2004-2005 Issue

A walk through Colorado forests with a fire ecologists brings home ecological realities.

If you compare a photo of the Boulder, Colorado, foothills in 1905 to the same landscape today, the difference is stark. The upswept, rocky ridges that mark the meeting of the Great Plains and the Rocky Mountains were once covered in tall grasses. Now, those foothills are a blanket of green, densely spaced conifers.

Walking along a tree-strewn mesa just south of Boulder, fire ecologist Rosemary Sherriff points to a nearby stand of ponderosa pines. These trees, once sparse in the grassy foothills, probably owe their existence to the fire suppression efforts of the last century. "Eighty years ago," she says, "most of these trees probably didn't exist."

Only a thousand feet higher in elevation, the scenario is quite different. In the steep valleys that weave their way up to the Continental Divide, the terrain is rockier and steeper. Sherriff, who recently received her doctorate from the University of Colorado, compares a wooded valley with a 1905 photo of the same area. Today, ponderosas cluster densely on the valley's north-facing slopes. A century ago, the northern half of the same valley was bare of vegetation, wiped clear by a severe fire that took out the forested hillside long before Americans got into the business of fighting forest fires.

Under the tutelage of biogeographer Tom Veblen, Sherriff and her fellow graduate students in the University of Colorado Department of Geography have studied the effect of disturbances such as fire and insect outbreaks on tree species across the spectrum of Rocky Mountain ecosystems. Traveling through the Colorado woods with hand drills, they take samples from trees to determine their ages, counting annual rings backwards from the rough bark to the smooth brown pith that marks the tree's center. "An annual tree ring thicker than the ones on either side of it represents an exceptionally wet growing season; a narrower band indicates a drought year," Sherriff explains. By taking samples from trees marked with the dark, triangular-shaped scars where flames surrounded the trunks as fires passed through, researchers can estimate the year in which a fire occurred. By sampling trees throughout a given area, they can also determine how many fires swept through that stand over the centuries and how severe those blazes may have been.

Veblen and his students use these samples to reconstruct the history of fires, and their research may force timber managers and policymakers to reexamine the prevailing wisdom about forests' "natural" state. For instance, Sherriff's work supports other scientists' findings that lower-elevation ponderosa forests were historically subject to frequent low-intensity fires that now burn hotter, faster, and larger as a result of 80 years of fire exclusion. But she has also found evidence that big fires blew through higher-elevation ponderosa stands long before humans began successfully fighting forest fires in the early 20th century. Her research indicates that differences in elevation, aspect, slope steepness, and other conditions determine whether a ponderosa stand is subject to low-intensity fires or patchy, severe fires that can take out all the vegetation in an area. Even in areas only a few miles apart, there can be tremendous variation in fire regimes. "Every time you turn a corner," says Sherriff, "you've got a different kind of forest because of the ways that topography shapes fire behavior."

In the subalpine environment of lodgepole pine and spruce-fir stands that populate Colorado's forests above 9000 feet, however, fire behavior becomes somewhat more predictable. Sherriff points to a mountain ridge blanketed in deep green, coarse-textured forest. On the other side of an almost imperceptible slope break, the foliage is more yellow and fine-textured. Where the untrained eye may see only a wooded hillside, Sherriff and her colleagues read centuries of ecological history: the lighter swath of trees took root after a crown fire blazed across the hillside, losing momentum only when it hit the ridge that now marks the edge of the older, greener stand. This subtle variation in color and texture tells the classic story of fire in subalpine forests: The trees burn rarely, but when they do burn-always in times of drought-they burn big.

The vast majority of lodgepole pine forests will go 100 to150 years between fires, while spruce-fir forests often go as long as 500 years. "We had high-magnitude burns long before we had any fire suppression," notes Veblen. "This is a point that is underestimated by vegetation managers: 2002 was the biggest fire year in the recorded history of Colorado, and it was also the driest year in the recorded history of Colorado. During extreme droughts you can count on severe-and widespread-fires."

After the devastating fires of recent years, however, these realities have sometimes escaped the notice of policymakers, who from 2000 to 2004 received $1.8 billion to reduce the risk of fire on the nation's forests, and timber managers, who have been charged with spending that money. In some cases, timber managers have proposed to use federal funds to treat high-elevation forests where even the most ambitious thinning project is unlikely to prevent a fire from burning everything in its path. In the Pike-San Isabel National Forest near Leadville, Colorado, for instance, the U.S. Forest Service has released a plan to treat more than 5,000 acres of subalpine forest, arguing that intensive thinning was needed to restore forest structure disrupted by clearcutting and mining in the late 19th century.

A group of scientists convened to study the project, however, concluded that the Forest Service could not use the goal of ecological restoration to justify thinning in lodgepole pine. "Any fire ecologist will tell you that as you go up in elevation in the Rockies, fire behavior changes and makes thinning an inappropriate fire restoration treatment," says Wilderness Society Senior Forest Scientist Greg Aplet, who was a member of the panel. "This isn't to say there shouldn't be treatment in forests above 9,000 feet, but let's start the thinning closer to the communities that are at risk."

Most forest scientists now agree that treatment should occur in low-elevation ponderosa pine forests, around homes first. Unfortunately, ecology and realism are seldom the sole considerations taken into account when making decisions about managing the nation's forests. "There is a natural tendency for vegetation managers to want to take action when the public perceives that there's a fire hazard or an insect outbreak hazard," says Veblen. "And it's hard to convince them that, at broad landscape scales, management is not going to have much of an influence. Add political pressure at the national level-with money being thrown at the Forest Service to 'do something' about fire hazard. The challenge is to use that money strategically, where it will do good."

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