Tuesday, August 15, 2023

How Invasive Grasses Killed Hundreds of People in Hawaii During America's Deadliest Fire in a Century

Wildfire near Yosemite National Park, United States, in 2013. (Wikipedia, public domain)

Invasive grasses have long been known to create grass-fire cycles that allow these species to expand rapidly across the environment. In such a process, the grasses provide abundant tinder material for fires, whether started naturally via lightning or through artificial means. Generally, they are optimized for utilizing and promoting fires due to several factors:

(a) their creation of high fuel loads, which refers to how much fuel is present and available to burn. Grasses in general, and invasive grasses in particular can form very high density clusters.

(b) their low live to dead biomass. Annual invasive grasses in particular create vast fields of very dry dead matter after they flower and seed.  

(c) their high surface to volume ratios. Grasses generally have thin blades, which means they have a lot more surface area compared to their interior volume. Higher values are correlated to shorter fuel ignition times, and hence faster fire spread rates.

Add in the fact that grasses typically recover rapidly following fires, and the result is that fires kill off most of their potential competitors, and allow the grasses to expand their range and continue the cycle ad infinitum.

A prime example of such a cycle is one undergone by Taeniatherum caput-medusae (medusahead grass), which along with Bromus tectorum (cheatgrass) and others have devastated vast areas of sage habitats in the Great Basin.

Hawaii is no stranger to invasive grasses, most of which were deliberately introduced as potential forage or as ornamentals, but have since escaped into the wild and have outcompeted the natives in the islands. These include Megathyrsus maximus (called Guinea Grass in that area), Melinis minutiflora (called Molasses grass in some places), and Cenchrus setaceus (called fountain grass in some places).

Megathyrsus maximus spikelets (inset inflorescence), with purple stigmas and yellow-orange anthers

Last week, a massive fire destroyed the town of Lahaina in Maui and became the deadliest fire in the USA in a century. As of this writing, 99 people have been confirmed dead, but officials warn that the final tally could be double or triple that.

The initial cause of the fire is not known, but changes in the climate in Hawaii (including long-term declines in average annual rainfall and drought), coupled with the spread of invasive grasses to produce vast grasslands, are thought to be major factors in creating and maintaining the large wildfires after its initial start.

Escaped Cenchrus setaceus rows in Mulholland Dr, in Los Angeles, California

It's been a few years since I travelled to Hawaii, but I watched the popular TV series Lost a couple years back. This show was mostly shot in Hawaii, and it was interesting to see how many times I saw what seemed to be large expansive stands of Megathyrsus maximus, which is a large species and thus easier to see identify. The clusters were not only in savanna habitats, but also seemed to exist as smaller groups within forested areas.

I also remember visiting California this year, and seeing masses of escaped Cenchrus setaceaus along the roadsides in San Diego and even in Mulholland Drive in Los Angeles, where they doubtless increase the likelihood of spreading and exacerbating any incoming fire.

In the end, this major tragedy again highlights the importance of not only implementing measures to curb and address global climate change, but it should also focus people's attention on the importance of stopping the spread of invasive grasses. 

Perhaps most people may (sadly) not care about the destruction of sage brush habitats by such invasive grasses, but they surely will care about people's homes and lives being lost to the same types of invaders. A good start would be to enact legislation that will address this problem now, including banning or regulating the sale of invasive ornamentals and forage.

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