Southwest forests
Some of the most extensive and detailed records of past fire activity come from the southwestern United States. Over thousands of years, this region’s widespread ponderosa pine forests have been shaped and structured by fire. Historically, frequent low-intensity ground fires maintained open, park-like forests with grassy understories. Although such fires are often very local in nature, a broad historical perspective reveals regional-scale patterns of fire incidence and intensity, driven by climatic variability.
Dr. Craig Allen has studied the history and effects of fire in the Jemez Mountains of northern New Mexico since 1986. He and his collaborators employ several methods for reconstructing the fire history of the Jemez and neighboring Sangre de Cristo mountains. Fires that do not kill a tree often leave a scar, which is recorded in the tree’s annual growth ring. By carefully examining the tree rings, researchers can determine the year and often even the season in which the fire occurred.
Allen’s team has put together over 4,500 fire dates, from over 600 trees, logs and stumps. “The Jemez is one of the better-sampled landscapes of its size anywhere,” says Allen. “Very few areas have as much fire history.”
Analysis of tree rings is carried out in cooperation with Dr. Thomas Swetnam of the Laboratory of Tree Ring Research at the University of Arizona. The Jemez data form part of the lab’s regional tree ring network for the entire Southwest, which contains regional fire history and climatological records for over 1,000 years.
Now Allen is using a different method to extend the Jemez fire history record back even further. In collaboration with Dr. Scott Anderson of Northern Arizona University, he is reviewing pollen and charcoal deposits in soil cores extracted from several northern New Mexico bogs. The cores contain a record of sediment deposition going back over 10,000 years. In one Jemez sample that has already been analyzed, Allen says, the contrast between the current century of fire suppression and the millennia that preceded it are clearly visible. “There is abundant charcoal throughout the core, except for the last couple of centimeters, corresponding to the past 125 years,” he says. “It shows that fire has been an important ecological process here in the Jemez for at least 8,000 years.”
Although humans have long shaped their landscapes through deliberate use of fire, Allen says fire patterns in the Southwest have largely been driven by the region’s weather patterns. “Human ignitions were probably less important here than in most places on the planet,” he says. Recent data show that the Jemez Mountains average about 16,000 lightning strikes per year, and Allen’s analysis of fire suppression records for roughly 5,000 fires since 1909 indicate about 75 percent were of lightning origin.
Fire scars indicate that historically, blazes were most frequent in the dry spring and early summer period, before the arrival of the late-summer monsoon rains. Most burned only along the ground, clearing away debris and maintaining open, montane grasslands over large areas. Some of the trees Allen has sampled experienced more than 30 fires over the course of a few hundred years, without being killed.
Swetnam and climate change scientist Dr. Julio Betancourt, of the USGS Desert Laboratory, have shown that patterns of fire incidence in Allen’s Jemez data are often mirrored across the broader Southwest region. The episodic occurrence of “regional fire years” appears to be associated with El Niño and La Niña events. Much of the Southwest is strongly affected by the weather patterns that characteristically follow these shifts in equatorial Pacific Ocean currents. El Niño years bring above-normal precipitation to the region, while La Niña years — which often follow on the heels of El Niños — are dry.
As might be expected, fire activity historically is greatest during La Niña events and droughts. But wet El Niño episodes play a role as well. In an environment in which water is often a limiting resource, wet years result in a rapid build-up of herbaceous understory vegetation. Years of intense regional fire activity often occur at the end of an El Niño-La Niña cycle, when this extra plant growth becomes a blanket of dry fuel across southwestern mountain ranges.
In the late 19th century, however, other factors came to dominate the region’s fire regime. Allen says that in the arid Southwest, grazing has played at least as big a role as fire suppression in altering the natural pattern of frequent, low-intensity burns. Beginning in the 1880s, large numbers of cattle and sheep were introduced into southwestern forests. As grazers consumed the grasses and other herbaceous vegetation fires need in order to spread, fire activity dropped off. In addition, said Allen, the trails created by livestock over time probably constrained the spread of fire as well by breaking up the continuity of the surface fuels.
“The initial cessation of fires preceded active fire suppression by several decades,” Allen says. As grasses were reduced and fires ceased to spread across the landscape, more trees were able to get established. The effects of grazing were then multiplied when fire suppression became the norm early this century.
Eventually, open areas were replaced by dense tree stands. Allen says that historically, a typical density of ponderosa pines in the Southwest was around 100 stems per acre. Today, densities at many sites exceed 2,000 stems per acre.
In these dense forests, destructive insect outbreaks are common. And fires, when they do get established, now often leave the ground and climb “ladder fuels” into the treetops. “We did not start to see extensive crown fires in ponderosa pine forests until around the 1950s,” Allen says. “It took that long for the forests to get dense enough and for the fuel conditions to change.”
Tree ring records show that, in addition to the El Niño-La Niña cycles, periodic droughts and wet periods of much longer duration are also part of the normal climatic variability in the Southwest. A prolonged drought during the 1950s contributed to outbreaks of large, destructive fires at that time. However since then — particularly over the last 20 years — precipitation totals across most of the Southwest have been abnormally high — a fact that Allen says should be cause for concern.
“It’s been extremely good for tree growth, and a lot of extra forest biomass has accumulated on the landscape,” he says. “The next time we have a significant drought of any sort, we can expect some very severe fire behavior. It’s the scale and associated ecological effects of potential crown fires that we worry about. The forests across whole mountainsides can just go up, burning entire watersheds and resulting in severe post-fire erosion and flooding. Once such crown fires are in progress, we can’t stop them through direct suppression methods.”
The solution, says Allen, is to thin the forests, but how this should be done in overcrowded ponderosa pine and mixed conifer systems is the subject of much debate. Many fire ecologists feel that prescribed burning is the best way to restore a more normal ecosystem structure; others argue that in at least some areas, trees are so dense that some mechanical thinning should be done first. Allen says current fuel conditions are so hazardous that prescribed burning needs to be done with caution. “You’ve got to get it hot enough to change the structure of the forest,” he says. “But the difference between burning it hot enough to kill some trees and burning it so it kills all the trees . . . that’s a challenging prescription.”