To increase the prescribed “burn window” for reaching restoration goals, land managers are now burning during winter (the dormant season) as well as spring and summer (the growing season) and fall.
Management goals often include fuel reduction, oak regeneration, habitat improvement for target wildlife species, and forest restoration to conditions once created by Native Americans and Euro-American settlers.
A recent study led by USDA Forest Service scientists Katie Greenberg and Tara Keyser, along with colleagues from the Pisgah National Forest, investigated how season of burn affects breeding birds in the Southern Appalachians.
From 2011 to 2016, the team measured overstory tree density and breeding bird communities before and after prescribed burns. They looked at nine oak-dominated, closed-canopy stands within the Bent Creek Experimental Forest.
Three units were burned during the dormant season, in early March, three were burned during the growing season, in late April, and three were left unburned as reference sites, or controls.
Regardless of season, the researchers found that low-intensity burns had a negligible effect on forest structure — as well as species composition and abundance of breeding bird communities in upland hardwood forests. Their results were published in the journal Forest Ecology and Management.
For Greenberg, these results were not surprising. “Structural heterogeneity in forests and across landscapes encourages wildlife diversity,” says Greenberg. Some birds need mature forest overstory, while others depend on open, young-forest habitat. A landscape with different-aged forests and a variety of structural characteristics supports many different species.
In any season, low-intensity burns generally don’t kill overstory trees. This means that changes to forest structure and composition are minor and transient. Shrubs may be killed aboveground, but they rapidly re-sprout. Leaf litter is reduced, but it is replenished the following autumn as leaves fall from trees.
An intact forest canopy limits the establishment of disturbance-dependent species, such as the Indigo Bunting, Eastern Towhee, or — at higher elevations — the Chestnut-sided Warbler.
In contrast, natural disturbances or forest management techniques that reduce canopy cover promote breeding bird diversity. In a study focused on oak regeneration, Greenberg and others found that shelterwood harvests — those that reduce the forest canopy by 70 to 80 percent — increased breeding bird density and the number of species (or species richness) within those forest stands.
That is because disturbance-dependent species such as Indigo Buntings and Chestnut-sided Warblers moved into the shelterwood harvests. At the same time, many generalist species remained in both shelterwoods and mature forests. Treatments that only altered understory light conditions, such as midstory herbicide application, had little effect on breeding bird diversity.
High-severity fires that kill a lot of trees also increase breeding bird diversity. In a 16-year study on avian response to fuel reduction treatments, Greenberg and colleagues found that a combination of mechanical understory removal followed by high-severity burns increased breeding bird species richness by up to 70 percent within three years. Other treatments, including repeated low-intensity burns or mechanical understory removal, had little effect on breeding bird communities.
Greenberg’s research — one study at a time — and the research of others, reveals that altering the forest canopy, whether by harvests or by high-severity burns, creates a temporary space for disturbance-dependent bird species to thrive. Low-intensity disturbances do not effectively create those open-canopy conditions.
Greenberg stresses that canopy reduction is not a forest management prescription for all species. While most cavity- and canopy-nesters remain abundant after high-severity burns or shelterwood harvests, the abundance of ground-nesting species often decreases temporarily. This is likely in part due to the temporary reduction in leaf litter during fires or harvests. For example, many studies report fewer Ovenbirds after high-severity burns or harvests that substantially reduce canopy cover.
“Across a landscape and over time, there will always be places that are better for some species but worse for others,” says Greenberg.
The good news is that forests are dynamic. Trees regrow, leaf litter re-accumulates as leaves drop from trees each fall, and habitats change. Landscape-scale species diversity is a product of this shifting mosaic of temporal and spatial heterogeneity.
For more information, email Katie Greenberg at firstname.lastname@example.org.