Do fuel reduction treatments increase resistance to insects and drought?

Not always, according to researchers

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Prescribed fire at Mount Rushmore National Memorial
Prescribed fire at Mount Rushmore National Memorial, April 29, 2020. Photo by Paul Horsted.

Intuitively we might think that fuel reduction treatments and prescribed fire would lead to more resistance to drought and attacks by beetles. While that is sometimes the case, it turns out that following the extreme 2012-2016 drought in California, prescribed burning increased beetle infestation rates and increased mortality of red fir and sugar pine in an area studied by scientists.

Researchers studied 10,000 mapped and tagged trees in a mixed‐conifer forest following mechanical thinning and/or prescribed burning treatments in 2001 through the extreme drought in California. The work was conducted in the Teakettle Experimental Forest (36°58′ N, 119°2′ W) located in the High Sierra Ranger District of Sierra National Forest, in California’s Sierra Nevada. Elevation of the forest ranges from 1,880 to 2,485 m.

While prescribed burning is an important tool for increasing resistance to wildfire their results suggest prescribed burning does not necessarily also instill drought resistance.

Below is an excerpt from a paper  titled, “Do forest fuel reduction treatments confer resistance to beetle infestation and drought mortality?” It was written by: Z. L. Steel, M. J. Goodwin, M. D. Meyer, G. A. Fricker, H. S. J. Zald, M. D. Hurteau, M. P. North, and published by the Ecological Society of America January 22, 2021.

Management challenges
Density reduction treatments that rely on mechanical thinning alone had neutral to positive effects on conifer survival during the 2012–2016 drought (Figs. 7, 8). The overstory treatment that removed medium to large trees (e.g., ≥25 cm) was most beneficial to residual individuals, suggesting such a strategy could be used broadly to increase drought resilience for some species (i.e., Jeffrey pine and white fir). While removal of smaller trees (e.g., ≤25 cm) may be less effective at mitigating drought mortality, treatments focused on ladder and surface fuels may still be preferred when considering non‐drought objectives such as reducing fire hazard or maintaining wildlife habitat (Stephens et al. 2012).

Prescribed burning appears less effective than mechanical thinning at reducing drought mortality and in some cases can lead to higher beetle infestation and mortality rates (Fig. 8). This is most striking in the case of large sugar pines which died at much higher rates in prescribed burn plots during the drought. The negative effect of burning on tree survival is somewhat surprising given that the fire regime under which these forests developed was characterized by frequent (i.e., 11–17 yr) low‐ to moderate‐severity fire (North et al. 2005, Safford and Stevens 2017), and that the prescribed burn occurred approximately a decade prior to the drought.

Mortality and probability change
Indirect effect of forest treatment on drought mortality. Treatment abbreviations are UU for Unburned/Understory Thin; UO for Unburned/Overstory Thin; BN for Burned/No Thin; BU for Burned/Understory Thin; and BO for Burned/Overstory Thin. Value distributions represent change in probability of mortality relative to controls for two tree sizes. The scale of the x‐axis varies among species. (From the research)

Further, van Mantgem et al. (2016) observed decreased tree mortality associated with prescribed fire elsewhere in the Sierra Nevada following the initial two years of California’s drought, and Meyer et al. (2019) found no difference in mortality between paired burned and unburned plots in red fir forests during the middle and late periods of the drought. The forests Meyer et al. (2019) sampled were at higher elevations than Teakettle where soil moisture is substantially higher and temperatures lower.

The results presented here could be unique to the Teakettle Experimental Forest, but we suspect they are more likely attributable to the historic severity of the 2012–2016 drought. When beetle populations are less than epidemic such as at higher elevations, during moderate droughts, or early in severe droughts, previous fire and its associated reduced density may be neutral or ameliorating for conifer mortality.

Our sugar pine results may indicate a tipping point beyond which the combination of extreme water stress from drought, bark beetle outbreaks, and fire result in increasingly high rates of tree mortality (Nesmith et al. 2015), and subsequent forest structural changes outside the natural range of variation (Young et al. 2020).

These results suggest cautious low‐intensity and small (i.e., stand) scale prescribed burning, as it is often applied by managers, may only benefit forests under short duration drought stress while contributing to higher mortality in red fir and sugar pine during prolonged and exceptional droughts.

High mortality rates of large sugar pines may be related to prescribed fires consumption of deep litter and duff layers that have accumulated around the base of pine species under fire suppression, suggesting removal of litter and duff through raking could protect individual trees. Nesmith et al. (2010) found raking increased survival and reduced bark beetle activity when fire intensity was moderate (<80% crown scorch) and when fuel depth was ≥30 cm. Thus, protecting individual trees of high ecological value may be possible prior to prescribed burns. However, such targeted measures are infeasible at broad scales in fire‐prone landscapes of the Sierra Nevada. In the long run, retaining sugar pine in these pyrogenic landscapes may hinge on fostering sunny, bare mineral soil conditions favorable for sugar pine regeneration and in the future reducing surface fuels on a regular basis.

Infestation probability
Marginal effects on beetle infestation. (C) host species basal area within a 10‐m radius, and (D) whether a tree experienced a prescribed burn treatment. Beetle and tree species abbreviations are jpb for Jeffrey pine beetle; rtb for red turpentine beetle; mpb for mountain pine beetle; eng for fir engraver; pije for Pinus jeffreyi (Jeffrey pine); pila for Pinus lambertiana (sugar pine); abco for Abies concolor (white fir); and abma for Abies magnifica (red fir). For C, thick lines show mean effect estimates with labeled solid lines represent relationships where the 90% credible interval does not include zero. To illustrate the spread of credible effects, 30 model posterior draws are also drawn as faint lines. Note the y‐axis scale differs for (D). (From the research)

Treatment effects on large diameter trees are often the focus of management restoration efforts since these structures have been reduced from past logging, take a long time to develop, and are associated with important ecosystem services (e.g., sensitive species habitat and carbon storage). Treatments using only thinning consistently reduced mortality of large (>75 cm DBH) trees across species, albeit with different effect sizes. For incense‐cedar and especially white fir, there was a greater reduction in mortality for small versus large trees, which are often the target of fuel reduction treatments. Prescribed fire has mixed effects, reducing mortality of large Jeffrey pine and slightly reducing small white fir mortality when combined with thinning, but increasing mortality of large red fir, incense‐cedar, and significantly increasing large sugar pine mortality.

While prescribed burning is an important tool for increasing resistance to wildfire (Stephens and Moghaddas 2005, Prichard et al. 2010), our results suggest such fuel treatments do not necessarily also instill drought resistance. There is general benefit to all species in reducing density, but the means (i.e., mechanical vs. prescribed fire) of treatment matters, suggesting caution in widespread use of fire in drought‐prone areas where managers want to retain large sugar pines and red fir.

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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.

5 thoughts on “Do fuel reduction treatments increase resistance to insects and drought?”

  1. What’s going on at: 36.943273258117756, -119.02291201018879 and vicinity?

    Where are the prescribed burn (corrdinates)?

    Is this 36.95303109640362, -118.98473820938777 a replanting? I haven’t done any real forestry in six decades, but this looks like a stand density of about 1,000 trees per acre. I presume the trees are sugar pines, since this is Sugarpine Hill. I could identify sugar pines quickly via black and white stereo pairs in the fifties. I can identify older sugar pines on the Google satellite pix, but with more difficulty and far less accuracy. Note that the stocking is far less dense off the plantation. When solar radiation strikes the soil surface, a lot of soil moisture is lost; conversely, a lot of soil moisture is lost through transpiration. On balance, it might be a draw, or close to it, or there might be a significant tilt in one direction or the other for each factor. As root systems develop, they mine a greater and greater cubic volume of soil, and at some stages of their lifespan the balance shifts from predominately near-surface moisture to deeper sources, such as interstices in the underlying geologic formations. I cored a lot of trees (sugar pines) in my work back then, and I noted striking differences in annual ring counts of trees of similar apparent condition and size. Around the coodinates supplied, the stocking appears to be mixed conifer with fir (white, or maybe some red?), yellow pines (P. jeffreyii and/or a ponderosa?), with a few sugar pines–I’m guessing of course. Study this?

    Too few relevant data can screw up conclusions. It’s complex and hard work, but it should be a helluva lot easier and cheaper with computers than the thirties technology we were using in the fifties.

    It’s not necessarily “drought,” especially when measured by reservoir levels. It’s more about STRESS. For example, a lot depends on location and geology rather than AVERAGE precipitation. The vagaries of storm event sequencing in time (spaced more closely or farther apart). That is, tree stress is more dependent upon AVAILABLE water (capillary water and high soil-atmosphere humidity coupled with fluctuations in temperature, and plant physiological characteristics (e.g., a young pine might survive where understory vegetation that slows or shuts down transpiration shades the soil but not in a comparable site absent such vegetation). Study this?

    A look at the 2021 Google Map data (claimed–I have noted that data dates are sometimes incorrect) show few standing dead trees at the subject study area.

  2. This sounds a bit like “researchers re-doing old literature findings;” with most of their info having been known and or taught in forestry schools 40-50 years ago. At best, one could say they have “tweaked the data and or given it more thoroughness.”
    Since when did any forester not know that fuel reduction work does nothing to reduce moisture stress in residual stands of timber, or not know that that there would be added potential for insects and bugs, given that prescribed burning in stands leaves dead and or dying trees in it’s wake?
    All of this said, what’s needed is a return to managing our federal forests; yes, that means thinning and harvesting. Through that process, we will improve “overall” forest health, in particular, if our prescriptions for cutting are biased toward leaving and or improving wildlife habitat conditions. The end results will reduce fuel loading and break up continous masses of fuel and enhance fire suppression abilities.

    1. Fuel reduction work can reduce moisture stress in residual stands of trees if thinning through prescibed fire or mechanical techniques is applied — fewer stems means less evapotransipirational demand. I think the point of this paper is that in the presence of historic drought that may not be enough for sensitive species like sugar pine.

      While I agree we do need to take a more active approach to management on some areas of the federal ownership, that won’t really solve the problems we are facing. That’s because too much of the problem exists outside the feds control and what they do control is severely underfunded for the level of work needed.

      We do not need to return to the timber harvest is king mentality of the 50’s-80’s. That kind of short-rotation, intensive forest management won’t solve the current problems and will likely make them worse. Selective harvests, thinning, and forest restoration management should be the primary focus and that’s going to cost money.

      1. Tim, I concur we certainly don’t need to return to what was going on in the 80s; can’t comment on the 50s. As you may know, in the 50s forward, we used Multiple Use Management Plans, then came Unit Plans, and finally Forest Plans (NFMA). I worked with and often “in” timber management on federal lands and there was never what you term “short rotations.” That kind of intensive forest management was on private lands, not public lands. We could all comment on “what’s needed today” for overall forest health, but what is not needed is the current lack of forest health treatments, and, yes, I include timber harvests in that mix. With the current assessment of 80,000,000-90,000,000 acres of dead and dying forests on public lands, I’d say we need to kick the USFS & BLM in the butt and get back to practicing some form of solid professional management and salvage of otherwise wasted wood. Wildlife needs this; watersheds need it; fuel reduction needs are greater than ever; and much more. Congress has been DEAD IN THE WATER on this disaster in the West. Thirteen States are in deplorable condition relative to the above and little to nothing is being done about it. Expect more multi-million acre fires in the coming years.


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