How Invasive Plants Fueled an Inferno in Maui

Researchers and scientists have been tracking and anticipating more frequent and larger wildfires across Hawaii for years. While the speed and scale of the wildfire that devastated Lahaina and killed at least 36 people this week was a surprise, the fact that the state, Maui, and especially the western part of the island was susceptible to fires was not.

In 2019, fire burned some 25,000 acres on the island. A government report on the 2019 fires concluded that “Wild/brush/forest fires present a growing threat to Maui County citizen safety and property. Island communities are particularly vulnerable because populations tend to be clustered and dependent on single highways, often located on the island edge,” almost directly anticipating the disaster in Lahaina.

Research by Clay Trauernicht, a fire specialist at the University of Hawaii, and others has shown that the scale and frequency of wildfires have been increasing across in Hawaii from the early 1900s to the 2010s. The researchers also identified a major culprit: non-native plants.

“Wildfires were most frequent in developed areas, but most areas burned occurred in dry non-native grasslands and shrublands that currently compose 24 percent of Hawaii’s total land cover,” the researchers wrote. “These grass-dominated landscapes allow wildfires to propagate rapidly.”

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The non-native grasses were brought to Hawaii by cattle ranchers in the 19^th century, University of California Santa Barbara ecologist Carla D’Antonio told me. “They were selected because they were drought tolerant.”

They are also invasive. The abandoned sugar and pineapple farms across the state are quickly taken over by non-native grasses. “When the land gets abandoned, the grasses are the first invaders. All you need is a little drought to have a flammable landscape.” Maui is currently in a drought.

The grasses are an especially potent fuel, D'Antonio explained, because they grow quickly when it rains and then stick around, deeply rooted into the soil, as dry, dead organic matter, becoming a “standing layer of very ignitable fuel.”

Then after a fire, these non-native plants tend to do better than native ones, thus increasing future fire risk. Fire “has generally been shown to decrease the abundance of native woody plants because nonnative, invasive, fire-adapted plants out-compete natives for resources in the post-fire environment and tend to dominate post-fire communities,” according to a United States Forest Service review.

These grass fires can also grow and move quickly, endangering residents and firefighters. “They see fire at a distance and the next thing they knew the building is on fire,” D’Antonio said.

The 2021 County of Maui report recommended “reduction of alien plant life that serves as fuel,” in order to prevent future wildfires, noting that “grasses serve as tinder and rapidly invade roadside shoulders.” Fire authorities should “implement an aggressive plan to replace these hazardous fuel sources with native plants to reduce combustible fuel while increasing water retention,” the report said.

If grasses provide the fuel for fire in Hawaii, then strong winds can help turn them into devastating wildfires, both by spreading fire and by sucking moisture into the storm and away from land.

“People really need to think about how they live in a flammable environment,” D’Antonio said. “They’re living with a legacy that’s going to be impossible to reverse.”

Read more about the Maui fires:

Your Biggest Questions About the Deadly Maui Fires, Answered