Explained | How invasive plants caused the Maui fires to rage

When Hawaii’s last sugar cane plantation shut down in Maui in 2016, it marked the end of an era when sugar reigned supreme in the archipelago’s economy. But the last harvest at the 36,000-acre plantation underscored another pivotal shift: the relentless spread of extremely flammable, nonnative grasses on idled lands where cash crops once flourished.

Varieties such as guinea grass, molasses grass and buffel grass — which originated in Africa and were introduced to Hawaii as livestock forage — now occupy nearly a quarter of Hawaii’s landmass. Fast-growing when it rains and drought resistant when lands are parched, such grasses are fueling wildfires across Hawaii, including the blaze that claimed at least 93 lives in Maui last week.

“These grasses are highly aggressive, grow very fast and are highly flammable,” said Melissa Chimera, whose grandmother lived on the Hawaiian Commercial & Sugar Co.’s plantation in Maui after emigrating from the Philippines.

“That’s a recipe for fires that are a lot larger and a lot more destructive,” added Chimera, who coordinates the Pacific Fire Exchange, a Hawaii-based project sharing fire science among Pacific island governments.

Investigators are still scouring for clues as to what ignited the Maui blaze, which became the deadliest American wildfire in more than a century. But as the planet heats up, it is becoming apparent that even a tropical place such as Hawaii, known for its junglelike rainforests and verdant hills, is increasingly susceptible to wildfires.

The islands have long had arid stretches of lava fields and drier grasslands, with rainfall varying from one side of an island to the other. But in recent years, the state has also seen long-term declines in average annual rainfall, thinner cloud cover and drought induced by climbing temperatures. Seizing on data showing a spike this century in Hawaii’s destructive fire activity, specialists in mitigating wildfire hazards had been issuing warnings for years about Maui’s growing vulnerability.

In 2020, for instance, a hazard mitigation plan prepared for Maui County said that the area of West Maui — where Lahaina, the town devastated by the blaze last week, is located — had the highest annual probability for wildfires of all communities in the county.

The document listed West Maui as having a “highly likely” probability, or a more than 90 per cent chance, of wildfires each year on average. Half a dozen other Maui communities were ranked lower, at anywhere from 10 per cent to less than 90 per cent.

After West Maui was hit in 2018 by a round of fires that destroyed 21 homes, Clay Trauernicht, one of Hawaii’s most prominent wildfire experts, warned in a letter t to the Maui News that the island was facing a hazard it had the potential to do something about. “The fuels — all that grass — is the one thing that we can directly change to reduce fire risk,” he wrote.

Fast forward to 2023, and Trauernicht, a specialist in wildland fire science and management at the University of Hawaii at Manoa, said the deadly Maui blaze has shown clearly how nonnative grasses — many of them on former plantation lands that have been left substantially unmanaged by large corporate landowners — can cause what might be an otherwise manageable fire to balloon in size.

In Lahaina, much of which was destroyed during last week’s fire, invasive grasses cover the slopes above town, growing right up to the edge of housing areas.

“We’ve entered a post-plantation era,” Trauernicht posted last week on X, formerly known as Twitter.

Fears over the risks from such grasses have been climbing since plantations began declining in the 1990s, marking the end of an agricultural model that lured immigrant laborers from around the world, shaping Hawaii for nearly 200 years. As tourism eclipsed the plantations in importance, the shift away from sugar cane and pineapple plantations allowed tropical grasslands to grow untended, bolstering what fire specialists call a “grass-fire” cycle.

Heavy rains that fall across the Hawaiian islands can cause nonnative grasses to grow in some cases as much as 6 inches in a day. Then the dry season arrives, and the grasses burn. Moreover, after fires ravage certain areas, the nonnative grasses quickly sprout and spread, displacing native plants less adapted to wildfires, making the cycle more destructive.

Nonnative trees such as mesquite, wattles and, at higher elevations, pines that were planted in the 20th century to stop erosion and provide timber, pose additional wildfire risks.

“We have an issue with a lot of conifers on Maui,” said Lissa Strohecker, an education specialist with the Maui Invasive Species Committee, an organization seeking to contain high-threat invasive species.

When a number of conifers were set ablaze in a fire on Maui in 2018, it caused their cones to explode, intensifying the blaze, Strohecker said. Updrafts then carried the seeds to new locations, producing saplings — and new fire risks — in other parts of Maui.

There are ways that authorities can limit this destructive cycle, tropical fire specialists emphasize. They include building firebreaks, introducing vegetation that is more resistant to fire and allowing livestock to keep grasses at a manageable level.

For years, Trauernicht and other experts have been calling for such moves to mitigate Hawaii’s wildfire risks. And in 2021, Maui County’s own wildfire prevention report noted that “grasses serve as tinder and rapidly roadside shoulders” while for calling for the “reduction of alien plant life.”

The need for more assertive wildfire mitigation efforts has been a matter of debate for years in Hawaii; across the islands, curbing the spread of invasive plants can be costly and logistically complex. Hawaii also competes for federal wildfire grants with more than a dozen other Western states where huge fires generally receive greater attention; some officials have urged the state government to provide more of its own funding for the fight against invasive grasses.

Hawaii holds other challenges, such as its highly varied terrain. Firefighters have to operate across zones including tropical forests, semiarid scrublands and chilly elevations on the slopes of volcanoes, sometimes resorting to costly rented helicopters to battle blazes.

There are also human factors in a place where activities such as campfires, fireworks and sparks from motor vehicles account for most fire ignitions. Hawaii’s acute housing shortage, reflected in a large homeless population that often cooks food outside, increases the risks of more ignitions, researchers say.

The hazard mitigation plan prepared for Maui County in 2020 by Jamie Caplan Consulting, a Massachusetts-based firm that specializes in natural hazard mitigation, also warned that steadily rising temperatures were affecting Hawaii’s vulnerability. “Wildfires could become more frequent in the future as drought conditions become more frequent and more intense with climate change,” it went on to say.

Maui County experienced 80 wildfires between 1999 and 2019 — an average of about four fires a year, according to the report, the largest one in 2009 that scorched more than 8,358 acres on the island of Molokai.

As for West Maui, the report painted a picture of a demographic particularly vulnerable to the ravages of wildfires.

It said West Maui had the highest rate of non-English speakers in the county — nearly 6 per cent.

“This may limit the population’s ability to receive, understand and take expedient action during hazard events,” the plan states.

It said the area also had the county’s second-highest rate of households without a vehicle, almost 7 per cent, which could make it harder for people to flee from a blaze.