Hawai‘i’s Native Forests Are Vanishing. Here’s How to Help.

Pests and diseases are decimating ‘ōhi‘a trees and others, but simple actions can help stop the decline.

Native trees are an essential part of Hawai‘i’s ecosystem, but they’re rapidly vanishing statewide, says Travis Idol, a professor of tropical forestry and agroforestry at UH Mānoa. He says multiple factors are to blame: Nonnative species compete with native plants for nutrients like water and sunlight. Often, they regenerate faster, grow taller and take up more space than native plants. In direct competition, the invasive species often overpower the native trees.

That means fewer native plant seedlings and fewer native plants being naturally grown.

The introduction of pests and diseases such as Rapid ‘Ōhi‘a Death has also devastated native trees and forests. As the name suggests, the disease causes significant damage to ‘ōhi‘a trees and kills them in a matter of days or weeks.

It’s difficult to contain pests and diseases once they’re introduced. “Invasive species, you can sort of get in there and control them. With pests and diseases, once they’re out there, it’s almost impossible to keep them under control,” says Idol.

Many diseases and pests active in Hawai’i are relatively new: ‘Ōhi‘a rust was first found here in 2005; Rapid ‘Ōhi‘a Death was identified in the last few years and has been in the state since at least 2014. And new diseases and pests are found every couple of years, Idol says.

A destructive invasive pest, the Erythrina gall wasp, nearly wiped out several native species, notably wiliwili trees. The release of a biocontrol agent – an even smaller parasitic wasp that preys on the gall wasp – reduced the population of the invasive pest and helped re-store wiliwili trees.

On their own, each pest and disease harms native species, but the compound effect is far more intense, Idol says.

“It really is the combination of disturbances to which our native plants are not adapted, combined with nonnative species that can take advantage of those conditions, that leads to the introduction, spread and dominance of nonnative species.”

Before Entering Forests

Local guidelines help reduce the spread of damaging pests and diseases in Hawai‘i. One guideline is: Avoid moving ‘ōhi‘a wood. If you don’t know where the ‘ōhi‘a wood or ‘ōhi‘a parts are from, don’t move them. A State Department of Agriculture quarantine prevents moving ‘ōhi‘a off Hawai‘i Island.

Another guideline: if you are hiking or working in the woods, clean your gear, tools, shoes and clothes before and after entering forests. Brush all soil off tools and gear, then spray with 70% rubbing alcohol. Wash clothes with hot water and soap.

New pests and diseases enter the state through imports, according to Idol, and the best way to stop them is to heavily restrict the importation of live plants and animals.

“Bringing in live plant material from out of the state is a risk if it hasn’t been cooked or sanitized before it gets here. There are more serious measures we could take to try to reduce the accidental importation of pests, diseases and even other potentially invasive plant species,” he says.

It’s important to keep Hawai‘i isolated from nonnative species, as native species serve a special role in our islands’ ecosystem, Idol says. “Many tropical trees can grow in Hawai‘i, but native species evolved as a community … of plants, animals and microbes that thrive together, support the health of the land, water and people, and therefore make Hawai‘i the unique and special place that it is for those of us who live here and the millions who visit every year.”

The Pacific Ocean served for millennia as a barrier to invasive species, pests and diseases. Native species benefited from that seclusion while it lasted, but now they have almost no resistance to those outside forces.

Idol says Australia and New Zealand’s strict import regulations on plants offer a model for Hawai‘i. Read about their regulations at tinyurl.com/Aus-Rules and tinyurl.com/NZPlantRules.

Learn more about local conservation efforts at hawaiiconservation.org.