Environmental scientists show link between rapid ‘ōhi‘a death mortality levels and hoofed mammals

A research team has discovered that patterns of ‘ōhi‘a mortality show significant differences in areas with and without hoofed mammals, suggesting that ungulate exclusion is an effective management tool to lessen impacts of rapid ‘ōhi‘a death.

Aerial view of forest canopy with interspersed brown tree crowns.
Aerial image shows the telltale sign of rapid ‘ōhi‘a death: browning of affected tree crowns. Photo courtesy of Ryan Perroy.

By Susan Enright.

Head shot of Ryan Perroy
Ryan Perroy

An environmental scientist at the University of Hawai‘i at Hilo, along with colleagues from the National Park Service, U.S. Forest Service, and U.S. Department of Agriculture, has published a paper on the spread of rapid ‘ōhi‘a death (ROD) associated with the presence of ungulates or hoofed mammals.

Lead investigator of the study, Ryan Perroy, an associate professor of geography and environmental science, specializes in innovative ways to use remote sensing such as aerial imagery, in this case to detect ‘ōhi‘a mortality at an individual tree level.

One of the paper’s conclusions is that the spatial patterns of ‘ōhi‘a 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 Hawai‘i.

In the paper, “Spatial Patterns of ‘Ōhi‘a Mortality Associated with Rapid ‘Ōhi‘a Death and Ungulate Presence,” Perroy and his coauthors share results from their integrated monitoring program based on aerial imagery, field sampling, and laboratory testing to detect and monitor ‘ōhi‘a mortality across four representative sites on Hawai‘i island. This is the first published paper on their findings.

“The results from this work show us that the impacts of ROD can vary across the landscape,” says Perroy, who directs the UH Hilo Spatial Data Analysis and Visualization (SDAV) Research Laboratory and also won a prestigious award in 2019 from the National Park Service for his aerial image work on ROD. “We hope this information can be useful in managing and caring for our native forests.”

Since 2014 when it was first discovered, rapid ‘ōhi‘a death has killed hundreds of thousands of mature ‘ōhi‘a trees (Metrosideros polymorpha) on Hawai‘i Island. It is also found on Kaua‘i, Maui, and O‘ahu. ROD is caused by two invasive fungi, Ceratocystis huliohia and Ceratocystis lukuohia, that if left unstopped, could irreversibly change Hawaiian ecosystems and cultural traditions by diminishing the keystone native tree in Hawaiian forests.

The study

The study is collaborative—working on the project along with Perroy is the Hawai‘i Island-based team of Timo Sullivan and Daniel Duda also from the UH Hilo Spatial Data Analysis and Visualization (SDAV) Research Laboratory; David Benitez, an ecologist at Hawai‘i Volcanoes National Park; Flint Hughes, an ecologist at the 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.

Based on collected aerial imagery, the researchers developed a rating system of canopy condition to identify ‘ōhi‘a trees suspected of infection by the fungal pathogens responsible for rapid ‘ōhi‘a death.

“We developed a custom imaging system for helicopter operations to map thousands of hectares (ha) per flight, a more useful scale than the ten to hundreds of ha typically covered using small, unoccupied aerial systems,” state the authors in the paper summary.

Two images: helicopter with camera set up in payload, and aerial of tree canopy showing a dead tree in forest.
Left, custom imaging payload developed for Hughes 500 helicopter mapping operations in Hawai‘i. Right, aerial image of ‘ōhi‘a mortality on Hawai‘i Island. Courtesy photos.

The researchers used this system to quickly generate and share suspect tree candidate locations with partner agencies to rapidly detect new mortality outbreaks and prioritize field sampling efforts. The studies revealed disparities between sites, illustrating challenges to definitively determine the cause of ‘ōhi‘a mortality from aerial imagery alone. The integrated approach of imagery, field sampling, and lab work proved better at effectively discerning causative factors.

The researchers write that “spatial patterns of ROD-associated ‘ōhi‘a mortality were strongly affected by ungulate presence or absence as measured by the density of suspected ROD trees in fenced (i.e., ungulate-free) and unfenced (i.e., ungulate present) areas. Suspected ROD tree densities in neighboring areas containing ungulates were two to 69 times greater than those found in ungulate-free zones.”

An unexpected event at one of the study sites showed the impact of ungulates in a fenced area. A fence line breach occurred during the study period, and feral pigs entered an area previously ungulate-free. “Following the breach, suspect ROD tree densities in this area rose from 0.02 to 2.78 suspect trees/ha, highlighting the need for ungulate control to protect ‘ōhi‘a stands from Ceratocystis-induced mortality and repeat monitoring to detect forest changes and resource threats,” state the authors of the paper.

The authors write that the integrated investigative approach of “multi-platform remote sensing in conjunction with field sampling and confirmatory laboratory testing to characterize new ROD outbreaks” illustrates the importance of fencing, feral ungulate removal, and felling of infected trees in suppressing ‘ōhi‘a mortality levels across affected regions.

The paper includes recommendations for large-scale land management in addressing ROD.


Story by Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories. She received her bachelor of arts in English and certificate in women’s studies from UH Hilo.

Share this story