How invasive plants are fueling California’s wildfire crisis

The fire is still there shaped the landscape in California. But today it burns hotter, colder, and more widespread than ever, a change driven by human development, climate changeand the prevalence of invasive species, which are non-native plants that have negative effects on local ecosystems. Herbs and trees brought to California in order agriculturelandscaping, or by accident, have transformed the fire dynamics of the state.

“Many non-native species can spread fire faster than native plants,” says David Acuña, battalion chief for Cal Fire, the state department of forestry and fire protection. This transformation is a neglected engine for the increasingly destructive fires in California and around the world.

Southern California is dominated by shrubs known as chaparral. This landscape was historically characterized by short plants and shrubs, and every native grass was perennial, retaining moisture and staying green for most of the year. Fires, when they did occur, were rare because lightning was infrequent. When the fires started, they burned hot, but did not spread far, because the open spaces between the plants acted as natural firebreaks.

The introduction of non-native grasses in the 1700s fundamentally changed this balance. Brought by European settlers, these grasses have evolved alongside heavy livestock grazing and routine burning, making them highly resistant to disturbance. They compete with native species and fill gaps in the shrubbery, creating a continuous carpet of flammable material, especially along disturbed areas such as roads – frequent starting points for fires.

Unlike native perennial trees, these non-native grasses are annuals, meaning they die each year and regenerate from seeds. Its short life cycle leaves a dense layer of dry, dead vegetation in late spring. “They have such a high surface to volume and they are very flat and thin, so they keep a lot of material standing dead, almost all year round,” says Carla D’Antonio, a researcher of the plant community and professor at the University of California. , Santa Barbara. In May, dead grass covers the ground. “It’s so flammable that it takes any kind of ignition — a cigarette, a spark from someone dragging a chain on the highway, or lightning,” says Hugh Safford, a vegetation and fire ecology researcher at the University of California, Davis.

The grass fills every available space – a phenomenon called fuel continuity. When the fire spreads, the unbroken line of dry vegetation acts as a wick, carrying the flames into the bushes. “People underestimate the destructiveness of grasses because you can go hack them up quickly with a hoe, whereas a shrub is quite difficult to cut down,” says D’Antonio. “But if the sparks and embers fly through a bunch of weeds, then — boom — everything around you just grows like gasoline. It spreads so fast and it’s so continuous. It’s like and throw the tissue paper on the fire.”

Eucalyptus trees, introduced to California in the mid-19th century from Australia, add another layer of fire risk. Known for their aromatic scent, these trees are incredibly flammable, oily leaves. Its papery bark peels off and flaps in the wind, carrying the embers half a mile away. The problem comes when people plant them right next to their house, says Acuña. “You can put a very hot, very vigorous burning plant like a eucalyptus next to a house, which is mainly composed of petroleum materials. It is a very strong fire,” he explains.