What causes old forests to burn?

Gil is right that we’ve had this discussion, and we agreed to disagree.  He reaffirmed his belief that acres burned is the result of  lower logging levels:

“Your wisdom is so infinitely better than mine, so I must be seeing things when I look at a graph that shows acres burned by year. Naturally, someone who doesn’t understand the scientific principles behind forest ecosystems would say that it is just coincidence that acres burned by year shows a very significant up turn since the 80% reduction in harvest levels.”

Here is the basis for a different belief.  This comparison of acres burned to the Pacific Decadal Oscillation cycle is taken from the Assessment for the revision of the Nez Perce-Clearwater Forest Plan in Idaho.  It led the planning team (who presumably understand scientific principles) to conclude:


“When PDO data are overlaid on the fire statistics an interesting correlation is seen. A period between 1940 until 1980 was in the cool wet phase, which would have limited wildfires while at the same time promoted tree growth, regeneration, and significant increases in forest density. Clearly cool wet trends resulted in lower wildfire occurrence regardless of the fuel loading across the region. Climate is the most controlling factor for wildfire and the one we can least influence.” (my emphasis)

Every scientist knows that correlation is not causation, and there are at least two opinions possible based on these facts.  I was, and am still, asking for some more definitive research results that would justify Gil’s confidence that more logging is the answer.



9 thoughts on “What causes old forests to burn?”

  1. The nature of old forests is that there is an accumulated quantity of snags and downed wood; i.e., fuel. As we’ve put out fires over the last century and, over the past 2-3 decades, we’ve not done much harvesting on federal forests, it seems probable that we’ve added fuel to our forests.

    If the Pacific Decadal Oscillation (1940-1980) was indeed a “cool, wet phase”, then are we currently in a warmer, drier phase? Further, if this cool, wet phase “promoted tree growth, regeneration, and significant increases in forest density”, then it seems we’d have grown and added fuels to the forest.

    Taken together, would it be logical to conclude that a warmer, drier phase when taken together with more fuels in the forest leads to more wildfires? If “climate is the most controlling factor for wildfire and the one we can least influence”, then what can we more easily influence? If we find wildfires, insects, disease, etc. unacceptable, would it be fair to say that harvesting and the judicious use of prescribed fire (i.e., fuel removal) are the only real alternatives left?

    • This is where site-specific science comes in. Some of the things we can influence, (but maybe not easily and cost-effective), are tree densities compared to annual precipitation and species compositions. We should not be preserving a lodgepole understory, underneath old growth pines. We could do a lot more prescribed fire, if only we could do a LOT more prescribed fire. Currently, according to NIFC, the South does 72% of all prescribed fire acres. The West, including everything west of the Black Hills, does a paltry 11%. The eastern section does about 12%. (And those in charge don’t seem to care about changing that practice.)

      I tend to think that it is a whole “suite” of impacts and practices that has led to large annual burned acreages but, there is also the issue of burn intensities, the explosion of bark beetle “habitats” and firefighting tactics, including Let-Burn. Yes, there are some National Forests which proudly display their lack of a Let-Burn policy. However, setting aside up to 100,000 acres in Maximum Management Areas, to Let-Burn, should have proper and formal NEPA analysis. Indeed, if people demand that 5000 acre thinning projects deserve an EIS, shouldn’t we do the same for 100,000 acres of Let-Burn?? No wonder our forests continue to be messed up. Some people’s solution is to blame dead foresters, which is highly ineffective at fixing the problem.

  2. Ignition is probably a function of basic temperature. If it’s a hundred degrees, that’s forty degrees closer to ignition than sixty —
    Second is dryness, which is not only a function of temperature but the uptake of water into the vegetation and the evaporative cycle. If you have dried out the durt, there’s no water to take up.
    So, we have liftoff and ignition, and I’m not going to argue that. But I will say that over a given area, the more fuel you have, the hotter your FIRE is going to be as the wood radiates in our little box.
    Even if we can’t completely avoid ignition or fire, fuel management will sure as heck mitigate the negatives of an “inevitable” fire — its rate of spread, intensity, deposition into the atmosphere, soil immolation etc adnsm.
    And don’t forget that fuel moisture can be also affected by the amount of vegatation relative to the water available for the year — or the new, climate-changed water cycle.
    Timbco, anyone?

  3. JonH

    The possibility of a rigorous statistical study to provide a definitive answer is an impossible task since we can’t go back in time and set up parallel universes and do replicated studies with controls. So how do we come up with a definitive answer? We look to the statistically sound studies that have led us to know that excessive stand density weakens all trees when the sunlight, moisture and nutrient carrying capacity for a site can not support the excessive load. We look to established entomology and plant physiology science to know that weakened stands are much more easily susceptible to attacks by insect and disease. We know that death from excessive competition for sunlight, nutrients and moisture and or death caused by insects and disease taking advantage of the weakened stand all lead to fuel build up. We know from established fire science (as mentioned by others above) that, when all other factors are held constant, the more fuel the hotter the fire and the lower the chance of keeping it from igniting other fuels within a given proximity. In addition, we know that excessive stand density means greater proximity which allows fires to preheat the proximate fuels to a greater degree and increase the rate of spread and decrease the ability to bring it under control (all other factors being equal). So you don’t need a massive time replicated definitive study to know that we are dealing with a causal effect rather than a coincidence. The fundamental, undeniable proven sciences of plant physiology, fire science, entomology and pathology lead you to but one definitive causal relationship. Yes, ENSO, PDO, climate change, solar flares and a myriad of other exogenous variables all impact the total wildfire acres burned. But within a specific configuration of exogenous variables, the scientific fundamentals of plant physiology, fire science, entomology and pathology drive the total acres burned. If we are in the middle of global warming, yes, total acres burned are going to increase but, if we manage our forests so as to keep them below an appropriate stand density, they won’t burn up as many acres as they would if stand density wasn’t managed.

    So global warming, enso and pdo don’t change the need for forest management to minimize catastrophic losses. In fact, global warming only makes managing stand density and fuels even more critical. But let’s talk about these exogenous variables briefly:

    A) 2ndLaw’s effort to blame the changes in wildfire acres burned on the ENSO impact was refuted here https://forestpolicypub.com/2014/05/03/more-on-wildfire-and-sound-forest-management/comment-page-1/#comment-46716

    B) Your attempt to negate the scientific fundamentals of plant physiology, fire science, entomology and pathology based on a single example from one national forest because “the planning team (who presumably understand scientific principles)” jumped on a correlation is underwhelming.

    Re: Their conclusion that: “When PDO data are overlaid on the fire statistics an interesting correlation is seen. A period between 1940 until 1980 was in the cool wet phase, which would have limited wildfires while at the same time promoted tree growth, regeneration, and significant increases in forest density. Clearly cool wet trends resulted in lower wildfire occurrence regardless of the fuel loading across the region. Climate is the most controlling factor for wildfire and the one we can least influence.”
    –> Now, let’s consider what is wrong with this and look on the national level rather than focusing on a possible anomaly/outlier/coincidence:
    —- 1) Does this mean that if the harvests had been reduced there would have been even fewer fires? No, because there would have been increased stand density leading to more fuels and greater proximity between fuels so the lightning strikes and man-made fire initiations would have had increased success and resulted in larger acres being burned.
    —- 2) Let’s look at harvest levels for the USFS on the next to last graph at http://www.fs.fed.us/forestmanagement/documents/sold-harvest/documents/1905-2012_Natl_Summary_Graph.pdf
    ——- You will note that harvest levels steadily increased from 1940 to 1965 and then leveled off until 1990. So the period from 1980 to 1990 wasn’t in the “cool wet phase” yet if we look at the two links given in #3 below, you will notice that in spite of the “cool wet phase” being over, national acres burned was still dropping from 1980 to 1990. So how does that fit in with extrapolating the Nez Perce-Clearwater Forest to the national level? Doesn’t look like a great idea to me.
    —- 3) Now, lets look at Total Acres Burned at:
    ——- http://www.google.com/imgres?imgurl=http%3A%2F%2Fwww.fs.fed.us%2Fresearch%2Fsustain%2Fimages%2Fcriteria-indicators%2Findicatorimages%2Ffigure16-1.JPG&imgrefurl=http%3A%2F%2Fwww.fs.fed.us%2Fresearch%2Fsustain%2Fcriteria-indicators%2Findicators%2Findicator-316.php&h=401&w=494&tbnid=5Tbc84EdL-2WNM%3A&zoom=1&docid=t5nvdqh8Bhv3iM&ei=7gD0U73lLciAygSv0IKgBA&tbm=isch&client=firefox-a&ved=0CGkQMyhBMEE&iact=rc&uact=3&dur=11670&page=3&start=44&ndsp=25
    ——- and at http://www.google.com/imgres?imgurl=http%3A%2F%2Fopenparachute.files.wordpress.com%2F2013%2F01%2Fus-acres-burned-web.jpg&imgrefurl=http%3A%2F%2Fopenparachute.wordpress.com%2F2013%2F01%2F28%2Fno-cause-for-alarm-if-you-cherry-pick%2F&h=289&w=632&tbnid=-OMlJAAc208HlM%3A&zoom=1&docid=6g1cnzocHiWTqM&ei=7gD0U73lLciAygSv0IKgBA&tbm=isch&client=firefox-a&ved=0CCEQMygDMAM&iact=rc&uact=3&dur=1016&page=1&start=0&ndsp=19
    These two charts here in #3 indicate success or failure of wildfire management and are for comparison with all of the other numbered items.
    —- 4) Now, let’s look at the Pacific Decadal Oscillation Index (PDO) at http://en.wikipedia.org/wiki/File:PDO.svg
    —-> Low and behold 1980 to 1990 was hot as blue blazes and the “cool wet phase” was over yet acres burned didn’t change as mentioned above in item #2. So climate didn’t have the last say after all. Hmm, the planners seem to have ignored an inconvenient problem with their conclusion unless acres burned on the Nez Perce-Clearwater Forest went up significantly. But if it did, it would show that the Nez Perce-Clearwater Forest data was an aberration compared to the national numbers in #3 and not suited for extrapolation to the national level.
    —- 5) Now, let’s look at the red line showing the filtered values for US precipitation at http://www.google.com/imgres?imgurl=http%3A%2F%2Fwww.ncdc.noaa.gov%2Fsotc%2Fservice%2Fnational%2Ftimeseries01%2F110-00%2F201007-201007.gif&imgrefurl=http%3A%2F%2Fwww.ncdc.noaa.gov%2Fsotc%2Ffire%2F2010%2F7&h=534&w=650&tbnid=G6vRDb0WewoNuM%3A&zoom=1&docid=qhFwSufTDfsxcM&ei=7gD0U73lLciAygSv0IKgBA&tbm=isch&client=firefox-a&ved=0CGUQMyg9MD0&iact=rc&uact=3&dur=1062&page=3&start=44&ndsp=25
    —-> I tried to get something more recent but time ran out and I seemed to be running into possible contradictory info. So this is still open to question. If the chart is correct then, things have been gettin wetter instead of drier since 2000.
    —- 6) Then there is this: “the chief of the Forest Service, told Congress in June that between 65 and 82 million acres of Forest Service lands “are in need of fuels and forest health treatments — up to 42 percent of the entire system.” Across all federal land holdings, 231 million acres are at moderate to high risk of damage from wildfires, according to a 2011 Congressional Research Service report. “Since many ecosystems need to be treated on a 10-35 year cycle … current treatment rates are insufficient to address the problem,” the report found” – See: http://thinkprogress.org/climate/2013/07/31/2312591/climate-change-wildfires/
    —> So if the planners on the Nez Perce-Clearwater Forest understand scientific principles, then so do the people who feed the chief of the USFS the numbers on the need for “fuels and forest health treatments”. I mean you can’t have it both ways. Is the chief and those scientists who feed him the information definitive enough for you. Or are you going to define what is definitive? If so, I am waiting for your definition. In addition, all of this only shows that picking a single variable to refute “the scientific fundamentals of plant physiology, fire science, entomology and pathology” is a self deceiving exercise in cherry picking data to find a data set that supports one’s suppositions.

    I may have to clean this up some tomorrow but I don’t think that I’ve made any major blunders so I’ll go ahead and pass it on – Hopefully, this should bring us closer – I really don’t want to have wasted my time here again.

    • 10 to 35 years? Sounds like Indians!
      When I did my photo shoot trip for the ITC issue in Evergreen (prior to the latest) the takeaway I got from every forester was that they consider re-entry at about 20 years. Do something, come back in 20 years to see how it looks, what it’s doing, and respond. Not hard. And 20 years gives SOME institutional memory feedback — yo, whippersnapper, this is what we did then, you’re here now and you can tell YOUR whippersnapper whassup in another 20 years.

  4. One thing that makes old forests burn is back burns set by the forest fire managers, let’s play with the fire because we have lots of money to spend (fires managed for resource benefit) , and lets watch it burn till the east wind comes up (and we get a real wildfire we can’t control) decisions by those same wildfire managers.
    I don’t know who could disagree that fuels reduction is a good idea, whether it be by controlled burning,(which I have seen very few examples of positive results here in the west) or timber harvest.
    I like the idea of active management. Once again with so much to do in our forests we fiddle while “Rome” burns. We waste all this time and money on NEPA documents that don’t really mean anything except in court.
    I don’ t think it would be to hard to find the correlations between our fire policy under that Northwest Forest Plan and the increase in forest acreages burned. It does seem though we have reached critical mass and it is getting a bit scary. Every time there is lighting storm these day we are lucky if it doesn’t turn into a major fire and we don’t end up spending hundred of millions of dollars burning up the old forests, I mean fighting fires.

    • One thing that I ALWAYS see when Let Burn fires escape is the excuse of “unforeseen weather conditions”. That simply isn’t a valid excuse, in these days of advanced weather equipment and satellites. Well, when a fire burns for days or weeks in the high mountains, there WILL be winds and excellent burning conditions, at some point. The big problem is deciding when to go for full suppression. Rarely does that happen before a big blow-up. When it does blow up, the excuse is firefighter safety. All too often, the fire managers are willing to “let it ride”, trying to burn up as much as possible before the weather inevitably changes. They never have a plan when fires do escape. except to back off even further, light more backfires and torch off budget funds. Indeed, they have a blank check to “play with fire”, touting all those dubious firestorm “benefits”. If a landscape truly needs to burn, why not torch it off in the fall, in advance of wet storms, and when suppression resources are plentiful? Instead, it seems like they want to “collect” big fires, all burning at once. Their slogan should now be “Only You Can Preserve Wildfires”.

  5. In a mixed-conifer, mixed-severity fire regime study area in SW Oregon, Crystal Raymond found that “Fire severity was greater in thinned treatments than untreated. … The additional fine wood left from the thinning operation (despite whole-tree yarding) most likely caused higher fire intensity and severity in the thinned treatments. … [T]he presence of activity fuels increased potential surface fire intensity, so increases in canopy base height did not decrease the potential for crown fire initiation. … [C]rown fire is not a prerequisite for high fire severity; damage and mortality of overstory trees in the wildfire was extensive despite the absence of crown fire, and the low predicted crown fire potential before and after the fuel treatment. Damage to and mortality of overstory trees were most severe in thinned treatments (80 – 100% mortality), least severe in the thinned and under-burned treatment (5% mortality), and moderate in untreated stands (53-54% mortality) following a wildfire in 2002. Fine fuel loading was the only fuel structure variable significantly correlated with crown scorch of overstory trees. Percentage crown scorch was the best predictor of mortality 2 years post-fire. Efforts to reduce canopy fuels through thinning treatments may be rendered ineffective if not accompanied by adequate reduction in surface fuels.” Crystal L. Raymond. 2004. The Effects of Fuel Treatments on Fire Severity in a Mixed-Evergreen Forest of Southwestern Oregon. MS Thesis.

    Similarly, Hanson and Odion (2006) compared wildfire behavior in seven previously thinned mixed-conifer forests vs. adjacent unthinned forest in the Sierra Nevada and found — “Contrary to our hypothesis, the mechanically thinned areas had significantly higher fire-induced mortality (p =.016, df = 6) and combined mortality (p =.008, df = 6) than the adjacent unthinned areas. Thinned areas predominantly burned at high severity, while unthinned areas burned predominantly at low and moderate severity … Possible explanations for the increased severity in thinned areas include persistence of activity fuels, enhanced growth of combustible brush post-logging, desiccation and heating of surface fuels from increased insolation, and increased mid-flame windspeeds. Given that sampling transects in thinned versus unthinned areas were only 100 m apart in each experimental unit, fire weather should have been the same for the thinned and unthinned areas sampled in each site. Thus, mechanical thinning on these sites appears to have effectively lowered the fire weather threshold necessary for high severity fire occurrence.” Hanson and Odion. 2006. FIRE SEVERITY IN MECHANICALLY THINNED VERSUS UNTHINNED FORESTS OF THE SIERRA NEVADA, CALIFORNIA 2006 Fire Congress Proceedings.

    A study in mixed-conifer forests in California showed that forest reserves were more effective than logging in terms of reducing fire hazard. “[T]he efficacy of seven traditional silvicultural systems and two types of reserves used in the Sierra Nevada mixed conifer forests is evaluated in terms of vegetation structure, fuel bed characteristics, modeled fire behavior, and potential wildfire related mortality. The systems include old-growth reserve, young-growth reserve, thinning from below, individual tree selection, overstory removal, and four types of plantations. These are the most commonly used silvicultural systems and reserves on federal, state, and private lands in the western United States. Each silvicultural system or reserve had three replicates and varied in size from 15 to 25 ha; a systematic design of plots was used to collect tree and fuel information. The majority of the traditional silvicultural systems examined in this work (all plantation treatments, overstory removal, individual tree selection) did not effectively reduce potential fire behavior and effects, especially wildfire induced tree mortality at high and extreme fire weather conditions. Overall, thinning from below, and old-growth and young-growth reserves were more effective at reducing predicted tree mortality.” Scott L. Stephens and Jason J. Moghaddas. 2005. Silvicultural and reserve impacts on potential fire behavior and forest conservation: Twenty-five years of experience from Sierra Nevada mixed conifer forests. Biological Conservation 125 (2005) 369–379.


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