Research guides efforts to protect ohia forests in Hawaii
Ohia are the pioneers — the first trees to grow on bare lava. Ohia are also adaptable — they grow from sea level to tree line. Ohia are critical in capturing fresh water, supporting threatened and endangered species, and maintaining traditional cultural practices like hula. But these remarkable trees are at risk from Rapid Ohia Death, a disease that kills ohia trees. ROD has killed over one million ohia across Hawaii island. Diseased trees have been found on Kauai, Oahu, and Maui. Already, hula halau are staying out of the forests to protect ohia.
Since 2014, a team of highly dedicated scientists — ecologists, plant pathologists, geographers, and foresters — have been studying the origin, impact, and spread of the two newly identified species of fungus that cause Rapid Ohia Death: Ceratocystis lukuohia and Ceratocystis huliohia. Ceratocystis is a common — and sometimes devastating — plant pathogen, but these two species of Ceratocystis are new to science and new to Hawaii.
Researchers Flint Hughes of the USDA Forest Service, Ryan Perroy of UH Hilo, Greg Asner of Arizona State, and others are using a combination of remote-sensing and field observations to gauge ohia death across Hawaii island. They have found that more trees are killed by ROD in areas where non-native hooved animals are present when compared to areas protected from those animals.
Invasive animals wound the bark of ohia, creating an entry point for the fungus. Damage from goats, sheep, and cattle is obvious — bark is missing. But the team also observed higher numbers of ROD-killed trees in forests with high pig populations. It is possible that pigs are damaging the roots of the trees opening it up for infection.
In a greenhouse study, pathologist Marc Hughes of the University of Hawaii and the USDA Forest Service mulched potted ohia seedlings with sawdust made from infected trees containing Ceratocystis. The trees were healthy until Hughes cut some of the roots with a knife. Those trees then died, indicating that injuries to roots can also create opportunities for the fungus to infect trees.
“Wounds can only be infected for a short time. Once they dry out the fungus can no longer infect them. So protecting a forest helps, even if it has some ROD now” says J. B. Friday, extension forester with the University of Hawaii Cooperative Extension Service.
Protecting the forests also helps ensure seedling survival. Ecologist Stephanie Yelenik of the USGS conducted experiments looking at what happens to ohia seedlings under ROD-affected trees, in plots where invasive plants and animals were removed and in plots where the pests remained. As seedlings died, researchers tested them for the presence of ROD — no sign of the pathogen was present. Seedlings were six times more likely to die in plots where invasive species were left unchecked. Invasive plants and animals posed a greater threat to the seedlings than ROD did during the study.
The first step in addressing ROD on Maui is to prevent it from arriving. HDOA has established a quarantine on the movement of ohia trees and plant parts from Hawaii island to other areas in the state. Hikers should brush off soil and clean boots or shoes with rubbing alcohol to remove any ROD-contaminated soil when traveling between islands.
Early detection, finding trees affected with ROD before the disease is widespread, is also key to protecting our ohia. Key signs include leaves that rapidly turn brown and appear frozen in place. On Maui, the only known ROD-affected tree was reported by an alert resident in Hana. The tree has since been destroyed. The Hawaii Department of Land and Natural Resources surveys our ohia forests looking for symptomatic trees. Conservation crews and Maui residents can report ohia trees that appear to have died quickly. To date, residents have reported 14 suspect trees over the last year. Though ohia die for many reasons, including drought, injuries from mowers or yard equipment, and herbicide, reporting is essential to finding ROD-damaged trees early. Fortunately, aside from the one tree, all other samples collected from Maui have tested negative for ROD.
Help protect our ohia forests from rapid ohia death: Support watershed protection efforts to remove invasive plants and animals. Respect the inter-island quarantine. Keep reporting suspect ohia trees to MISC (573-6472). Join the Ohia Love Festival, Nov. 16-21. Sign up for the virtual event through rapidohiadeath.org.
* Lissa Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. “Kia’i Moku,” “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.