More than one million trees on Hawaiʻi Island have already been lost to the fungal disease Rapid ʻŌhiʻa Death, and Hurricane Lala may worsen that epidemic.
Scientists began investigating the relationship between large storms and ROD after Hurricane Iselle blew through Hawaiʻi Island in 2014.
At that time, the aggressive fungal pathogen that causes ROD had just been identified.
"When Iselle hit, we didn't really think anything of how that might impact ʻōhiʻa in terms of Rapid ʻŌhiʻa Death," said Rob Hauff, the state protection forester with the Department of Land and Natural Resources.
But after the storm, ROD worsened in Hawaiʻi’s ʻōhiʻa forests, and new outbreaks of ROD were recorded across the island. In some instances, it seemed like the fungus had jumped from one spot to another miles away.
Storms may spread ROD
Researchers posit that storms like Iselle and Lala may exacerbate the spread of ROD in a couple of ways. First, ROD is a wound disease, meaning the fungal pathogen enters an ʻōhiʻa through some sort of opening in the tree’s bark.
“So with a hurricane like this, you have a lot of broken branches across the landscape. … Those are all vulnerable to infection,” said J.B. Friday, the extension forester for the University of Hawaiʻi, based in Hilo.
Once a tree is sick, it becomes a target for little insects called ambrosia beetles, which drill into a tree and produce a powdery frass, almost like sawdust. That frass is full of fungal pathogen spores and easily carried by wind, making it a possible vector for new cases of ROD.
Friday said tree damage and high winds from Lala could prove to be a deadly combination.
“But in order to get (an) infection, you have to have all the pieces in place. You have to have the frass blowing around, and you have to have the broken branch at the same time,” he said.
Testing the hypothesis
Researchers have yet to conclusively link storm conditions to an exacerbation of ROD. Ryan Perroy, who directs the Spatial Data Analysis and Visualization Research Lab at UH Hilo, said Hurricane Lala provides another chance to test this hypothesis.
The SDAV lab plans to use aerial and satellite imagery to document wind damage and map ROD outbreaks on Hawaiʻi Island. That data will be compared to atmospheric modeling of Lala’s wind patterns to understand if the storm may be driving new ROD infections.
“Really the sort of active question is, are you able to connect an outbreak, which may happen months or even potentially years later, with a storm event?” Perroy said.
Damaged fences
Understanding Hurricane Lala’s impact on ROD mortality could take years. In the meantime, conservationists have a more immediate concern: Lala damaged fences around many of Hawaiʻi Island’s native forest preserves, creating openings for destructive ungulates like cattle, goats and pigs.
These ungulates can injure ʻōhiʻa with their hooves, leaving the trees vulnerable to ROD infections. They also eat native plants and spread invasive species.
Friday said fencing is a crucial part of protecting native forests. Without them, ungulates are quick to trample delicate ecosystems.
“In some areas, you can look on a fence, and you can see the trail on the outside of the fence as the animals walk around trying to find a way to get in,” he said.
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