A new study shows rapid ohia death, a fungal disease which has killed off hundreds of thousands of mature ohia trees on the Big Island, can be exacerbated by the presence of animals with hooves, or ungulates.
The study’s lead investigator, Ryan Perroy, an associate professor of geography and environmental science at the University of Hawaii at Hilo, specializes in remote sensing and was integral in obtaining aerial imagery to detect ohia mortality at an individual tree level.
“For this paper which we published, there were four sites, and three of the sites were in (Hawaii Volcanoes National Park), and the other site was within the Laupahoehoe Forest Reserve,” Perroy said Tuesday. “What we observed is increased mortality (of trees) in the areas where the ungulates were present, compared to where they aren’t present.
“Now, for what is the mechanism for causing that mortality, more work needs to be done, looking at different types of ungulates. ‘Are pigs different than sheep?’ Those types of questions. I think it’s important to show that whatever the mechanism is, we do see significantly more mortality of ohia where ROD is present and where you have ungulates versus not having ungulates.”
The paper also illustrates the importance of removing infected trees to suppress ohia mortality levels across affected regions.
Perroy worked alongside a Hawaii Island-based research team, which includes: Timo Sullivan and Daniel Duda from the UH-Hilo Data Visualization Laboratory; David Benitez, an ecologist at HVNP; Flint Hughes, an ecologist at the U.S. Forest Service’s Institute of Pacific Islands Forestry; and Lisa Keith, Eva Brill and Karma Kissinger, plant pathologists from the Daniel K. Inouye U.S. Pacific Basin Agricultural Research Center.
“We developed four helicopters … in partnership with the National Park Service and Volcano Helicopters, and we also used drone imaging in this project, as well as satellite imagery,” Perroy said. “We were looking at imagery collected starting back in 2016, so this is about four or five years worth of data we were looking at.”
The study’s co-authors collected field samples and conducted laboratory testing using data from impacted areas within the four research sites. The spatial patterns of ohia mortality observed across all four sites included in the study show significant differences in areas with and without ungulates, suggesting that ungulate exclusion is an effective management tool to lessen the impacts of ROD in forested areas in Hawaii.
“The results from this work show us that the impacts of ROD can vary across the landscape,” said Perroy. “We hope this information can be useful in managing and caring for our native forests.”
Asked if it could be concluded from the study that eradication of feral ungulates such as swine, sheep and goats should be a part of the strategy to save the forests from ROD, Perroy replied, “Not necessarily.”
“I think it’s up to the land managers in our communities to make those types of decisions,” he added. “The forest provides different types of services. Increased fencing and ungulate removal can be quite controversial. I’m certainly not calling for widespread aggressive moves like that.”
ROD was first discovered on the Big Island in 2014 and has since obliterated hundreds of acres of once sprawling ohia. It is also found on Kauai, Maui and Oahu.
The deadly disease is caused by two invasive fungi, ceratocystis huliohia and ceratocystis lukuohia, and, if unchecked, could irreversibly change Hawaiian ecosystems and cultural traditions by diminishing the keystone native tree in Hawaiian forests.
“How the trees are getting infected? This study’s not able to tell that,” Perroy said. “But it does show that where you see ungulates present, you’re seeing striking levels of increased mortality.
“We do know that the fungal pathogens responsible for rapid ohia death get into ohia trees through wounds. And so it may be that the animals are wounding the trees somehow, either stripping bark or breaking branches or interfering with roots. But it’s unclear at this point — more work needs to be done, and other folks are working on this. Are the ungulates themselves delivering the pathogen, or is the pathogen getting in through the air or some other mechanism?
“What this study tells us is where we don’t have ungulates, the forest seems to do a bit better.”
