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How invasive plants are fueling California’s wildfire crisis

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How invasive plants are fueling California's wildfire crisis

Fire has always shaped the California landscape. But today it burns brighter, more frequently and spreads more than ever – a change driven by human development, climate change and the prevalence of invasive species, which are non-native plants that have negative effects on local ecosystems. Grasses and trees brought to California for farming, gardening, or by accident have transformed fire dynamics in the state.

“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 transformation is an overlooked factor in the increasingly destructive wildfires in California and around the world.

Southern California is dominated by shrublands known as chaparral. This landscape was historically characterized by low shrubby plants, and the native grasses were perennial, maintained moisture, and remained green for most of the year. Fires, when they occurred, were rare because lightning was infrequent. When the fires ignited, they smoldered but did not spread far because the open spaces between the plants acted as natural firebreaks.

The introduction of non-native grasses in the 18th century fundamentally altered this balance. Brought by European settlers, these grasses evolved alongside heavy livestock grazing and routine burning, making them highly resistant to disturbance. They outcompeted native species and filled gaps in the bush, creating a continuous carpet of flammable material, especially along disturbed areas such as roads, frequent fire starting points.

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

Grasses fill all available spaces, a phenomenon called fuel continuity. When a fire breaks out, the unbroken line of dry vegetation acts like a wick, carrying the flames into the bushes. “People underestimate the destructive power of grasses because you can cut them quickly with a hoe, whereas a bush is quite difficult to cut,” says D’Antonio. “But if sparks and embers fly in the middle of a bunch of introduced weeds, then, boom, everything around you just burns like gasoline. It spreads so fast and is so continuous. “It’s like throwing tissue paper on a 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 have oily and incredibly flammable leaves. Its papery bark peels off and catches in the wind, carrying embers up to half a mile away. The problem arises when people plant them right next to their house, says Acuña. “A very hot and very vigorous burning plant, such as a eucalyptus, is placed next to a house, which is composed mainly of petroleum-derived materials. “It is a very strong fire,” he explains.

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