fire is always there Shaped the landscape in California. But today it burns faster and spreads faster than ever before—a change driven by human evolution, Climate changeand the prevalence of invasive species, which are non-native plants that have negative impacts on local ecosystems. grass and trees brought California For AgricultureLandscaping, or by accident, has changed the state's fire dynamics.
“Many non-native species can spread fire faster than native plants,” says David Acuña, battalion chief for Cal Fire, the state's department of forestry and fire protection. This change is an overlooked driver of increasingly destructive wildfire in california And around the world.
Southern California is dominated by shrubs known as chaparral. Historically the landscape was characterized by small, bushy plants, and all native grasses were perennial, retaining moisture and remaining green most of the year. Fires, when they occurred, were rare because lightning was rare. When fires broke out, they burned but did not spread far because the open spaces between the plants served as natural fires.
The introduction of non-native grasses in the 1700s fundamentally changed this balance. Introduced by European settlers, these grasses evolved along with heavy livestock grazing and regular burning, making them highly resilient to disturbance. They outcompete native species and fill gaps in the bush, creating a continuous carpet of flammable material, especially in altered areas such as roadways – which are often the starting points for fires.
Unlike perennial native grasses, these non-native grasses are annuals, meaning they die each year and regrow from seed. Their short life cycle leaves a dense layer of dry, dead vegetation by late spring. “They have such a high surface area to volume and they're very flat and thin, so they retain a lot of dead material almost all year long,” says Carla D'Antonio, a plant community researcher and professor at the University of California, Davis. , Santa Barbara. By May, dead grass covers the ground. “It's so flammable that anything can ignite it – a cigarette, a spark from someone pulling a chain on the highway, or lightning,” says Hugh Safford, a vegetation and fire ecology researcher at the University of California, Davis.
Grass fills every available space – a phenomenon called fuel continuity. When a fire breaks out, the uninterrupted line of dry vegetation acts like a wick, carrying the flames into the bushes. “People underestimate the destructiveness of weeds because you can cut them down quickly with a hoe, whereas it's much more difficult to cut down a bush,” says D'Antonio. “But if sparks and embers fly in the middle of a haystack, everything around you goes off like gasoline. It spreads very fast and continues continuously. It's like throwing tissue paper on fire.”
Eucalyptus trees, introduced to California from Australia in the mid-19th century, add another layer of fire risk. Known for their fragrant aroma, these trees have incredibly flammable, oily leaves. Their papery bark becomes loose and blows in the wind carrying embers up to half a mile away. The problem, Acuna says, comes when people plant them right next to their homes. “You have a very hot, very fast-burning plant like a eucalyptus tree planted next to a house, which is composed primarily of petroleum products. That’s a lot of fire,” he explains.