fellow, both from UH Hilo’s biology department. Uehana and Pang-Ching are recent alumni of UH Hilo’s graduate program in tropical conservation biology and environmental science.
The study
Hawai‘i Island’s tropical forests evolved without large herbivores and were ill-prepared to withstand their arrival in the 18th century. Cattle were introduced to the island in 1793, and their numbers increased rapidly, with feral cattle roaming the island freely. By 1960, 65 percent of the island was grazing land, most of which was formerly forest, and the cattle also were allowed to graze the remaining forested areas. Following the introduction of cattle, feral pigs were introduced and also caused extensive damage to forests through their rooting behavior and eating of seedlings and saplings.
In 1985, Hakalau Forest National Wildlife Refuge was established to preserve approximately 13,000 hectares of higher-elevation wet forest that had been impacted by cattle and pigs. The conservationists fenced off large tracts of the preserve and by 1992 had removed all remaining cattle, most of which had become feral. Subsequently, thousands of feral pigs were removed, mainly through hunting.
In the mid-1990s, after the cattle were removed, Hart, who was then a doctoral student studying the refuge as a bird habitat, measured and tagged 7,000 trees in some of the fenced-off areas. For the 2020 paper, Hart and his colleagues returned to the same areas he had studied 25 years earlier.
The goal of the Hakalau Forest National Wildlife Refuge project was to allow the forest to passively regenerate, mainly to improve habitat for a number of threatened and endangered native Hawaiian forest bird species, but also to increase native plant diversity and native Hawaiian forest cover.
The new paper by Hart and his colleagues demonstrates that the refuge has been largely successful. “We were able to demonstrate that passive restoration can work in these upper elevation areas,” says Hart. “It’s a positive story because the bird habitat is regenerating.”
“When we returned to those areas that I had studied 25 years ago, we found 4,000 new recruits [young trees] five cm in diameter in those same plots,” says Hart. “Going up there 25 years ago you would see mostly medium to large trees with very little understory, but now you see lots of keiki.”
Hart was particularly pleased by the fast recovery of the native koa trees (Acacia koa). “The fencing allowed this new cohort of koa trees to come up just in time,” he says. “The koa trees get big, they grow fast, and can live for 200 years, so they’re very important for the bird species up there.”
Hart says one lesson from the team’s research is that in certain cases, passive conservation management can be effective. “Often in Hawai‘i when the non-native plants come in and out compete the native species, it requires active management and active weeding,” says Hart. “In this case, the refuge is basically doing passive management, although some maintenance of the fencing is required.”
Hart notes that the reason the restoration was successful without active management may be because the forest had not yet been completely converted into grassland, and there was still relatively intact forest left nearby, which helped the native trees to come back on their own. “Once the natives make a canopy they can shade out the invasives,” he notes.
Story by Leah Sherwood, a graduate student in the tropical conservation biology and environmental science program at UH Hilo. She received her bachelor of science in biology and bachelor of arts in English from Boise State University.
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