Invasive plant species increase wildfire risk

Invasive plant species increase wildfire risk

11 April 2010

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USA —    No matter where you live in California, wildfire has been a widespread issue over the past several years, and it doesn’t seem to be getting better. Nearly four years of statewide drought coupled with hot, dry weather haven’t helped the problems set out for us by nature, but another facet to the wildfire formula often is ignored: invasive plants destroying or replacing native species, a process known as “type conversion.”

This occurs when human activity prompts a change in natural vegetation patterns, leads to a loss of native chaparral ecosystems and their replacement – scientists call it invasion – by seasonal and often more flammable plants. With more flammable plants around us, wildfires can start much more quickly than if the stout, woody chaparral claimed its natural place.

You might be thinking to yourself, “Why don’t we just get rid of all of it so we don’t have to worry about things catching on fire near our houses?” Therein lies the problem.

On U.S. Forest Service land, where many of the state’s big fires start, the strategy Forest Service officials have adopted in many cases is creating fuel breaks – areas stripped of vegetation to prevent fires from spreading – and keeping chaparral from growing too old. The theory behind this is that fuel breaks will stop fires from traveling farther than the gap they create and that keeping chaparral small and young will mean fires that don’t burn as big or as hot.

However, a lot of scientific research shows the value of leaving things just the way they are and keeping brush-clearing confined to the space around our houses. Manipulating growth patterns, say scientists, is only making the problem worse by causing type conversion. In other words, scientific data show that the age of chaparral can affect the temperature of a fire, but it doesn’t affect its area, destructiveness or the speed with which it is ignited. And many of us have seen fire jump a fuel break when the wind picks up as if the gap weren’t even there.

Rick Halsey, director of the California Chaparral Institute in Escondido (San Diego County), contends that dry, non-native grasses that have replaced chaparral over the years caused the tinder-box conditions that led to huge fires that burned large swaths of San Diego County in 2003 and 2007. In Santa Barbara County, both the Jesusita Fire last May and the mammoth Zaca Fire in 2007 were started by sparks from maintenance work among dry, non-native grasses.

The Forest Service has a lot of funding for aggressive fuel management – usually consisting of “prescribed” (controlled) burning or mechanical removal of chaparral – but that’s not the best way to deal with the problem, Halsey says. Where one plant has been removed, unless aggressive weed abatement is pursued, something else, often something that catches fire more quickly, will take its place. In Southern California, the new vegetation is usually some kind of seasonal grass. Farther north, says UC Berkeley wildfire researcher Max Moritz, scotch broom, pampas grass and other more flammable nonnatives move in where some variety of chaparral once stood.

After a big fire has occurred, people tend to view chaparral as a pretty scary-looking plant. It’s dense, and when it burns, the fires are big and hot. But consider for a moment that chaparral takes longer to catch fire than do most of the nonnative grasses and other fluffy invaders that take its place when removed.

In a 2004 paper entitled “Fire Management Impacts on Invasive Plants in the Western United States,” Jon Keeley, a biology professor at Occidental College in Los Angeles, declared that the presence of nonnative annual grasses – caused in many cases by years of prescribed burning and vegetation removal – has increased the cost and effort needed for fire prevention and suppression.

Researchers from universities all over the Western United States have studied historical records from the past 200 years and prehistoric data taken from tree ring and ocean floor carbon-deposit sampling. They have found that despite our best efforts to prevent large fires with prescribed burns and brush removal, fires have always been a regular occurrence on the land that today makes up California and other Western states.

Almost all Californians have some risk of losing everything in a wildfire, but modifying the landscape in a knee-jerk defensive reaction has, research indicates, unintended impacts. Unfortunately, making sure your house doesn’t burn down has more to do with the weather than it does with that scraggly stand of chaparral on the hillside across the street.

That doesn’t mean we have to be apathetic about fire prevention, but instead of frantically chopping down every flammable-looking plant in sight as a measure of defense, we ought to consider the years of scientific research showing that the solution lies within our own communities.

We are already familiar with the concept of defensible space around buildings, but if you live near wildland, you simply need to accept a certain level of risk. Investing in roof sprinklers and fire-retardant gels is going to work a lot better (and cost everyone else less) than hacking up the very natural environment that is probably the reason you bought a house in such a location to begin with. Halsey suggests that citizen emergency response teams – a concept often brushed aside by authorities – are a great way to deal with spot fires while getting people involved in their own fire protection efforts.

When it comes to fires, Californians are generally adept at getting together to take care of a problem, but as climate and water supply change and the frequency of fires increases, a special effort is called for. Let’s not leave it up to someone else to engage in environmentally damaging fire suppression programs that in the long run do more harm than good. Assess your level of risk, get educated, and get on deck to help out.



Chaparral: A shrub endemic to California and north Baja California, Mexico, with variants existing in Mediterranean climates around the world.

Fuel breaks: A strip of land where vegetation and debris have been removed to prevent fire from crossing it.

Prescribed burn: Fire applied in a somewhat controlled manner in order to manage flammable vegetation.

Type conversion: The process by which a nonnative species replaces a native one, either by destroying or replacing it.

Defensible space: An area around a structure cleared of vegetation and flammable materials to slow the advance of wildfire.


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