Hawaii's red-blossomed 'ōhi'a is tough enough to colonize recent lava flows, but until this summer the iconic native tree seemed doomed. Four years ago, an invasive fungus began to kill 'ōhi'a (Metrosideros polymorpha) on the island of Hawaii; by now, the blight has spread across 800 square kilometers. The news got worse in May, when dying trees tested positive for the fungus on the neighboring island of Kauai, fueling fears that rapid 'ōhi'a death (ROD) would span the state.
But the picture brightened at a meeting on Oahu late last month. Aerial surveys and studies on land and in the lab now suggest that some 'ōhi'a will survive. The killer fungus turns out to be two distantly related species, one of them less deadly to 'ōhi'a, and some trees seem to have a native resistance to both strains. Management practices such as fencing out animals also appear to slow the spread of the fungus. "We are not going to see an extinction of 'ōhi'a," predicts Flint Hughes of the U.S. Forest Service's Pacific Southwest Research Station in Hilo, who is coordinating ROD research. "As we understand it more, our management tools are improving and we are learning about the potential weak points of the fungus and the strengths of 'ōhi'a."
At the annual Hawaiian Conservation Conference in Honolulu from 24 to 26 July, Lisa Keith, a research plant pathologist with the U.S. Department of Agriculture's Agricultural Research Service in Hilo, described key differences between the two 'ōhi'a killers: Ceratocystis lukuohia, which slays trees in weeks by clogging their circulatory systems, and C. huliohia, which causes canker sores and seems less deadly. Related fungi cause rot in sweet potatoes and pineapples, and affect cacao and mango trees, among other plants. Keith's genetic studies determined that the 'ōhi'a's deadlier foe comes from Latin America, whereas the less aggressive fungus, which is infecting trees on Kauai, may have been imported from Asia or Australia.
Some trees are also more resilient, Keith and her colleagues found when they infected each of Hawaii island's five varieties of 'ōhi'a with the two fungi. "There's definitely variability" in how the seedlings respond, she says, with some still alive after 1.5 years, even after being infected with the deadlier species.
Aerial drone surveys confirm the lab findings. For the past several years, Ryan Perroy and Timo Sullivan from the University of Hawaii (UH) in Hilo have flown cameras above the treetops in four 40-hectare sites on the island. The progress of the disease is easy to track in the drone images: Affected trees turn dark red, then silvery gray as they die. At two of the sites, Sullivan reported at the meeting, ROD's spread has greatly slowed, and the number of infected trees has hovered between 30% and 40%, compared with nearly 100% in the initial outbreak.
'Ōhi'a are tough enough to thrive on new lava but can be downed by invasive fungi.
Larger trees seem most susceptible, possibly because they are more injury-prone and provide a greater target for windborne saw-dust carrying the pathogens. In addition, Sullivan says, "Fences appear to make a difference," with more trees surviving in enclosed areas. Because the fungi seem to infect only wounded trees—such as those on which animals have rubbed, munched, or rooted around the base—those behind fences are less vulnerable, he suggests.
Gregory Asner, an ecologist with the Carnegie Institution for Science in Palo Alto, California, has seen the same pattern in aircraft surveys of Hawaii island. He detects early signs of the blight with an onboard spectrometer sensitive to changes in 'ōhi'a leaf water, sugars, and defensive chemical content. By overlaying those data on land-use maps, Asner says he, too, has turned up "some really strong evidence that feral animals are spreading the fungus," again by wounding trees and making them more susceptible to infection. Domestic livestock can also do damage. Studies on the ground provide further support: Hughes's team set up 200 0.1-hectare plots across the island of Hawaii, fenced and unfenced, and found the protected trees were healthier. "Knowing that fences can have a very positive impact is very exciting," he says.
Ironically, another Hawaiian icon—the Kilauea volcano—may aid the 'ōhi'a. The fungus has done the most damage in the Puna district of the island of Hawaii. That area happens to be where this spring lava began spewing out of a giant fissure, destroying hundreds of homes. "It's absolutely terrible," says Rebecca Ostertag, a tropical forest ecologist at UH. "But there is a small sliver of a silver lining." In many areas devastated by ROD, foresters are worried 'ōhi'a won't be able to return because fast-growing invasive trees, such as strawberry guava, will crowd them out. But that won't happen on Puna's fresh lava fields. 'Ōhi'a is always the first—and often, the only—tree to get a foothold in such landscapes. So, Hughes says, "Those new lava flows will give 'ōhi'a a chance to recover."