Burning dead and dying timber in parts of Boulder’s Open Space and Mountain Parks area increased plant species and pruned dense ponderosa pine forests that pose a serious wildfire threat to nearby homes, a Colorado State University study found.
The two-year study, led by forest sciences professor Rick Laven, focused on the effects of prescribed burning–the intentional setting of small fires to help clear overstocked forests of debris–on two open space parcels where ponderosa pine forests border grasslands.
The study found that prescribed burns in these two specific areas thinned out dominant plants so that other plant species could emerge, resulting in a healthier and more diverse ecosystem. It also proved a useful method in thinning out forest debris that has accumulated after years of fire suppression.
"Fire triggers a thinning and rejuvenating process in ecosystems, and also maintains a border between ecosystems," Laven said. "Without fire or thinning forests by mechanical means, a wildfire is likely to be much more intense and harder to control. This is especially a concern for Colorado’s Front Range, where so many people have built homes in these areas within grassland and ponderosa pine ecosystems."
Operating under a $50,000 grant from the City of Boulder’s Open Space and Mountain Parks Department, Laven and graduate research assistant Sarah Gallup measured tree densities and inventoried plant species before and after prescribed burns in the two areas. Unburned areas were used for comparison.
The two burns behaved differently, one creeping through grass at low intensity and the second burning at a much higher intensity. A year after the plot was burned by intense fire, researchers documented an increase in plant species compared with other plots, while grasses decreased. The area with less intense fire showed a decrease in plants that reproduced only by seed.
Laven said historical photographs of Boulder’s open space show that prior to human settlement, similar areas were apparently thinned out naturally with low-intensity surface fires that may have occurred as frequently as every 40 years.
As urban populations increased and fire was continually suppressed, the ponderosa pine ecosystems became increasingly dense and began to encroach on native grasslands. That changed the number of native plants and other plant species present in the two ecosystems, Laven said.
Laven pointed out that suppressing fire in lower-elevation ponderosa pine forests, such as those in the study, posed more of a wildfire threat than suppressing fire in higher elevation forests marked by species such as lodgepole pine or spruce.
"At higher elevations, hundreds of years often pass between forest fires because there’s greater moisture and the climate is less conducive to frequent burns," Laven said. "At lower elevations, we suspect that fuel build-up on the forest floor occurs much faster. By suppressing fire, more woody debris is available to act as a fuel source."
Laven’s study dovetails with a recent debate in Congress over President Clinton’s proposal to burn as many as 1.3 million acres in national forests to weed out areas overloaded with fallen timber. The U.S. Forest Service suggested that low- intensity fires will help reduce the threat of wildfires that quickly grow out of control.
Laven cautioned that his study focused solely on the effects of prescribed burning in two specific areas within Boulder’s open space, and cannot be used to reach a general conclusion about the use of fire.
"The effects of fire are not easy to generalize. Each fire behaves differently and conditions are different, which makes replicating fires for experiments extremely difficult," Laven said. "We hope this study adds to our limited knowledge of the ecological consequences of prescribed fire and helps managers choose the most appropriate land management options."
Laven has studied the use of fire and forest ecology for more than 20 years and is co-author of "Introduction to Wildland Fire," a book covering all aspects of prescribed burning and its ecological effects.