There’s good news for besieged ohia trees

Researchers for the first time have shown that ohia seedlings can survive for at least a year in forests afflicted by rapid ohia death — a glimmer of hope for a beleaguered species that has lost more than a million individuals since the disease was discovered in Hawaii a decade ago.

“This is one of the first good-news stories about the ohia in a long time,” said Stephanie Yelenik, an ecologist with the U.S. Geological Survey and lead author of a study published recently in the science journal Restoration Ecology.

Ohia is the dominant tree in Hawaiian forests and one of the most cherished and culturally important plants in the islands.

But the fungal disease known as rapid ohia death has continued to spread across the landscape of Hawaii island and has recently landed on the other main islands, threatening ecosystem disturbances affecting water supply, natural resources and cultural traditions.

This study, however, suggests that restoration of forests with active rapid ohia death infections may indeed be possible.

To test how ohia seedlings would fare in areas where the disease is prevalent, the researchers planted seedlings directly underneath adult ohia infected with either Ceratocystis huliohia or lukuohia, the fungal pathogens that cause rapid ohia death.

The seedlings were monitored in plots that were fenced to keep out animals and weeded to remove invasive trees and shrubs. Other seedlings were planted in areas with no fencing or weeding.

Seedlings that died were tested for the DNA of Ceratocystis and for evidence of fungal spores.

Turns out that none of the dead seedlings tested positive for Ceratocystis, indicating that ohia can survive in afflicted forests if protected from wild pigs and goats and invasive trees and shrubs, according to the study.

Additionally, seedlings were six times more likely to die in plots where weeds were allowed to grow than in areas where weeds were cleared around the seedlings.

The results, Yelenik said, suggest that controlling invasive plants and animals has more of an impact on survival in the tree’s first year than exposure to the disease.

Yelenik said land managers and homeowners have been wondering whether it’s even worth planting ohia seedlings on their property “when they’re just going to die from rapid ohia death anyway.”

Now the evidence suggests that they’re probably not going to die and certainly not with the appropriate protections, she said.

The results of the study also indicated that ohia seedlings are not infected by the disease through the soil.

“Those results are encouraging for two reasons,” J.B. Friday, extension forester with the University of Hawaii Cooperative Extension Unit, said in a news release. “First, it means that even in forests with invasive trees and shrubs, ohia may possibly be re-established. And second, it means that in our high-elevation, pristine native forests, natural ohia regeneration could be possible, even in forests hit by (rapid ohia death), if those areas are protected.”

In January the state updated its plan to deal with rapid ohia death, calling for more than $20 million for research and other measures over the next five years.

Yelenik, who works for the USGS’s Pacific Island Ecosystems Research Center, said longer-term studies of ohia seedling survival in disease-affected forests are still needed, but these early results demonstrate that active planting could successfully help maintain ohia forests.

“While that one-year survival of seedlings is great news, this species lives centuries and there’s currently no treatment once the tree becomes infected,” she said.

The paper’s co-authors are Kylle Roy of the USGS and Jeff Stallman of the Hawaii Cooperative Studies Unit of the University of Hawaii at Hilo.