HONOLULU — New findings reveal the spread of a fungal disease killing off hundreds of thousands of mature ‘ohi‘a trees (Metrosideros polymorpha) on Hawai‘i Island could be exacerbated by the presence of ungulates or animals with hooves. A geographer 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, published a collaborative study that closely examines the link between the disease known as rapid ‘ohi‘a death (ROD) and ungulates such as feral pigs.
Lead investigator of the study, Ryan Perroy, an associate professor of geography and environmental science at UH Hilo, specializes in remote sensing and was integral in obtaining aerial imagery to detect ‘ohi‘a mortality at an individual tree level. The study’s co-authors collected field samples and conducted laboratory testing using data from impacted areas within Hawai‘i Volcanoes National Park (HAVO) and the Laupahoehoe Forest Reserve.
The spatial patterns of ‘ohi‘a mortality observed across all four sites included in the study showed 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.
“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.”
Perroy worked alongside a Hawai‘i Island-based research team, which includes Timo Sullivan and Daniel Duda from the UH Hilo Data Visualization Laboratory; David Benitez, an ecologist at HAVO; 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.
The researchers discovered the number of suspected ROD trees in unfenced areas were significantly higher (more than 50 times greater for one location) than those found in fenced areas that prevent hoofed animal access.
ROD was first discovered on Hawai‘i Island in 2014 and has since obliterated hundreds of acres of once sprawling ‘ohi‘a. It is also found on Kaua‘i, Maui and O‘ahu. The deadly disease is caused by two invasive fungi, Ceratocystis huliohia and Ceratocystis lukuohia, and, if left unstopped, could irreversibly change Hawaiian ecosystems and cultural traditions by diminishing the keystone native tree in Hawaiian forests.
Study enhances ROD data collection
Based on collected aerial imagery, the researchers developed a rating system based on tree canopy condition to identify ‘ohi‘a that may be infected. They developed a custom imaging system for helicopter mapping operations, which enables a much wider surveying scale.
Using this system, the researchers 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 ‘ohi‘a mortality from aerial imagery alone. The integrated approach of imagery, field sampling and lab work proved better at effectively discerning causative factors.
This research identifies effective strategies for HAVO and other protected areas, where study co-author Benitez considers ROD to be a top threat to native forests and ecosystems. “The focal sites include forests near the summit of Kilauea, including Nahuku and the ‘Ola‘a Tract rainforests and the Kahuku Unit,” Benitez explained. “We are very excited about this collaborative research, and the resulting technologies such as a safer and more cost-effective airborne mapping system which can pinpoint ROD outbreaks across vast areas on all Hawaiian Islands.”
Need for feral animal control
An unexpected event at one of the study sites showed the impact feral pigs caused after they breached a fenced area. Suspect ROD tree densities in the area rose from practically none to nearly three trees per hectare (nearly 2.5 acres) highlighting the need for ungulate control.
“There are lots of remaining questions that still need to be addressed relating to ungulate species-specific behavior and impacts (pigs vs. goats vs sheep vs. cattle), differences between domesticated and feral animals, and lots of remaining questions regarding non-ungulate factors we think play a role in ‘ohi‘a mortality, from boring beetles to storm events,” Perroy said.
The paper also illustrates the importance of removing infected trees to suppress ‘ohi‘a mortality levels across affected regions.
Pigs has nothing to do with rapid Ohia death. Here we go again, blaming pigs for something humans did. The nature conservancy let that fungus go back in the early 2000, to kill off the rose apple trees. Every hunter on every island seen the death of the rose apple. We all wood wait for rose apple season because the pigs would all flock to the rose apple patches, but that was the nature conservancy idea of controlling the pig population, get rid of the food sources. There was no such thing as rapid Ohia death or that fugus befor the rose apple started dieing. Then in the early 2000’s every rose apple tree on every island started dieing from a fungus. Then all of a sudden the lehua started dieing and now all the Java plum trees are also dieing. The funny thing is the rose apple, the ohia lehua, and Java plum all belong to the same family. The myrtle family, go ask Susan Case about it. She headed the nature conservancy at the time. They let that fungus go to try and kill off the food supply for the pigs and did a big mistake. Sombody never do their homework and look that the rose apple and ohia lehua are the same family and now they like blame pigs even further, that they are spreading the fungus. That is the biggest bunch of bull if I ever heard one. Do your guys homework. Go look it up. The rose apple tree, ohia lehua, and Java plum. They all dieing. They all belong to the myrtle family and the nature conservancy probably the state knew about it to. Let that fungus go to control the pigs. their solution was you get rid of the food sources for the pigs you get rid of the pigs. But dummies never look that they belong to the same family as the ohia lehua and now they spending thousands and thousands more of our money for study something that they know where it came from. Do your homework, look up the truth. Talk to any hunter who spends their whole life deep in the forest , we all seen the same thing. No trust somebody who sit behind a desk with glasses telling more lies. Our government is full of liars with agendas.