New Study: Wildfires can burn hot without ruining soil

Here’s a link to a short article (and video) about the new study, “Hot fire, cool soil,” with a brief excerpt below. The American Geophysical Union demanded that we remove a copy of the actual study, which they provided me earlier in the day, from our website….so I’ve done that.  Sorry folks.

When scientists torched an entire 22-acre watershed in Portugal in a recent experiment, their research yielded a counterintuitive result: Large, hot fires do not necessarily beget hot, scorched soil.

It’s well known that wildfires can leave surface soil burned and barren, which increases the risk of erosion and hinders a landscape’s ability to recover. But the scientists’ fiery test found that the hotter the fire—and the denser the vegetation feeding the flames—the less the underlying soil heated up, an inverse effect which runs contrary to previous studies and conventional wisdom.

Rather, the soil temperature was most affected by the fire’s speed, the direction of heat travel and the landscape’s initial moisture content.

And here’s the abstract:


Wildfires greatly increase a landscape’s vulnerability to flooding and erosion events by removing vegetation and changing soils. Fire damage to soil increases with increasing soil temperature and, for fires where smoldering combustion is absent, the current understanding is that soil temperatures increase as fuel load and fire intensity increase. Here, however, we show that this understanding that is based on experiments under homogeneous conditions does not necessarily apply at the more relevant larger scale where soils, vegetation and fire characteristics are heterogeneous. In a catchment-scale fire experiment, soils were surprisingly cool where fuel load was high and fire was hot and, conversely, soils were hot where expected to be cooler. This indicates that the greatest fire damage to soil can occur where fuel load and fire intensity are low rather than high, and has important implications for management of fire-prone areas prior to, during and after fire events.

11 thoughts on “New Study: Wildfires can burn hot without ruining soil”

  1. I don’t think that anyone is contesting the idea of a soils damage “mosaic”. However, a loss of all organic matter within a soil cannot be a good thing. It is the cumulative effects of high-intensity wildfire that makes it so damaging to our environment. As well as us humans.

    Then again, “If only” humans didn’t live in forested areas ………

  2. I t will be interesting to hear more about this study. The issue of soild damage from wildfire has been researched over the years by several research oraganizations including the Forest Service’s research stations. A short summary would be, dry vegetation and dry soils will burn in hot fires or smouldering fires. The damaqge depends on soil types, time of the year of the fire and other cosiderations.

    When controlled burning started become a management tool in the 1950 and 1960’s one of the criteria for burning was to evaluate the potential for soil damage. Most burning was done in the Spring or after first Fall rains to reduce the damage to soils.

  3. Bernard Bormann, a soils scientist with the USDA PNW Research Station, set up a number of research plots in the Biscuit Fire area BEFORE the fire took place. He was then able to revisit his plots after the fire to measure the differences. One surprising finding was that most of the topsoil was gone in many plots — it had been transferred to the ocean via fire-generated winds, smoke clouds and prevailing east winds. I don’t recall how the effects of the wind-removed topsoils compared with those actually combusted during the fire (and maybe there was no way to accurately measure the differences between the two), but it was a significant amount.

  4. Fires can burn intense (crown fire), but not have severe effects (soil damage). Conversely, fires can have severe effects and not burn intensely, such as where there are heavy surface fuel loadings. Intesity and severity aren’t the same but often get confused and interchanged.

    Is an “entire” 22 acre watershed really enough to make scientific conclusions from? No judgement, just curious.

    • JZ: A 22-acre area is certainly large enough to generate scientific conclusions, but they would be mostly site-specific conclusions — which, as Larry keeps pointing out, can provide very useful management information (“for that site”). Such a sampling size is probably not worth generalizing to a much larger area, such as 10,000 or 10 million acres. Certainly not to another river basin or continent.

      As John Marker points out, there has been a massive amount of research done on this topic. It is hard to understand why the scientists were “surprised” by the results of their “fiery test,” which they reported showed “an inverse effect which runs contrary to previous studies and conventional wisdom.” So far as I know, their findings were generally common knowledge in most types of prescribed burn and wildfire conditions — at least among professionals — and consistent with (not “contrary” or “counterintuitive”) to most findings on most continents.

  5. JZ – one way to look at new research studies is to say.. does this reflect what I have observed in reality?

    If all the practical kinds of things (wetter areas also have more fuel in dry climates and will burn not so well until they completely dry out ) seem to be already known by people in the field, that’s OK, but one wonders why it is published as new. That can easily happen when academics don’t have their papers and proposals reviewed by practitioners (that’s the “sounds new to me” criterion of whatever reviewers were selected.)

    It is only observations that contraindicate what we practitioners observe in nature that make us wonder “what was different about those conditions of this study such that we haven’t observed this behavior?”

    And in my opinion, discussing those issues is where real mutual learning and the growth of joint knowledge occurs..

    • I was looking at Google Maps and zoomed into a burned area, seeing where the bigger logs were consumed. Plants were growing in between those linear zones of sustained heat. We have always known about flashy fuels burning fast and cool. Thick duff has also been known to burn hot enough to kill cambium, especially if it has been smoldering for a long time. The Biscuit Soils Study is worth reading, although there is a little hint of “pro-management” in there. Clearly, there was significant loss of soil productivity through high-intensity wildfire. I doubt any study could duplicate real world firestorm conditions, like in the Biscuit Fire Study.

  6. Larry, you’re missing the point. Loss of soil productivity would be the resuly of high severity fire, not high intensity. One is about fire behavoir, one is about fire effects, to oversimplify.

    Sharon, as far as mutual learning….if I were to ascribe a motive to Matt’s post, I think it would lie in the fact that many forest management (harvest) projects use an ill-conceived justification of “reducing the risk of ‘catastrophic’ wildfire” in the purpose/need for the project. It’s a catchy, politically/socially appetizing phrase that will unfortunately continue to be used in conjunction with terms like “restoration” to justify responsible forest management.

    It’s my hope that someday we can be rid of these euphemisms, since the end result on the ground is ultimately the same, regardless of the “sales pitch”. We’ve strived to limit discussion to fire “behavoir” (intensity) in our effects analysis, since this is something that is absolutely within our control (acknowledging that CLIMATE is the driver of fire, not fuels, blah, blah, blah, Matt). Discussions of effects (severity) are far too speculative given the inconclusive and conflicting science and have little value in project level justification for harvest. In my (salesperson) opinion…of course.

    • I often post new research findings here on the blog. My ‘motivation’ with this one wasn’t so much as JZ describes it, but rather was related to the often repeated – somewhat simplistic – claim that “fires burn hot and destroy the soil.” Turns out, just like with most things in the natural world, it’s considerable more complicated than that.

      • So, that mindset is also offset by those times where fires did not burn that hot, and still does very significant soils damage. So, it still seems highly likely that soils damage will occur when a wildfire burns in an overstocked and fuels-laden forest, very similar to what we have in our western public forests. Dang, these wildfires seem to cause all sorts of likelihoods of damage and destruction. Fires do not need to be “re-introduced”. They never left, although their characteristics have changed.

        “If only” we could let fires be “free range”………

    • Hi JZ: I think you’re probably right about the intensity vs. severity thing — the definitions kept changing there for a few years. When I was still in school, “severity” mostly meant “mortality.” How many plants (mostly) and animals got killed when a fire moved through? A “stand replacement event,” therefore, was really severe. “Intensity” was the actual heat and flames generated by the fire — which could bake the soil and create winds — and varied from moment to moment as fuels were consumed, winds picked up, slope changed, etc.

      Mostly I remember one of my Committee members, Kermit Cromack, making me read several different definitions for each, including his own interpretations.


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