This is the second in a series of two Public Lands Blog posts regarding the idea that commercial thinning of frequent-fire-type forests should take place before reintroducing fire. Part 1 set the stage with a review of past thinking about thinning. Part 2 examines the new science and its implications for policy.
Figure 1. Understory burning on the Deschutes National Forest, Oregon. Source: USDA Forest Service.
As that great quote magnet Winston Churchill (or maybe it was John Maynard Keynes, or actually his student Paul Samuelson) said: “When the facts change, I change my mind.” The facts regarding the efficacy of thinning plus prescribed burning versus prescribed burning alone to reduce fire severity in frequent-fire-type forests have changed. Here we will look at the findings of a new meta-analysis that “provides up-to-date information on the extent to which active forest management reduces wildfire severity and facilitates better outcomes for people and forests during future wildfire events” (Davis et al. 2024).
Findings of a New Meta-Analysis of Forest Treatments
Comes now a new Tamm review (“a high-profile series of invited reviews on key issues in forest ecology” named in honor of Swedish forest ecologist Carl Olof Tamm and published in the journal Forest Ecology and Management) entitled “Tamm review: A meta-analysis of thinning, prescribed fire, and wildfire effects on subsequent wildfire severity in conifer dominated forests of the Western US” (Davis et al. 2024). The lead author is with the Forest Service’s Missoula Fire Sciences Laboratory in Montana. The second author is a post-doc at the University of Montana who was previously a Nature Conservancy fellow. The other five authors work for the Nature Conservancy, which has been quite hawkish on “active management” to “restore” frequent-fire forest type stands, including the liberal use of logging (generally more than I have been comfortable with).
A meta-analysis is an examination of data from a number of independent studies of the same subject in order to determine overall trends. Davis et al. 2024 considered data from California mixed conifer, interior mixed conifer, lodgepole pine, ponderosa/Jeffrey pine, and subalpine forest types. All types, save for lodgepole pine and subalpine, have historically experienced “frequent to moderately low- or mixed-severity fire.” It’s unfortunate that this meta-analysis included lodgepole pine and subalpine forest types, as these historically have experienced infrequent stand-replacing fire, which is the norm and normal, aka characteristic, in these two forest types. There is no ecological need to reduce tree densities in these two and all other infrequent-disturbance forest types.
Nonetheless, Davis et al. 2024 is useful in examining the efficacy of measures intended to reduce the density of overly dense forests in frequent-fire forest types in the American West. These measures were examined (the percentages following each measure indicate the mean reduction in fire severity):
• thin + prescribed burn (72 percent)
• thin + pile burn (62 percent)
• thin only (27 percent)
• prescribed burn only (62 percent)
• prior wildfire (25 percent)
Let’s ignore both thin only and prior wildfire because, as the paper notes, the numbers were not “significantly different than zero.” The authors conclude: “All treatments except ‘thin only’ and ‘prior wildfire’ significantly reduced wildfire severity in CA [California] mixed conifer, interior mixed conifer, and ponderosa/Jeffrey pine forest types.”
The Forest Service often chooses to pile the slash left in the woods after thinning because it feels there is too much slash to call for a “broadcast” burn or, in the terminology used in Davis et al. 2024, a prescribed burn. Ironically, the reduction in fire severity is less with pile burning (62 percent) than with broadcast burning (72 percent).
If reduction of fire severity is your only goal, thinning before burning wins out. However, if you have other goals such as cost effectiveness, biological diversity, watershed integrity, and/or the transfer of carbon in the atmosphere to the biosphere (aka forests), it’s not quite so simple. Let’s focus on thin + prescribed burn versus prescribed burn only as we examine these goals.
Looking at Cost Effectiveness
Doing prescribed burning only rather than thinning + prescribed burning is the most cost-effective way to achieve a reduction of fire severity in fire-starved frequent-fire-type forests.
Notice that:
• Only 10 percent of the reduction in fire severity is attributable to thinning (72 percent – 62 percent = 10 percent).
• Fully 82 percent of the reduction in fire severity from thin + prescribed burn is attributable to prescribed fire, not thinning (62 percent / 72 percent = 82 percent).
Assuming an average thinning cost of $1,004/acre and an average prescribed natural fire cost of $149/acre, the cost of a thin + prescribed burn is $1,153/acre, while the cost of a prescribed burn alone is $149/acre.
In other words, a 62 percent average reduction in fire severity costs $149/acre. To boost the average reduction in fire severity to 72 percent costs 7.4 times as much. Is it not better to embrace a 62 percent reduction in fire severity on 7.4 times ($1,153 / $149 = 7.4) as many acres as can be treated if thinning is done along with prescribed natural fire? After all, the Forest Service has said that the scale of the treatments needs to be at the landscape level to be effective.
Figure 2. A hypothetical illustration comparing the effects on fire severity of various treatments. Notice how the end result (“Post-wildfire”) is almost identical for a prescribed (Rx) burn and a thin + prescribed (Rx) burn. The 10 percent additional reduction in fire severity means that fewer snags (which are excellent wildlife habitat) are left. The surviving live-tree component is the same. Source: Davis et al. 2024.
Looking at Ecosystem Health
It is better for the biosphere, the hydrosphere, and the atmosphere for forest carbon to stay in the forest.
• Biological diversity. Thinning before burning results in less biological diversity, most particularly in fewer snags (standing dead trees). Snags may be unsightly to a forester but are not to a woodpecker. Logging equipment (as well as domestic livestock grazing) can facilitate the spread of invasive species onto the site.
• Watershed integrity. Logging involves the use of heavy equipment that churns up the forest floor, which causes more erosion of sediment into streams.
• Atmospheric carbon. Big Timber and the Forest Service like to tout the carbon kept out of the atmosphere by storage in long-lived wood products. Yet, only 21 percent to 22 percent of the wood removed from a forest that goes to a mill ends up in long-lived wood products—products that have a half-life of thirty to sixty-five years (for every half-life, half of the stored carbon ends up in either landfills or the atmosphere). The timber-industrial complex conveniently fails to account for all of the carbon that was safely stored in the forest and is now in the atmosphere due to logging, including emissions from slash, from operating logging equipment, and from transporting the logs to the mill. In eastern Oregon national forests, large (21+ inches in diameter at breast height) trees comprise only 3.1 percent of trees in the forest by number but contain 42.2 percent of the aboveground carbon (living and dead vegetation). Sending a large log to the mill is a carbon bomb to the atmosphere, but so is sending a lot of smaller logs.
Big Timber’s Reduced Capacity to Do the Thinning
Big Timber isn’t as big as it used to be, especially in the arid Intermountain West, so it cannot be consistently relied upon to do the thinning before the burning anyway.
Stemming from its we-must-log-the-old-growth-forest-to-save-it mentality, the Forest Service recently identified the lack of mill infrastructure as a major “threat” to mature and old-growth forests (Figure 3), noting that for forests “in firesheds where wood processing capacity is low, but current threats, including severe fire, are high . . . . these areas may struggle to practice active management to reduce such threats.” As I noted in a previous Public Lands Blog post, “The Continuing Reduction in the Number of Sawmills in the Pacific Northwest,” milling capacity is declining in spite of increased logging levels on the National Forest System. Milling capacity is also shrinking elsewhere in the American West.
Since I wrote on sawmill reductions in the Pacific Northwest, Forisk has reported that while six lumber mills have closed or downsized since June, seven have increased their milling capacity, resulting in a minor (-61 million board feet) change in net lumber milling capacity. At the same time, milling capacity for oriented strand board, pellets, pulp/paper, plywood/veneer, and other engineered wood products has gone up significantly.
Figure 3. How milling capacity stacks up against current threats to od-growth forests, according to the Forest Service. To properly interpret this map, one must first suspend reality and agree with the Forest Service contention that a lack of milling capacity is a threat to old-growth forests. For example, the brown timbersheds are “low [milling]capacity/high threat [to old-growth forests].” Where the Forest Service most wants to log the old growth to save it are often the areas with low or non-existent milling capacity to carry out the deed. Source: USDA Forest Service
The Bottom Line on Thinning
• It is less cost effective to thin a degraded frequent-fire-type forest before restoring it by burning it.
• Thinning, even if for a supposedly more noble purpose, is logging, and logging causes water pollution and soil erosion. It also causes more carbon to go into the atmosphere at a time when it is imperative to keep as much carbon in forests as possible and out of the atmosphere.
• Big Timber can no longer be consistently relied upon to do the thinning before the burning.
Relying on thinning + prescribed burning to reduce fire severity in frequent-fire forest types is increasingly problematic. Such is neither prudent nor rational. Better to rely on Rx burning, aka prescribed fire.
Burn, Baby, Burn
One of the major reasons the Forest Service doesn’t do more prescribed burning is that the bureaucracy is afraid of fire. The good news is that the Forest Service is rational about fire all year except for during the fire season, when it reverts to its historic form. The Forest Service currently spends unlimited amounts of money (actually, it is limited by the availability of fire crews to be hired and equipment to be rented) to
• successfully extinguish fires early in the fire season, when they should be encouraged to burn, and
• unsuccessfully fight fires at the peak of the fire season, when all efforts are futile.
The solution is to reprogram the fire-industrial complex. Rather than fighting fire, the Forest Service should be fomenting fire.
The Forest Service should spend its money doing prescribed burning at scale (meaning a lot of it) during the fire season—especially near the start and near the end of each season. The Forest Service has the resources to go big on prescribed burning. It just needs to spend the money it is already spending, but at different times in the fire season.
Bottom Line: The most ecologically rational and fiscally prudent course is to eschew thinning before reintroducing fire into fire-dependent forests.
Figure 4. Crew on a prescribed burn. Prescribed burning creates jobs. Source: USDA Forest Service.