How Common Were Low Severity Blazes in Western Ecosystem?

A recent article in the Arizona Republic, “The only way to save Arizona forests is to let them burn,” repeats the misguided idea that low-severity/high-frequency fires keep the forest open and park-like, with limited fuels to sustain tree-killing wildfires. In other words, if a fire kills most trees, it is “lost” and “destroyed.”

In the Arizona Republic article, historian Stephen Pyne suggests that fire burned Arizona’s ponderosa pine forests every 3–8 years.

One problem with Pyne’s estimate is that almost no fire studies, even in ponderosa pine forests, have such short fire intervals. In a survey of 342 fire studies by William Baker in dry western forests (which includes ponderosa pine, dry mixed conifer, which can consist of Douglas fir and grand fir among other species), the shortest fire rotation reported in Arizona was 7.2 years; most Arizona studies indicated fire frequency of about 10–25 years.

Remember that Arizona and neighboring New Mexico have the West’s shortest dry forest fire rotation. Most of the other parts of the West’s pine forests are dominated by longer fire intervals with a percentage of high-severity patches.  

A frequent fire regime does not negate the occasional, but less frequent, high-severity blaze that kills most trees. Even in Arizona, dense forests and high-severity fires occur under the right climate/weather conditions, such as extreme drought and high winds.

The idea that historically dry forest landscapes were dominated by old-growth forests kept open and park like by frequent blazes is primarily due to fire scar studies. These studies have numerous methodological issues that tend to “shorten” fire regimes challenged by General Land Office reconstructions, early photographs, sediment, and pollen evidence. 

To the degree it is accurate for ponderosa pine, research shows that short (less than 25 years between blazes) only applies to about 14% of all dry forests, while 86% experienced multi-decadal fire intervals. The mean fire frequency of all 342 sites in dry forests was 39 years.

A recent paper, “Countering Omitted Evidence of Variable Historical Forests and Fire Regime in Western USA Dry Forests: The Low-Severity-Fire Model Rejected,” challenges this dominant paradigm.

A further problem with this generalization is that this fire model doesn’t apply to most plant communities, including spruce, lodgepole pine, fir, cedar, hemlock, aspen, chaparral, juniper, and sagebrush, which are characterized by infrequent fires that are often many decades to hundreds of years between major blazes. Yet the media and even too many agency representatives fail to make this distinction, giving the impression that low-severity, high-frequency fires dominate most Western landscapes.

Yet, much of the current fire policy is based upon this flawed assumption that frequent low-severity fires were historically the norm for much of the western landscapes. Both logging/thinning and prescribed burning are advocated to “restore” these forests. Such “restoration” has numerous ecological impacts.

The focus on logging as the preferred response to fire has many other ecological impacts—far worse for forest ecosystems than any wildfire. Most western plant communities are adapted to wildfire to some degree. Still, logging is an entirely new influence that brings many ecologically harmful issues, such as sedimentation from logging, disturbance of sensitive wildlife, and loss of biomass and structure from forested landscapes. It is also important to note that logging degrades forest genetics by indiscriminately removing trees that may be resilient to drought, disease, insects, and fire.

There are numerous ecological problems with promoting the idea that our forests require frequent blazes to be “healthy.” Too frequent fires can affect the nutrient cycling, habitat for wildlife, and the creation of down wood and snags by natural processes, from wildfire to drought to bark beetles.

Too frequent fires will also impact forest regeneration as most seedlings and saplings are killed by low-severity blazes.

PROBLEM WITH FIRE SCAR STUDIES

Even though Baker utilized primarily fire scar studies in his analysis, there are a number of methodological issues with them.

First, most fire scar studies count fires. They do what is termed “composite” fire studies, where each year in which there is a documented fire scar is counted as a “fire year.” But the number of fires doesn’t necessarily tell you anything about the ecological footprint of fire.

The average fire typically burns less than 5 acres, most less than an acre. Thus, you can document a lot of fires but not burn much landscape. At such a rate, it takes hundreds or even thousands of fires to burn any significant amount of the landscape.

Indeed, the evidence suggests that the vast majority of all acreage charred by wildfire annually results from a few large blazes. Eliminating these large fires (nearly impossible for various reasons) would dramatically reduce the fire influence at the landscape scale.

To give one hypothetical example of how this counting fire might skew the results, let’s pretend we have a 1,000-acre study area. In that study area, we found evidence for at least one fire every year between 1900 and 2000 or a fire interval of 1 year. However, if each of those fires only burned 1 acre in a hundred years, at most, only 10% of the 1000 acres would have been burned. At that rate, the fire rotation would be 1000 years.

One needs to have a geographical sense of fire. Is a fire scar in a particular year just one tree struck by lightning or a fire that burned across the entire study area? Should such small blazes even be counted?

The problem with targeted sampling is that it’s non-random. It’s like going into a brewery to poll people about whether they like beer.

In the 1930s, the bank robber Willy Sutton was asked why he robbed banks. Sutton is reputed to have replied with the self-evident “because that is where the money is.” That is how fire researchers have gathered their data on fires—they sample in places with many fire scars.

Places with abundant fire scars tend to have naturally low fuel loadings and frequent fires. However, these sites may not represent the surrounding landscape, such as north-facing slopes or valley bottoms, which may be wetter or have higher productivity and, thus, longer intervals between blazes. Extrapolating what appears to be a frequent fire history from targeted sites may not represent the fire history of the landscape.

This extrapolation is common in studies suggesting that Indian burning influences Western landscapes. Most of these studies are focused on places where tribal people concentrate, such as village sites, but are not representative of the fire regime on the landscape as a whole.

Remember that most fire studies are small—sampling a small amount of the landscape, but they are frequently used to justify landscape-scale active management. Unless scarred trees are sampled and positioned geographically throughout the study area, even filters will not eliminate the upward bias in the fire frequency in a given study area.

However, the main problem with the extrapolation of the low-severity/high-frequency model as a forest policy model is how fires appear to burn. In my experience with prescribed burns, most fires burn a very small percentage of the landscape without wind. Fires run into a foot-wide game trail and stop.

The idea that you can burn hundreds or thousands of acres at low severity is an exaggeration. Most fires burn a few acres at most, and often just individual trees. If I don’t account for these fires’ “geographical” footprint, I can give the impression that low-severity blazes have burned a large percentage of the landscape.

A complicating factor in basing forest policy on historic forest fire reconstruction is that the climate has shifted. We are experiencing some of the historically driest conditions and higher temperatures than in the past. Extreme fire weather is driving today’s wildfire regimes.

The Biscuit Fire in Oregon killed some forest stands with high-severity burns, but other patches burned at low to moderate severity. Photo by George Wuerthner

What I have seen, however, is fires burning under more extreme weather conditions, with significant high-severity patches, across thousands of acres. Even in such blazes, the bulk of the burn severity is moderate to low. In other words, getting a landscape-scale influence only occurs when you have the conditions for a high-severity blaze.

As for reducing fuels, my experience looking at prescribed burns in the pine forests around the West and elsewhere is that, within a few years to a decade or so, you have more fine fuel biomass, and the potential for a larger, more significant blaze is increased. Sure, immediately after a burn, the lack of fuel can obviously slow or stop a blaze, but in 3–5 years, I often see more grasses, shrubs, etc., on the burned site than before treatment.

The critical takeaway is that while low-severity/high-frequency blazes occur in some dry forest landscapes, they do not dominate landscapes where longer fire intervals are more common. Furthermore, the low-severity/high-frequency fire regime does not apply to most western plant communities, which are naturally dominated by mixed-to high-severity fire regimes controlled by climate, not fuels. Therefore, nearly all “active forest management” is inappropriate and will not achieve the stated goal of reducing large, high-severity wildfires.