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).

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

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, www.pnas.org/cgi/doi/10.1073/pnas.1112839109