In early September, a drone flew over the Waiakea Forest Reserve on Hawaii’s Big Island. It slowed its pace, lowered itself to a hover just feet from the canopy, and readied a device attached to its undercarriage. Two plastic “arms” rotated gently, grabbed a small branch, and, using a built-in saw, chopped it off. Having collected the sample, the drone flew away.
This could be the future of forestry. The operation, conducted by Ryan Perroy, a geographer at the University of Hawaii at Hilo, is part of a rescue mission to save a special tree — the sacred ʻŌhiʻa (pronounced “oh-HEE-ah”) that blankets Hawaii’s islands. For many Hawaiians, the ʻŌhiʻa is a symbol of nature, an ecological backbone, and the very essence of the forest. But the trees are under attack.
“It really is probably the most devastating thing I’ve worked on in my career.”
The U.S. Department of Agriculture (USDA) Agricultural Research Service, the USDA Forest Service, and the University of Hawaii at Manoa first identified a tree-killing fungus on Hawaii Island back in 2014. Last year, researchers from the University of Pretoria and other institutions confirmed in a scientific paper that there are two species of fungus causing havoc — and both are new to science. There are now 1,600 confirmed detections of the disease on the Big Island and 100 more cases across the other islands.
“It really is probably the most devastating thing I’ve worked on in my career,” says Lisa Keith, a plant pathologist at the USDA who has been stationed in Hawaii for nearly 20 years.
The fungus is a strangler. It causes a disease known as Rapid ʻŌhiʻa Death, which works its way into the vascular system of the plant and cuts off the internal water supply, starving the tree. Within weeks, the tree’s leaves turn a dull red-brown. Black smudges appear in the sapwood. Eventually, the leaves fall off, leaving a skeletal claw of diseased wood behind. The lifeless core of the tree, still reaching skywards, is doomed to rot. There is no known cure for the infection.
A major challenge to halting the spread of the fungus is monitoring its dispersal. Hawaii’s forests are vast and dense, and not easily surveyed on foot. That’s where the drones come in.
Since early 2017, Perroy has been assembling a small fleet of seven modified drones for fungus surveillance. One of the larger drones, a DJI Matrice 600 hexacopter, was modified in partnership with ETH Zurich to carry a branch sampler. Using a six-foot-long carbon fiber pole, the sampler grabs a branch and removes it without risking entanglement in the canopy.
Other drones in Perroy’s arsenal gather images of the forest through camera attachments. The drones fly in straight lines across a plot, taking hundreds or even thousands of photos until the whole plot is captured. This imagery can then be checked manually — or with an automated machine learning system — for signs of the disease, such as the characteristic red-brown leaves. This work has allowed Perroy’s team to map areas known to be affected by the fungus. Repeated surveys show the spread over time.
Perroy’s fleet also includes an eBee fixed-wing drone with a range of up to 500 hectares, but he and his team have to always maintain visual line-of-sight when flying the drones. This is required by Federal Aviation Administration (FAA) regulations. It is possible to claim an exemption from this rule, however — something that Perroy plans to apply for. If the exemption is granted, he is planning to build up his drone fleet to allow for more comprehensive surveillance efforts.
Perroy says the drones have become an important tool in the battle against Rapid ʻŌhiʻa Death. “They’ve definitely gone beyond nice-to-have to a pretty critical part of our monitoring program,” he says.
Using a six-foot-long carbon fiber pole, the sampler grabs a branch and removes it without risking entanglement in the canopy.
The team is currently attempting to identify diseased trees at an earlier stage of infection, when the leaves are just beginning to turn color. However, once visible symptoms are present, the ʻŌhiʻa is already as good as dead. Plus, the brown leaves could be a sign of dehydration caused by other things. What the scientists really want to know is whether the killer fungus is actually present. That’s why the branch sampler is so important.
“To get confirmation, you need to take a physical sample,” Perroy says.
Although ʻŌhiʻa are plentiful in Hawaii — making up 50% of the forest and 80% of the native forest — huge numbers of them could succumb to the fungus over the next few decades. That would fundamentally alter Hawaii’s natural environment. As one paper published in September noted, more than 140 endangered native plant species are currently growing in areas that are likely to be affected by the fungus. If the forest changes dramatically around those species, their chances of survival could be slashed.
Richard Camp, an ecologist at the US Geological Survey, worries that, should the disease spread up hillsides on the islands, it could affect the natural habitat of endangered bird species, such as the Hawaii Creeper or the beautiful orange-red ‘Akepa.
“This is the last refuge for these endangered species and they’re at low populations,” he says.
Lisa Keith praises Perroy for his pioneering drone work. “It’s absolutely critical,” she says. Forestry teams are involved in efforts to contain the disease — for instance by removing affected trees from the forest once they are identified. Using technology to more closely monitor Rapid ʻŌhiʻa Death is essential, Keith says: Such an approach provides “a fighting chance.”