WILDFIRE DANGER:
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.

The public has been misled to believe that trees and forests constitute a wildfire threat, and thus need to "managed."  Confusion and misinformation about which wildland vegetation is safe or “safer" in a wildfire is rampant.  "Management" in this context typically means killing wild plants and trees with chainsaws, bulldozers and herbicides, hence our use of quotations for these terms to alert readers to the frequent use of euphemisms to distort meaning, mislead the public, and hide the truth.  The truth being that "vegetation management," like "removal" of trees, and hundreds of trees, and using herbicides on both tree stumps and small plants, degrades soil, poisons and kills wildlife, and harms ecosystems — and doesn't even necessarily reduce 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 rarely 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, more effective than killing plants over 100 feet from your house.  READ MORE about Defensible SpaceREAD 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, including thistle, broom, ivy, poison oak and grasses, dry out in fire season, and are easier to ignite and faster burning than trees.  They produce the fast-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 made the 1991 Oakland Hills fire so destructive and unstoppable; it could only be contained after winds dropped.

 

The smaller plants like broom, thistle, ivy, and grasses dry out in our (increasingly) hot, dry summers and autumns and become what fire agencies call "fine fuels."  But you still might wonder about the far greater amount of wood contained in a forest; isn't it a greater amount of fuel for a fire?  Yes, a forest contains a greater mass and volume of material that can burn, but that's not the only determinant of what constitutes wildfire risk. 


Determining wildfire danger is complex, involving many factors, including these:

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;

4) Wind and windbreaks - the more wind feeding oxygen to a fire, the faster it burns and spreads.  Brush and smaller vegetation is exposed to winds, and thus burns and spreads rapidly, as with recent California wildfires in 2018 and 2019.  Forests, by contrast, block and disrupt winds that drive wildfires.  Thinning trees in a forest can also increase wind speeds within it and encourage wildfire spread.

3) Speed of a fire - the faster a material burns, the less time firefighters have to mobilize to stop it.  A forest, with its large number of thick-trunked, water-filled trees, is much less ignition prone, and burn more slowly, than grasses and chaparral, and is thus easier for firefighters to contain.

5) Types of wildfires - Grasslands and shrubs are considered

surface fires that spread quickly via direct contact from flames.
 

Crown or canopy fires refer to fires in the tops of trees.  These

generate huge numbers of lofted firebrands (embers) that winds

can carry long distances.  The firebrands can start multiple "spot

fires" by igniting fine fuels where they land.  This "spotting" is why

keeping  rooves and gutters clear of leaves and needles is vital

READ MORE about making your house Ignition Resistant.


PHOTO at right: Brush fire flames taller than trees. 
"Black Saturday" bushfires, Australia, Feb. 2009.

While cutting down a forest removes the risk of a canopy fire, it increases the likelihood of a wind-driven surface fire of the smaller, drier plants that grow in it splace.  Deforestation merely trades one type of wildfire risk for another.

6) Flame lengths -

7) Slope - wildfires burn faster up hillsides than on flat ground, and flame lengths increase.

 

 

 

 

 

 

 

 

 

 

 

 

 

Grassland and chaparral vegetation (like California's common coyote brush), being finer and drier than mature trees of any species, ignite more easily and burn faster than trees.  This means grassland fire can reach your door before firefighters can.  Stated in reverse,  forests, compared to chaparral and grasses, burn more slowly because of their greater volume, greater density,  greater moisture content, and their wind reducing affect. So firefighters have much more time to reach them, set up fire breaks, and contain them. 

Most people have been conditioned by decades of inaccurate and sensationalized media coverage.  Forests are often characterized as if they were stacks of dried firewood waiting to burst into flames.  And of course forests can and do burn.  But the water content makes all the difference (more on this below).  And forests in fact benefit from periodic fires to remain healthy.  READ: "The War Against Wildfire," by George Wuerthner.

 

But the majority of WUI (Wildland Urban Interface) fires start in grasses and brush, including California's chaparral.  (Compared with mountain wilderness fires which typically begin with lightning strikes.)  These surface fires move rapidly and can then ignite trees, forests and houses.  When driven by winds, they can become impossible for firefighters to contain.

 

Don't Cut Down the Forests; "Harden" your House

 

Ultimately, making your house Ignition Resistant is the most effective way to protect your home from a wildfire.  A forest's canopy fire can shower thousands of firebrands onto your house and property.  A fast-moving chaparral or grassland surface fire can quickly bring flames to your property or to your front door. You can protect yourself from both threats with simple, inexpensive "Home Ignition Zone" preparations to your house and the immediate surrounding area which fire agencies (and this website) recommend.  READ MORE about the Home Ignition Zone on this website.  READ MORE about hardening your home on FireSafe Marin.

 

No firefighting agencies advocate cutting down forests, or even stands of living trees, as a wildfire defense.  Those promoting doing so under the cover of euphemisms like "vegetation management" or "fuels reduction," have agendas other than wildfire protection.  Felling large numbers of trees has little or no effect on wildfire ignition of structures over 100 feet away, and can even increase the chances of  house/structure ignitions.  This is intentionally repeated on this page and website, to drive this key point home and counter a prevailing forest-are-to-be-feared narrative. 

 

Forests offer numerous wildfire-suppressing benefits including 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 — and this is irrespective of tree species, or its country of origin. 
 

Sensationalized and oversimplified media stories often carelessly conflate living trees with "wood," implicitly likening them 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, and also wood chips and dead logs left in the wake of "management" projects, have little water content, and are easily ignitable.  These materials are 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.  Each illustrates 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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