The fire is always shaped the landscape in California. But now it's hotter, more frequent, and spreading farther than ever—a change driven by human development, climate changeand the spread of invasive species, which are non-native plants that have a negative impact on local ecosystems. Herbs and trees brought to California for agriculturelandscaping, or unintentionally, has changed the state's fire dynamics.
“Many non-native species can spread fire faster than native plants,” said David Acuña, battalion chief for Cal Fire, the state's forestry and fire protection department. This change is an overlooked driver for the increasingly destructive wildfires in California and around the world.
Southern California is dominated by shrubs known as chaparral. This landscape was once characterized by short, bushy vegetation, and any native grasses are perennial, retaining moisture and staying green most of the year. Fires, when they do occur, are rare because lightning strikes are rare. When they do catch fire, they burn but don't spread far because the open spaces between plants act as natural firebreaks.
The introduction of non-native grasses in the 1700s fundamentally changed this balance. Brought by European settlers, these grasses evolved alongside heavy livestock farming and regular burning, making them highly resilient to disturbance. They outcompete native species and fill gaps in shrubs, creating a continuous carpet of combustible material, especially in modified areas such as roadways—often starting fires.
Unlike perennial native grasses, these non-native grasses are annuals, meaning they die each year and grow from seeds. Their short life cycle leaves a dense layer of dry, dead vegetation in late spring. “They have a very high surface area in volume and are very flat and thin, so they maintain a lot of dead standing material, most of the year,” said Carla D'Antonio, a plant community researcher and professor at UC Santa Barbara. By May, dead grass blankets the ground. “It's so flammable that it needs any kind of ignition—a cigarette, a spark from someone dragging a chain on the highway, or lightning,” said Hugh Safford, a vegetation and fire ecology researcher at UC Davis.
Weeds fill every available space—a phenomenon called fuel continuity. When a fire breaks out, the unbroken line of dry vegetation acts like a fuse, carrying the fire through the bushes. “People underestimate the damage of weeds because you can hack them quickly with a hoe, whereas a bush is a little harder to cut,” D'Antonio said. “But if sparks and embers fly into a bunch of introduced grass, then—boom—everything around you goes up like gasoline. It spreads so fast, and it's continuous. It's like you're throwing a tissue paper on fire.”
Eucalyptus trees, introduced to California in the mid-19th century from Australia, add another layer of fire danger. Known for their fragrant smell, these trees have incredibly burning, oily leaves. Their papery skin is shed and caught in the wind, carrying the embers up to half a mile away. The problem comes when people plant them right next to their homes, Acuña said. “You put a very hot, very strong burning plant like a eucalyptus tree next to a house, which is mainly composed of petroleum materials. That fire is very strong,” he explained.
