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Fire in the American West: A long-term perspective

A mountain on fire at night, with a city in the valley.

Mt. San Miguel fire, 2007. Photo by slworking2 on Flickr, used with permission.

Today’s post is by guest-blogger Dr. Jenn Marlon, a biogeographer and  paleoecologist who studies the history of fire in the American West and across the globe. Dr. Marlon discusses the implications of fire suppression in the west, as explored in a recent paper in PNAS.

Fire is fundamental to our planet, and we’ve been making good use of it for millennia. As the fire historian Stephen Pyne has noted, however, when compared with air or water, fire has received surprisingly little attention in science. We have hydrologists, atmospheric scientists and climatologists, but we have few fire scientists. Perhaps this isn’t surprising – fire is episodic and seems random; it usually affects one community at a time, and often remains hidden from view, burning in distant forests. Most of us don’t think much about fire, and most of us don’t need to. Or at least, that used to be the case.

 Fires are changing

In the past decade, eight states including California, Texas, and Oregon, experienced record-breaking fires. In 2004, 6.6 million acres burned in Alaska — more than 8 times the 10-year average. In 2007, a series of wildfires in southern California burned 1500 homes, caused over $1 billion in property damage, and forced 1,000,000 people to evacuate their homes — the largest evacuation in California’s history. Nine people died from those fires and 85 others were injured, including 61 firefighters. In 2011, over 3.7 million acres burned in Texas alone, 43 percent of the national total that year. In Arizona the same year, the Wallow Fire, which caused 10,000 people to evacuate their homes, surpassed the Rodeo-Chediski fire of 2002 as the largest fire reported in state history. Losses for wildfire activity across Texas, Arizona, and New Mexico included at least 5 deaths and over $1 billion, helping 2011 make its own record as the year with the most billion-dollar disasters ever. In January this year (2012), which is not usually considered part of the U.S. wildfire season, over a dozen large fires burned in Oklahoma, Texas, Florida, Missouri, and even in Minnesota and Montana, which is usually snow covered in winter but wasn’t in January this year due to unusually dry conditions.

A graph showing biomass burning over the last 3000 years, indicating that biomass burned in the last few hundred years is lower than average.

Fig. 1) 3000 years of burning in the West (Marlon et al. 2012). Click for a larger image.

Why is this happening?

The National Interagency Fire Center tracks changes in acreage burned and acres burned per fire, and has documented an increase in both over the past decade. Fire histories from satellite data, fire scars on trees and charcoal that has blown and washed into lakes are also being developed to track changes in fire activity over longer time scales. In a study I just published with colleagues in PNAS, we combine three kinds of fire-history data to describe and explain changes in fire over the past 3000 years across the West as a whole from charcoal data.

When compared with historical and fire-scar records (the data overlap for the past 400 years or so), all three kinds of data agree and show a general “hump” pattern for the past 400 years (Fig. 2). Particularly noteworthy is that burning in the 20th century was as low as it was during the “Little Ice Age,” about 400 years ago.

A graph showing fire histories reconstructed from different data sources, including tree rings and charcoal, indicating a period of low burning over the last 400 years.

Marlon et al. 2012. Click for a larger image.

Such low burning is not normal for the Western U.S., however. Despite a gradual decline in fire over the past 2000 years, burning was never as low as it was during the LIA or in the 20th century. Large fluctuations, lasting centuries at a time, however, were common, and largely track increases and decreases in temperature and drought. Fire was high, for example, during the warm, dry interval known as the Medieval Climate Anomaly, and again from 1800-1900 AD, thanks to both drought and the fire-happy early settlers of the West who intentionally and accidentally caused many fires through logging, camping, and railroading.

Things changed very quickly for fire around 1900 AD. There was no single cause, but most of the reasons relate to human behavior. Grazing was perhaps the most important factor for the decline in fire, at least initially. Hundreds of thousands of livestock were introduced to pine forests and grasslands in western states in the late 1800s. The massive herds reduced grassy fuel loads, compacted soils, and prevented fires from spreading. Road and trail building also created fire breaks that limited fire spread. Cultural changes were yet another factor that helped reduce burning; by 1900 AD, the western frontier had largely closed and several large catastrophic fires (e.g., the Peshtigo Fire in Wisconsin in 1871 that killed over 1000 people, and the fires of 1910 in Idaho and Montana) transformed public attitudes as well as policies towards fire. In 1891, the Forest Reserve Act was passed, which allowed the President to reserve forests from the public domain, and in 1905 the U.S. Forest Service was established with a primary mission of suppressing all fires that occurred on reserved lands. Responsibility for fire management was transferred from the Army to the National Park Service when it was created in 1916, and full suppression remained the policy for the next five decades (with greatly increased efficiency in the 1940s).

A poster of a forest fire and a cigarette, with the words "Fire wrecks a forest."

WPA poster, Library of Congress. Creative Commons.

The young forest stands that developed after all the logging and burning during the early settlement period, however, also helped to reduce fire. Young stands in sub-alpine forest are less susceptible to fire, and so are aspen stands, which were also expanding then.

Settlement of the West fundamentally changed forests after 1900 AD. Stand structure, species composition, understory vegetation and fuel loads, non-native species, and altered disturbance regimes created forests unlike no others in the past. Coupled with timber extraction and land clearance, the consequences were dramatic and set the stage for today’s unusual fire regimes.

Where are we now?

Our forests today are young due to the legacy of land-use changes, but that isn’t the only thing that’s novel about them.  Reconstructions of temperature, drought, and human populations show that all have accelerated in recent decades. As a result, low levels of burning in the West aren’t only odd because they are so low, they are odd because they are so low despite increasingly warmer and drier climate conditions. If humans were out of the picture, levels of burning would be higher now due to warm temperatures and drought than they have been in the past 3000 years. One way to think of this is that there is a “fire deficit” in western forests today because fire is in disequilibrium with climate. A statistical model that predicts fire from temperature and drought (Fig. 2, panel A) shows the “fire deficit” as a gap between what we observe from charcoal data (red line) and what we would expect given current climate conditions (dotted black line).

Interestingly, the statistical model that predicts fire from temperature and drought so well, also implies that Native American burning (prior to Euro-American settlement) had relatively little influence on levels of burning on the West as a whole, although finer-scale impacts may have had important ecological consequences.

Our forests are ripe for fire

Today, our forests are under stress — from insects, disease, drought, invasive species, logging, and development. Many of these factors, especially prolonged drought, warmer temperatures, water shortages, and extreme weather are the perfect conditions for fire.

In an era of shrinking budgets and limited resources, problems that suck up billions of dollars are problems for all of us, regardless of which state we live in. We need creative solutions to move our forests towards healthier conditions, to restore patterns and processes that not only reduce vulnerability but also promote resiliency. There are several tools available, but all come with trade-offs, and all take effort, resources, coordination, and

Photo showing widespread tree death caused by mountain pine beetle outbreak.

Outbreaks of pests such as mountain pine beetle have created a large amount of fuel in Western landscapes. Photo by the Forest Service-Northern Region.

leadership. Some of the most effective tools, such as prescribed burning, have undesirable but temporary effects, including smoke. Smoke can be limited, but it can’t be eradicated. The smoke from prescribed burns can be problematic, but it is far less damaging and destructive to human health and livelihoods, as well as to ecosystems and the services they provide, than the so-called “megafires” that prescribed fires can prevent. We will undoubtedly need a wide range of tools to improve the current situation, however, and greater awareness of the problem in the communities most likely to be affected is a critical ingredient to mitigation and adaptation to fire in a warming world.

Marlon, Jennifer R., Patrick J. Bartlein, Daniel G. Gavin, Colin J. Long, R. Scott Anderson, Christy E. Briles, Kendrick J. Brown, Daniel Colombaroli, Douglas J. Hallett, Mitchell J. Power, Elizabeth A. Scharff, & Megan K. Walsh. Long-term perspective on wildfires in the Western USA. PNAS Early Edition,

Categories: Commentary Guest Posts Papers I Wish I'd Written

Tagged as:

Jacquelyn Gill

3 replies

  1. Link is wrong (by one letter) for Nat. interagency fire center.
    Love the Pyne thought about not many fire scientists. I would add soil to that elemental list of air and water (having plenty of scientists).


  2. Excellent post! Since I spent last summer in New Mexico, and had my research at Los Alamos interrupted by the Las Conchas fire, I was certainly interested to hear a long-term perspective on the situation. Thanks for an informative look at the history of the fire situation in the US.


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