Why Native Plants are more Fire-Prone


Fear of fire has fueled the heated debate about native plant restorations. Native plant advocates want the public to believe that the non-native forest is highly flammable, that destroying it and replacing it with native landscapes would make us safer. Nothing could be further from the truth.


The fact is that any forest—whether it is native or non-native—is generally less flammable than the landscape that is native to California. In the specific case of the Sutro Forest in San Francisco, this general principal is particularly true: the existing forest is significantly less flammable than the landscape that is native to this location.

The “Mount Sutro Management Plan” was written by UCSF and is available on their website. It describes “native” Mount Sutro as follows: “In the 1800s, like most of San Francisco’s hills, Mount Parnassus [now known as Mount Sutro] was covered predominantly with coastal scrub chapparal [sic], consisting of native grasses, wildflowers, and shrubs…” (page 4)

A Natural History of California (Allan Schoenherr, UC Press, 1992) tells us that chaparral is not only highly flammable, but is in fact dependent upon fire to sustain itself: “Chaparral…is…most likely to burn. The community has evolved over millions of years in association with fires, and in fact requires fire for proper health and vigor. Thus it is not surprising that most chaparral plants exhibit adaptations enabling them to recover after a burn…Not only do chaparral plants feature adaptations that help them recover after a fire, but some characteristics of these plants, such as fibrous or ribbonlike shreds on the bark, seem to encourage fire. Other species contain volatile oils. In the absence of fire, a mature chaparral stand may become senile, in which case growth and reproduction are reduced. “ (page 341)


In July 2003, a prescribed burn on San Bruno Mountain raged out of control. The intention was to burn 6 acres. The fire eventually burned 72 acres and came perilously close to homes, according to an article in the Yerba Buena newsletter: http://www.cnps-yerbabuena.org/experience/other_articles.html#pageTop. (The article describes in detail the return of the natives after the fire and concludes, “A January 2004 field trip…revealed mostly natives returning to the landscape.”)

Native plant advocates are well aware of the importance of fire to sustain the native landscape. The San Francisco Natural Areas Program of the Recreation and Park Department plans prescribed burns to maintain their native plant restorations. Their plans for prescribed burns are described in the Initial Study of environmental impacts that was published in May 2009.

The local chapter (Yerba Buena) of the California Native Plant Society also acknowledges the value of fire to restore and maintain native plant populations. A wildfire fire on San Bruno Mountain in native grassland and coastal scrub “consumed about 300 acres” in June 2008 according to an article on their website: http://www.cnps-yerbabuena.org/experience/other_articles.html#pageTop . The article reports that “Fire is an adaptive management tool that, along with natural grazing and browsing, has been missing in promoting healthy grasslands that once covered much of the lower elevations of California…The threats to native grasslands are invasions of non-native grasses and forbs, and succession by native and invasive shrubs. Fortunately the fire scrubbed the canyons pretty clean of just about everything. This gives the land a shot of nutrients to recharge the soil and awaken the seedbanks that have long been lying dormant.”

Now let us compare the flammability of forests with that of the native scrub. The fire on Angel Island provides an opportunity for comparison. According to an “environmental scientist” from the California state park system, 80 acres of eucalyptus were removed from Angel Island 12 years ago. Only 6 acres of eucalyptus remain. (“Rains expected to help heal Angel Island,” SF Chronicle, October 14, 2008).

The fire that burned 400 acres of the 740 acres of Angel Island stopped at the forest edge: “At the edge of the burn belt lie strips of intact tree groves…a torched swath intercut with untouched forest.” (“After fire, Angel Island is a park of contrasts,” SF Chronicle, October 15, 2008). It was the native grassland and brush that burned on Angel Island and the park rangers were ecstatic about the beneficial affects of the fire: “The shrubs—coyote bush, monkey flower and California sage—should green up with the first storms…The grasses will grow up quickly and will look like a golf course.” Ironically, the “environmental scientist” continues to claim that the eucalyptus forest was highly flammable, though they played no part in this fire and there was no history of there ever having been a fire in the eucalyptus prior to their removal.

Unfortunately, the 1991 fire in the Oakland hills has enabled native plant advocates to maintain the fiction that eucalyptus is highly flammable. And in that case there is no doubt that they were involved in that devastating fire.

However, there were several factors in that fire that are not applicable to Mount Sutro. The climate in San Francisco is milder than the climate in the East Bay because of the moderating influence of the ocean. It is cooler in the summer and warmer in the winter. There are never prolonged, hard freezes in San Francisco that cause the eucalyptus to die back, creating dead, flammable leaf litter. The 1991 fire in the Oakland hills occurred in the fall, following a hard winter freeze that produced large amounts of flammable leaf litter. In fact, there were several fires in the Oakland hills in the 20th century. Each followed a hard winter.

When it is hot and dry in the Oakland hills, as it was at the time of the 1991 fire, it is cool and damp in San Francisco, particularly on Mount Sutro. Fogs from the ocean drift over Mount Sutro, condensing on the leaves of the trees, falling to the ground, moistening the leaf litter. These are not the conditions for fire ignition that exist in the Oakland hills.

The reputation of the eucalyptus as a fire hazard is also based on the assumption that the oils in its leaves are flammable. The National Park Service reports on its website that the leaves are, in fact, fire resistant: “The live foliage [of the eucalyptus] proved fire resistant, so a potentially catastrophic crown fire was avoided.” (http://www.firescape.us/coastliveoaks.pdf) Even if oils were a factor in flammability, there are many native plants that are equally oily, such as the ubiquitous coyote bush.

This observation is consistent with a comprehensive book about trees, The Tree (Colin Tudge, 2005): “…many trees are highly fireproof, like redwoods and eucalyptus…” (page 376) The living wood does not burn easily. Dead leaf and bark litter are the main source of fuel for fires. Fuel loads can be reduced by removing leaf and bark litter on the ground, without destroying the living tree.

High winds, accompanied by dry heat were the prerequisites of the 1991 fires in the Oakland hills. To the extent that trees create a wind break, they create a shield from a wildfire. Any fire that starts on the westward, windward side of Mount Sutro will move unhindered to the eastern, leeward side of the Mountain if the windbreak is destroyed. Many of the neighbors of the Sutro Forest understand the function that the windbreak performs in protecting their neighborhoods from both wind and potential fire. Hence their opposition to the destruction of trees on Mount Sutro.

Native plant advocates should not use fear of fire to justify the tree destruction that benefits their restorations. Those who know the natural history of California recognize the hypocrisy of such claims. Fire is integral to the native landscape. The same people who advocate for prescribed burns to maintain a native landscape cannot simultaneously use fear of fire to justify the destruction of healthy non-native trees.

June 15, 2009

1 Response to Why Native Plants are more Fire-Prone

  1. I’d call this an incendiary title! Shrubs are more fire prone than tall trees, but apart from that the title gives a misleading impression. In my experience fundamentalism/zealotry is generally perceived in the “opposition” to a greater extent than it actually exists. Seems like eucalypts and chaparral vegetation are similar in how they cope with fire and make use of it. Also – let’s not forget that for 1000’s of years, native people burned extensively to promote growth of food plants. Thanks for the interesting info on Eucalyptus – I’ll look it up. I live on a ridge in the Santa Cruz Mountains with chaparral, redwoods, and other trees, including three enormous blue gums that make me fearful – bombs waiting to explode, everyone says. I’ll look into it more carefully now. I realize I have swallowed opinions about eucalypts without looking for evidence. I was told that they suck up a lot of water, for example. BTW for a lot of great detail on fire and those California natives that evolved in a fire-prone environment, visit the Las Pilitas Nursery web site, run by a former fire fighter who has done a lot of testing – not science, but practical experience. http://www.laspilitas.com/fire.htm

    Webmaster: LOL at “incendiary.” Thanks for being open-minded, and thanks for the link. It’s not the native-vs-nonnative status that makes plants flammable. But dry grasses are extremely flammable whether native or not, shrubs (especially when dry in summer) are also very flammable. Trees, especially when they’re green, are less so. Eucalyptus included.

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