Friday, August 11, 2023

The Invasive Grass Fueling Wildfires in Hawaii

Hawaii didn't used to get pummeled by highly destructive wildfires. What has changed? A big part of the answer lies in the interaction between climate change and invasive species. 

Begin with a couple paragraphs buried in a NY Times article:

The area burned annually by wildfires in Hawaii has quadrupled in recent decades. Declining rainfall and rising temperatures have left the islands more susceptible to blazes, climatologists say.

Invasive grasses that are highly flammable have crowded out native vegetation in some areas, and climate change has exacerbated dry and hot conditions in the state, allowing wildfires to spread more quickly.

But what invasive species? A University of Hawaii website points to one that has been particularly destructive:
Guinea grass (Megathyrsus maximus), a nonnative invasive grass in Hawaii, forms dense stands that outcompete native plants and has very high fine fuel loads that greatly increase fire potential, spread, and severity.

Wikipedia describes guinea grass as a tough customer, growing ten feet tall. Though it can thrive in full sun, it can also tolerate shade, allowing it to invade native woodlands and thereby increase their vulnerability to fire during droughts. Native to Africa, the grass was introduced not only to Hawaii but also to south Texas.

How did guinea grass get to Hawaii (also spelled Hawai'i)? Wired provides an answer:

When Europeans arrived in the late 18th century and established plantations for growing sugarcane and pineapple, they also brought invasive grasses. Now the economics have changed, and those fields lie fallow. But the grasses have spread like a plague. “Those fire-prone invasive species fill in any gaps anywhere else—roadsides, in between communities, in between people’s homes, all over the place,” says Pickett. “At this point, 26 percent of our state is covered in these fire-prone grasses.”

This stuff is highly sensitive to short-term fluctuations in rainfall. The grass will grow like crazy when the rains come, then quickly desiccate when the landscape dries. “When we get these events like we’re seeing these past few days—when the relative humidity really drops low—all those fine fuels become very explosive,” says fire ecologist Clay Trauernicht of the University of Hawaiʻi at Mānoa.
An article in ABC News explains how the more intense and frequent fires affect the soil and human health: 
Elizabeth Pickett, co-executive director of the Hawaii Wildfire Management Organization, a nonprofit working with communities to prevent and mitigate fires, lamented the changes wrought by fire.

Invasive and fire-prone grass species have moved in over time and during a fire they can burn into native forests, which means the forests are replaced by more grass, Pickett said. The soil burns and sloughs off, leading to massive post-fire erosion that smothers coral, impacts fisheries and reduces the quality of the ocean water, she said.

The state is windy and the dust blows for years, harming human health, she added.

“When you lose your soil, it’s really hard to restore and replant. And then the only thing that can really handle living there in many cases are more of those invasive species,” Pickett said. “It’s systemic. Air, land and water are all impacted.”
A Philosophical Footnote
It's important to note that both climate change and the spread of invasive species are largely unintentional. Our world is threatened by excess carbon dioxide and other planet-heating gasses--lowly biproducts of our economy and lifestyles. We are used to thinking of collateral damage as minor and incidental, and tend not to judge people by what they do unintentionally. In fact, the cumulative impact of unintentional acts is the central threat we face. We live our days trapped in a predicament in which humanity, largely well-meaning, is allowed to collectively and unintentionally create problems, but not allowed to collectively and intentionally solve them. 

Additional reading: Thanks to a comment (see the critical comment and my response below), I found a couple more interesting articles about guinea grass. One gives a good overview of guinea grass as both an excellent, deeply rooted forage grass for cattle, and a weed that has disrupted ecosystems and croplands around the world. The other invests the grass with cultural connotations.

Other Invasive Grasses Fueling Fires in Hawaii

My friend Fairfax sent a link to another informative article that mentions three other introduced grass species fueling fires in Hawaii: fountain grass, buffel grass, and molasses grass. It also stresses that these and other highly flammable introduced grasses are altering fire ecology in the mainland U.S. as well.

What Guinea Grass Has in Common With Japanese Stiltgrass

Some people aren't aware the extent to which grasses affect our lives, for better and for worse. Corn is a grass, as are sugar cane, bamboo, and sorghum. In Princeton and up and down the east coast, the most dominant invasive grass is Japanese stiltgrass, which like guinea grass can grow in sun or shade, and uses what's called C4 photosynthesis to fix carbon from the atmosphere. Plants that use the C4 process--corn also being an example--are more efficient than other plants that use C3. Stiltgrass has invaded most areas of Princeton, growing from a zillion seeds each spring to blanket large expanses of woods. Wildlife don't eat it, so as it takes over, the landscape becomes increasingly inedible. I've long wished that someone would come up with a highly selective herbicide that would impact only C4 plants. If stiltgrass's impact on eastern habitats hasn't been sufficient to stimulate research, maybe the fire hazard in Hawaii will get researches to take a look.


  1. I don't buy the "African Grass fueling the fires" commentary. Why do we not hear of devastating fires in Africa or the Canary Islands from which these grasses originated?

    My understanding is that these non-native grasses have large root systems, tapping 4+ feet into the ground. This would work to RETAIN soil, not lose it.

    1. Typically, in the region where a species like guinea grass evolved, other species would have co-evolved to eat it and limit its rampancy, be they insects or, in Africa, megafauna. If guinea grass is adapted to fire, it likely evolved in an area where other species would also be fire-adapted, and so the ecosystem as a whole would survive those fires. From what I read, the forests in Hawaii are not adapted to fire. Guinea grass, being tall and thriving in partial shade, serves to channel fire up into the canopy where it can spread through the forest. The native species of Hawaii are not adapted to spring back after a fire, so the forest will not return and the invasive species will take over. A dense stand of guinea grass might hold soil well, but the burning of a forest, particularly on a steep slope, would expose soil that then would be highly prone to erosion. It sounds like guinea grass and other invasives might move in to claim the decimated area, but the damage is already done, and the native forest with all its diversity is gone. Here are a couple interesting articles on guinea grass's cultural and ecological aspects. It sounds like guinea grass is a great foraging grass for cattle, but the cessation of grazing allows it to grow unchecked.