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Roger Weller, geology instructor

Soil Affected by Fire
by Jessica Ruppel
Physical Geology
Fall 2015

Effects of Severe Fire on Soil

Good soil is comprised of more than just the various minerals at its core.  Organic material is also an integral part of soil’s ability to support vegetation and other life. This organic material is made of decomposed plants, animals and some minerals, and, while under normal conditions it generally benefits from wildfires, when conditions become more severe, the results can be detrimental. There are many examples of this, especially across the western United States. A few local to Arizona are the Monument, the Horseshoe Two, and the Rodeo – Chediski Fire. These fires shared some situation altering factors - high temperatures, high winds, and high fuel loading in the form of overgrown vegetation to name just a few. The combination of these and other factors produced fires that burned so intensely they greatly influenced not only the vegetation, but also attacked the organic material of the soil in the most severely affected areas. One result common among these fires was soil degradation in two varieties. The first is soil that has only been stripped of its plant life, and while this can mean the mass movement of destabilized soil, the disturbed soil can begin to grow over as soon as the movement stops. The second is soil that has become hydrophobic due to extremely high temperatures, it can no longer absorb water as it has been stripped down to bare mineral and coated in a waxy substance. This substance forms when combustion of vegetative materials creates a gas, which then cools down and leaves the waxy substance behind. Another contributing factor in soil hydrophobicity is topography. Steep, heavily wooded slopes, like those seen in the mountainous regions where these fires occurred, carry fire much faster, and produce much more heat, causing serious damage to the underlying soil. Hydrophobic soil is also easily moved, even more so than the previously mentioned, but more importantly it cannot begin to heal as soon as it stops moving. While the first soil discussed still has everything it needs to support plant and animal life, hydrophobic soil will have to undergo a century or more of transformation back into something that can hold water, before plants and certain animals can once again make this their home.

Severe, slow moving fires that have the intensity to cause soil damage, continue to affect the landscape long after the fire dies. Many times this comes in the form of debris flows. Debris flows are similar to mudslides, but can be caused by different forces. In the case of severe fire, they are caused by soil destabilization on a slope due to the loss of vegetation.  Below is a picture of a debris flow which occurred shortly after the Horseshoe Two Fire, in Hell’s Half Acre Canyon, in the north end of the Chiricahua Mountains. The fire that caused this flow burned with extreme intensity due to high, up-slope winds, and heavy fuel loading consisting mostly of manzanitta. This flow consists of 12-15ft of unconsolidated material. (Photo was taken approx. one month later.)

Photo credit: Douglas Ruppel, Range Staff,  U.S Forest Service

The next picture shows the scar of a debris flow that occurred under similar circumstances, on Rough Mountain, in the Chiricahua mountain range. The loss of the vegetation atop this mountain, and a single 4in. rain event, destabilized the soil and caused it to flow off the side of the mountain in the months following the fire.

Photo credit: Douglas Ruppel, Range Staff, U.S Forest Service

                      To conclude, while less severe, more frequent fire is beneficial to soil and the ecosystem of the landscape, when fires become fewer and farther between they can be damaging to the landscapes they occur in, especially under the extreme circumstances previously discussed.