What Can Be Done to Stop the Death of Native Hawaiian Trees on the Big Island?

In the battle to stop the spread of rapid ʻōhiʻa death that already has killed an estimated 1 million native Hawaiian trees on the Big Island, boots on the ground are a problem.

The pathogens that cause the new fungal disease — and the seeds of invasive species that can take over the land where the native trees die — can be trapped in the soles of shoes and boots and then spread to other areas.

To combat this assault, land managers have been deploying a simple weapon: boot brush stations.

State land managers have put 30 boot brush stations at trailheads and other public sites, including about 20 at Hawai‘i Volcanoes National Park and at all Nā Ala Hele trails, said Franny Brewer, acting program manager of the Big Island Invasive Species Committee, in an email to Big Island Now.

Plant pathologist Lisa Keith and her team at the U.S. Department of Agriculture Agricultural Research Service in Hilo collected soil samples from from 34 boot brush stations on the Big Island as part of a study and found that the pathogens that cause rapid ʻōhiʻa death were present at 28 of the stations, showing that the pathogens do move around on shoes. That’s a concern, as people moving from one hiking area to another could track rapid ʻōhiʻa death into new locations.

“Avid, well-meaning outdoor enthusiasts can pick up all kinds of seeds on their footwear and move them around — many seeds are specifically designed to be picked up in this way,” Brewer said. “So we support any mitigation measures the state is taking to halt the spread of invasive pests and diseases.”

On O’ahu in July, the state installed new boot brush stations that included a dispenser for sterilizing spray and a bench so users can sit down and spray the bottoms and sides of their shoes comfortably. Keith found that a 70% isopropyl alcohol mixture kills the pathogens that cause rapid ʻōhiʻa death.

The boot brush stations on the Big Island do not have the sterilizing spray or benches. Brewer said when the first stations were installed in the state, there wasn’t a way to include alcohol short of tying a bottle to the handles, which was not efficient or practical.

“Removing soil decreases the risk, so it is worthwhile, but a rinse with alcohol adds another layer of mitigation,” Brewer said, added at some point the Big Island’s boot brush stations will be replaced with the new designs. It’s just a matter of time and funding.

The Big Island Invasive Species Committee continues to monitor rapid ʻōhiʻa death on the island and assist various researchers with projects connected to the scourge. It also recently received a microgrant from Vibrant Hawai‘i for its Plant Pono program to host a workshop this fall to teach residents of the Big Island more about helping the native forest in their backyards.

“Like using a boot brush, it is a small thing to plant a single ʻōhiʻa, but every small contribution adds up,” Brewer said.

ʻŌhiʻa is a species that is famous for its genetic variation, a good thing when a species needs to evolve in response to a new phenomenon or pathogen, Brewer said. It’s likely there already is some resistance to rapid ʻōhiʻa death in the population on the island or that resistance could evolve.

“However, this takes time,” Brewer said.

And as time goes by, native trees die.

While researchers using field samples and remote sensing, including drone surveys and aerial and satellite imaging, estimate there are still about 200 million of the native Hawaiian trees on the Big Island, some forests, especially those in the lowlands, have been devastated.

“The rate of death is unsustainable,” Brewer said. “Ongoing studies in forests outside Hilo indicate a 10% per year mortality rate due to (rapid ʻōhiʻa death).”

Brewer said to see the impacts, drive along Stainback Highway outside Hilo.

Both fungi that cause rapid ʻōhiʻa death, C. huliohia and C. lukuohia, two versions of the fungus Ceratosystis, have been detected in all districts of Hawai‘i Island and in almost all forests. Rapid ʻōhiʻa death seems to be most prevalent in warm, wet forests, which Brewer said makes sense because fungi grow best in that type of climate. At higher elevations, where the climate is cooler, the fungi grow and kill ʻōhiʻa more slowly, and they seem not to spread as quickly in dry forests.

The key to winning the war against rapid ʻōhiʻa death is prevention. In addition to boot brush stations, the installation of fences that reduce the number of feral hooved animals from wounding the trees also is a weapon.

Long-term studies of forests inside and out of fenced areas show areas with feral pigs, cattle, sheep and goats have higher levels of rapid ʻōhiʻa death than those where the animals are not present. The animals strip bark and mangle roots, leaving wounds on ʻōhiʻa that allow the fungi that causes the disease to enter and grow.

“We do know through lab studies that there must be a wound on the tree for the fungus to get in,” Brewer said.

That’s one reason Puna, near Hilo, is thought to have been hit so hard by rapid ʻōhiʻa death in 2014-15, following the destruction of Tropical Storm Iselle that caused massive wounding to trees.

“Once inside the tree’s soft tissues, the fungus grows and slowly blocks the vascular system, cutting off transport of water and nutrients,” Brewer said. “The tree pretty much dehydrates to death.”

J.B. Friday, longtime Hawai‘i forester with the University of Hawai‘i College of Tropical Agriculture and Human Resources, said in an email: “The higher elevation forests, if protected from hooved animals, will be less affected by (rapid ʻōhiʻa death), where it could be a manageable problem rather than a disaster.”

In the worst-affected forests, non-native, weedy tree species such as albizia and strawberry guava often replace ʻōhiʻa, leaving no native forest behind.

“Invasive plants on our islands are a huge, huge problem, and they are very good at racing to fill in the gaps when a canopy tree dies,” Brewer said earlier this summer. “If they are able to claim the space before the ʻōhiʻa or other natives can, as you see throughout Puna, then the forest in that area is lost and unlikely to recover without serious, expensive, intensive effort.”

Fortunately, recent research the Big Island Invasive Species Committee collaborated on shows that ʻōhiʻa seedlings can and do regenerate in areas heavily affected by rapid ʻōhiʻa death as long as they are not swamped by other invasive plants.

There also are ways the average person can help in the battle:

• Avoid injuring ʻōhiʻa.
• Don’t move ʻōhiʻa wood or ʻōhiʻa parts.
• Don’t transport ʻōhiʻa between islands.
• Clean gear and tools, including shoes and clothes, before and after entering forests.
• Wash the tires and undercarriage of your vehicle to remove all soil or mud.

“I would also add: work to clear your community — or at least your backyard — of invasive plants,” Brewer said. “Even if you’ve go no ʻōhiʻa on your own property, invasive plants could be a source of seed that does end up in ʻōhiʻa forest via wind, birds, etc.”

The Plant Pono website has information about plants you can check before planting them. You also can buy plants from a Plant Pono-endorsed nursery.

By working together and following some simple advice, the community can not just fight back against rapid ʻōhiʻa death but hopefully thwart its advance.

“There is a very broad working group across the state, made up of folks from just about every agency in existence in Hawai‘i, who have done an insane amount of work in less than a decade to build up such a huge body of knowledge about a brand-new disease,” Brewer said. “The work is ongoing and no one is slowing down.”

There also is a human toll from watching ʻōhiʻa die and disappear.

“For many folks who’ve worked years, or decades, in the forests of Hawai‘i, this has been a hugely emotional experience,” Brewer said. “Trees in Hawaiian culture — and for those of us who work directly with them daily — are living beings and to watch them whither away can be crushing.”

It’s the same for residents who experience the slow, oppressive loss of ʻōhiʻa around them, she added. Phrases such as “losses of biodiversity” or “impacts to the ecosystem” sound very distant and abstract. Brewer said the consequences of rapid ʻōhiʻa death are not abstract ideas, they are real and painful losses.

“I have had people come up to me at public events and choke up when they talk about a special ʻōhiʻa they lost,” she said. “I am invigorated by the folks who have taken this blow and are fighting back with outplantings, and removing invasives, and taking their little ʻōhiʻa in a pot and carefully planting it in their backyard.”