Chaparral & Grasses / Trees & forests
In August 2016, a wildland fire burned quickly through brush and grasses without igniting trees, including eucalyptus trees (top left). So what vegetation brings fire to your door?
Chaparral and grasses pose the same - or greater - wildfire risk as trees and forests. Here's why.
Confusion and misinformation about which wildland vegetation is safe or “safer" in a wildfire is rampant. The public has been misled to believe that trees and forests constitute a wildfire threat, and thus need to "managed." "Management" is this context almost always means killing wild plants and trees, hence our use of quotations for these terms to alert readers to this frequent euphemistic distortion of meaning. The obvious intention is to hide the reality that "management" of vegetation, like "removal" of a tree, or even hundreds of trees, is far more destructive to soil, wildlife and ecosystems than these euphemisms sound. Which is exactly the point of using them, to increase public acceptance of a common practice that does both environmental damage and, if trees are "removed," often increases wildfire danger.
Modern fire science repeatedly demonstrates that forests — or any wild "vegetation" — aka, plants — over 100 feet from houses and structures — present little if any additional wildfire risk. There is usually no need to destroy them to make houses safer from fire. This website contains extensive information on this topic, including Defensible Space guidelines that all of today's fire agencies recommend for home protection, over killing plants more than 100 feet from your house. READ MORE about Defensible Space. READ MORE about fire risk & forests.
This is because when mature trees are cut down, or forests are destroyed, the smaller plants that grow in their place are usually easier to ignite and faster burning — producing faster-moving surface fires that have devastated California communities in recent years. Forests also act as wildfire-slowing windbreaks. High winds are what have driven California's headlines-making 2018 and 2019 wildfires. Relentless winds are what made the 1991 Oakland Hills fire so destructive and unstoppable; it was only contained after winds dropped.
The smaller plants that grow where trees are destroyed include various broom, thistle, ivy, and grasses. When then dry out in our hot, dry summers and autumns, they become what fire agencies call "fine fuels." You might wonder, though, about the far greater amount of wood in a forest; isn't it so much greater than in unforested landscapes like chaparral, and therefore more of a fire danger? Yes, there is more material that can burn in a forest, but that's only part of what constitutes a threat to houses. Consider these other factors:
1) Ease of ignition - the finer the material, the easier the ignition (other factors being the same);
2) Moisture content - the drier the material, the easier the ignition and the faster it burns — and spreads
3) Speed of a fire - the faster a material burns, the less time firefighters have to mobilize and work to stop it;
4) Wind and windbreaks - the more wind feeding oxygen to a fire, the faster it burns and spreads. Forests are tremendous wind breaks compared to grasslands
5) How fires spread: surface fires cf. crown fires
6) Flame lengths -
But this is also why they burn so much faster, and that means the fire can reach your door before firefighters can. Restated in reverse, because forests, compared to chaparral and grasses, burn more slowly because of their volume, density, and wind reduction, firefighters more time to contain them, set up fire breaks, etc. READ: Protecting Our Communities from Wildfire
The average person has been conditioned in numerous ways, especially by under-informed media coverage, which often exaggerates and sensationalizes, to see a forest and think it's a wall of wood waiting to catch fire. And of course forests can and do burn. But the majority of WUI (Wildland Urban Interace) fires start in grasses and brush, including California's chaparral. These can then ignite trees and forests and houses. The "surface" fires, when driven by winds are the ones firefighters can't contain.
Don't Cut Down the Forests; Fireproof the Houses
Ultimately, making your house Ignition Resistant is the most effective way to protect yourself from a wildfire. Put another way, "Don't cut down the forests, fireproof the houses." There are multiple reasons for this guideline. Forests are not the enemy, and never have been. No firefighting agencies advocate cutting down forests, or even stands of living trees as a wildfire defense. Those promoting doing so, whether with the euphemism "vegetation management" or under the guise of "fuels reduction," have other agendas. Because felling large numbers of trees has little, no, or even the opposite effect of reducing house/structure ignitions. As if intentionally being repeated on this page, and on this website, the numerous effect of forests including wildland shading, cooling, moisture-retention, and winds reduction.
All living trees, like all living humans beings, are, despite what your eyes tell you, primarily water. Fire scientists do NOT consider living trees over 3" in diameter to be fire risks. They are not considered "flammable" by fire scientists. It's important to note, this is irrespective of tree species, and a tree's country of origin. Sensationalized and oversimplified media stories often carelessly conflate living trees with "wood," implicitly likening living trees to firewood. The massive difference in water content between living trees and dead trees makes all the difference. You know this too, if you've ever put a "green" log on the hot coals of a campfire and watched it hiss and steam and not burn. In stark contrast, dried grasses and shrubs, with little water content, are easily ignitable, and are often what start WUI (Wildland Urban Interface) fires in the SF Bay area.
Two Bay Area wildfires are featured on their own pages on this website because they are so instructive about wildfire behavior, and illustrate what vegetation constitutes fire danger: 1) The 1991 Berkeley-Oakland Hills Fire, and; 2) recent wildfires on Angel Island, in the San Francisco Bay, before and after eucalyptus deforestation. (Click each fire's name to read more about each.)
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