Fire Behavior on the Stanislaus National Forest

The Rim Fire burned about 400 square miles, an area one-third larger than New York City. MAFFS air support laying fire retardant around Groveland Ranger Station. On the single worst day of the Rim Fire, about 52,000 acres, or about 81 square miles burned. A tree slowing burning several days after the flames passed through the area. The fire burned with such intensity along the face of this ridge that no vegetation remains. Smoke from the fire caused air quality to deteriorate to 'unhealthy' for ten days in one Nevada county. Crew members and support staff receive a morning briefing during the Rim Fire.

 

 

Wildland fires and the size they grow to are a result of fuel conditions, weather and topography

These factors determine the intensity of the fire, how fast it will move, and in what direction. A dense, dry stand where dead branches litter the ground and vegetation and small trees create a continuous ladder of woody vegetation to the upper canopy will burn differently than a dry stand where mechanical thinning or a controlled, prescribed burn has reduced the amount of available fuel.

On the Stanislaus National Forest, topography also plays a large part in the strategy of how wildfires are suppressed.

In sizing up a new fire start crews will ask:  How steep is the terrain?  Is the wind gentle or is it howling up a steep slope?  Is it hot and dry out and how long is the sun shining each day?  Are the fuels on the ground - dead limbs, bush, leaves and needles - dry? These are important questions because they affect the length of the burning period.  It’s also important to know which side of the mountain, or aspect, the fire is burning on. South slopes receive a lot more solar radiation and heating during the day, making them more fire-prone. 

Stand density, the number of trees within a specific area, also plays a role in fire growth. One researcher studying records discovered that during 1911 there were just 19 trees larger than six inches in diameter per acre. When he and his students were in the same area in July of 2013, they found 260 such trees per acre.

Heavy ground fuels buildup and tightly spaced trees create a scenario ripe for the formation of crown fires.  These are the most destructive and fastest moving fire that crews can encounter.  When ground fires generate enough heat and intensity they will drive from the forest floor up into the tops of the trees.  Half of the acres burned during the Rim Fire occurred in two days under these extreme crown fire conditions. 

An unstable atmosphere can also cause a fire to blow up by creating a suction-like force similar to that found around huge thunderstorm cells.  Heat and intensity drive into the bottom of the column and atmospheric conditions provide a lift, creating a huge plume.  This column of smoke is filled with ash and burning embers.  These plumes then begin to create their own weather, readily igniting spot fires with their erratic winds.  Flame lengths can soar to several hundred feet and often sound like the roar of a jet engine.  Stopping a firestorm of this nature is rather like trying to stop a hurricane. When a forest burns with such speed and intensity  there is very little fire crews can do to stop it. 

In the high intensity burn areas, the Rim Fire was so hot that it not only killed every tree but the top inch or two of soil with critical soil microfauna; seed stocks were also sterilized. Fire of this intensity is relatively rare in the moist middle elevations on western slopes of the Sierra Nevada and the native forests are not adapted to bounce back from this type of fire. Climate changehas been one of the major drivers of such landscape change.  

Hand crews are most effective when the flames are less than four feet tall.  Engine crews with water support and air attackcan assist to some degree to help protect key areas with retardant drops and much needed water. But population growth in and around national forestlands, combined with budget constraints on both federal and state agencies, greatly complicates the task of adapting management  policies to forests’ need for smaller, more frequent, less destructive fires.

The Stanislaus National Forest is in an extended drought along with most the southwestern states.  The same conditions that fueled the Rim Fire are present as we head into the recreational months of 2014.   





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