Rim Fire: soil severity and vegetation severity

Rim Fire Vegetation Severity

A U.S. Forest Service wildland fire ecologist that the Associated Press quoted as describing the area burned in the 250,000-acre Rim Fire in and near Yosemite National Park as “nuked” stirred up some controversy with his quoted remarks. It is difficult to use a subjective one-word description to sum up the varied fire effects on a huge fire that burned for weeks under an assortment of weather, vegetation, and topography conditions.

Most of the early assessments of burn severity on a large fire are derived from multiple sensors on satellites that are orbiting hundreds of miles above the earth. Using data from individual sensors, or combining information from multiple sensors, scientists can compare recent data with historical records to produce maps highlighting their area of interest. Two of the most common burn severity maps you will see are vegetation and soil severity.

The Burned Area Emergency Response (BAER) team assigned to the Rim Fire has publicized their version, soil burn severity maps which specifically focus on severity to soils and watersheds. The primary objective of the BAER team is to identify imminent post-wildfire threats to human life, safety, property, and critical natural or cultural resources and to take immediate actions to implement emergency stabilization measures before the first major storms. Their map of September 13 shows approximately 56% of the fire is either unburned or received a low-severity burn, 37% sustained a burn of a moderate severity, and approximately 7% burned at high severity.

The September 17, 2013 Rapid Assessment of Vegetation Condition after Wildfire (RAVG) map produced by the USFS’ Remote Sensing Applications Center (RSAC) analyzed vegetation severity produced by a change detection process using two Landsat Thematic Mapper images captured before and after the fire. They came up with very different numbers: 35% unchanged or low-severity, 27% moderate severity, and 38% high severity.

The maps from the two organizations are below. Larger versions can be seen HERE and HERE. Also included in the gallery are photos showing two areas burned in the fire, and a photo taken two hours after the fire started.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire. Google+

4 thoughts on “Rim Fire: soil severity and vegetation severity”

  1. Thanks for the continuing excellent and objective coverage here of this, particularly given the way the Rim fire has been used as a scare symbol elsewhere.

    Aside from the fact that most big fires have widely varying impacts inside the general area of the fire, there seems to not have been enough questioning of a why high severity burns are considered “bad” in wildland areas. Certainly high-severity burns can be worse for communities in the area, both due to erosion and other collateral effects, and the more direct threats to structures and people. But, forest-wise, high-severity burns are a part of the natural history of the Sierra. With appropriate building codes and defensible space requirements, much of the threat to structures can be mitigated. The wildflower meadows, grazing deer and nesting woodpeckers that will be found in some of the high-severity burn areas in a few years shouldn’t represent an environmental problem.

    1. We should all understand that one size of shoe does not fit everyone and we should also understand that one fire regime does not fit all vegetation types across the whole Sierra Nevada. The above comment suggests an over simplified one dimensional landscape whereas in the real world the landscape is complex both physically and biotically. The result of a complex landscape is a complex set of fire regimes that will vary among differing vegetation types and in differing topographic settings. Some areas will have predominantly high frequency surface fires, other areas longer intervals between surface fires, and other areas with mixed-severity patches of high severity. Additionally, what are the patch sizes of the high severity fires? Are they in the order of one to a few hundred of hectares or in the order of many thousands of hectares? The impacts will be very different. So making a blanket statement that high severity fire is a “natural” the fire regime across all the Sierra is as inappropriate as stating that low severity fire occurs across all vegetation types in the Sierra. A potential result of assuming high severity fire is appropriate Sierra wide will be detrimental impacts to many ecosystems, particularly with the added threat of global change and its impacts looming over us.

      1. TC,

        This is an important topic, and thanks for joining in. What I had said, that “high-severity burns are a part of the natural history of the Sierra,” is a fact. It does not mean that EVERY fire there, now or before European settlement, needs to be or was high-severity. Trying to impose a singular “fire regime” for the Sierra as a whole, of whatever sort, I agree would be foolish. I personally don’t believe that it’s even possible at present to predict too well, before the fact, what the overall burn severity of a given fire will be. But, that high-severity fires are a part of the Sierra’s natural history is a clear fact.

        As far as patch size of high-severity fires, they tend to vary. Part of the fear is that you won’t get natural conifer regen in large high-severity areas, but it isn’t clear that this is the case.

        I do agree that some human interventions that actively contribute to the potential for high-severity fires, such as plantation forestry, should be carefully regulated. Regulated does not in my view mean stopped, but it should be realized that these are equivalent to industrial uses of the land, and among other things the full cost of a more-active fire management policy, which may be appropriate for these areas, should be born by those benefitting economically from the relevant use. This is similar to ski areas being responsible for their area-specific avalanche-control work.

        The Rim fire will be a good case in point of what regen looks like as it occurs.

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