Prescribed burning is an important active management approach that can address the fuel buildup and wildfire hazards that currently face western forests after a century of fire exclusion and suppression. Although prescribed burns are applied widely across the United States, their effects aren’t always well-documented. That means managers can’t anticipate how well prescribed burns will, ultimately, achieve their ecological and restoration goals and under what treatment conditions. What’s more, unsuitable prescribed fire regimes may actually be harmful to some ecosystems and their species, potentially exacerbating other management concerns such as invasive species. Ponderosa pine forests of the intermountain West—which cover approximately 27 million acres—are one of the forest types where prescribed burn effectiveness is not fully understood.
Recognizing this need, Forest Service research ecologist Becky Kerns and Oregon State University senior faculty research assistant Michelle Day documented the effects of five different prescribed fire regimes in Oregon’s Malheur National Forest over time. Their study, published in the journal Ecosphere, spanned 15 years and examined the effects of prescribed fire regimes on plant communities and, in particular, the forest understory.
“There are relatively few long-term studies about prescribed fire outcomes in the western United States, and even fewer examining multiple prescribed fire regimes as our study did,” said Kerns, who is based at the Corvallis Forestry Sciences Laboratory in Oregon.
In their study, the researchers applied multiple prescribed fire regimes—single spring and fall burns, five-year spring and fall reburning, and no burning at all—to five stands that varied in their plant cover and type and management histories.
To gauge the treatments’ vegetation effects, the researchers measured the plots’ under- and overstory plant canopy cover, ground cover, and species composition. Their results suggest that contemporary ponderosa pine vegetation is somewhat resistant and resilient to fire, and even frequent spring burning did not appear to be detrimental.
“The vegetation we observed was not particularly responsive, a phenomenon we call ‘resist but not respond’,” Kerns said. “In other words, it might be more complicated than we think to get these forests to respond vigorously to prescribed burns. We suspect these forests might be somewhat ‘stuck’ in their current state and have a lot of inertia to overcome in addition to experiencing other stressors like invasive species, livestock grazing, and more frequent drought. So, pushing these landscapes into new states with just something like low-severity fire might be difficult.”
Contrary to their hypothesis, most of the plant responses the scientists observed were not significantly different among the prescribed fire regimes or even in comparison to no burning at all—many treatment results were neutral, subtle, or ephemeral. Burning in the spring, although considered outside of the historical wildfire season, does not appear to be strongly detrimental to plant communities, even when done frequently.
So what do these findings mean for managers of western dry forests working to reduce severe wildfire risk and reestablish historical fire regimes and vigorous understory plant communities in areas with a fire deficit?
“Our results suggest that common prescribed fire application under very low-severity prescriptions may not do the ‘work’ of wildfire or the work we expect given contemporary conditions, so management outcomes related to prescribed fire use, like a vigorous vegetation response or restoration of the understory, may not be as anticipated,” Day said. “With so many changing dynamics, the past might not be the best ‘baseline’ for the future.”