Barriers to keep pigs and other hoofed animals away from ohia trees could be the next line of defense against rapid ohia death, a University of Hawaii-led study in the journal Forests found.

Researchers in that study found a link between the presence of invasive hoofed animals and the spread of the fungal disease that causes rapid ohia death, or ROD, which has already killed hundreds of thousands of mature ohia trees in forests and residential areas, mostly on Hawaii Island.

The study was led by Ryan Perroy, an associate professor of geography and environmental science at the University of Hawaii Hilo. He said his lab began monitoring ROD via drone in late 2016, using aerial imagery to detect ohia mortality at an individual tree level.

The co-authors of the study used that data to pinpoint areas to collect field samples to conduct laboratory testing within Hawaii Volcanoes National Park and the Laupahoehoe Forest Reserve.

The results showed significantly higher levels of ROD in areas that were unfenced, with one test area having more than 50 times the number of infected trees.

“What’s come out with Ryan’s study recently is really a game changer,” said J.B. Friday, a UH Manoa extension forester who was not part of the study.

Friday said that high levels of hoofed animals in an area leads to high levels of ROD. He added that the current rate of mortality for ohia is at an unsustainable level.

“In a couple of generations, in some of these forests where we see a lot of disease, the ohia will be gone,” he said. “And it will just be a weed forest.”

While it is not yet certain how hoofed animals actually spread ROD, conservationists believe that the trees become vulnerable to fungal pathogens when pigs damage roots in the soil and goats and cattle strip away bark on the tree.

Kaye Lundburg and Rod Vanderhof, who together restored a 41-acre plot in the Hamakua district with ohia and other native plants, said that fencing has been crucial in their effort to restore the land.

Over the last seven years, there were only three instances where pigs broke into the plot, Vanderhof said, causing dozens of trees to get uprooted and damaged.

“If the pigs were able to access the ground, they would uproot everything,” he added. “It is definitely very important to fence.”

Perroy said that a fence was breached during the time they were studying the forest, allowing feral pigs to ransack an area.

“Shortly after those animals came in there, the mortality levels shot way up,” said Perroy, who added that fence maintenance is crucial to the success of any fencing project. “It drives home the point that putting up fencing is not enough. There needs to be this commitment afterwards.”

Erecting fences on Hawaii island to protect ohia trees is a priority for the state’s Natural Area Reserves program, said Emma Yuen, manager of the state’s Native Ecosystems Program, which is part of the state Department of Land and Natural Resources.

The legislature has been funding state fencing projects for more than nine years, providing an average of $4 million per year to the program. The DLNR also receives an average of $5 million a year from non-state sources, according to Yuen.

However, a recent draft of the state budget bill did not include funding for watershed protection projects, which primarily funds fencing construction.

Yuen said that if the state does not provide funding, the DLNR would need to default on its grants from non-state sources.

David Tarnas, chair of the House Water and Land Committee, said that he will be advocating for the funding to be restored. He added that the DLNR already has agreements with landowners to build fences.