Prescribed fire history affects pollinator diversity in southern forests

Pollinators, such as bees in the sweat bee family (Halictidae), play an important role in pollinating flowers like this tickseed (Coreopsis spp.). Photo by Michael Weatherford, used with permission.

Landscapes with diverse fire histories – or pyrodiverse landscapes – have higher diversity of pollinators, as a recent study by USDA Forest Service scientist Michael Ulyshen shows. The study was published in Frontiers in Ecology and the Environment.

Tall Timbers Research Station, the study location, is nestled in the Red Hills Ecoregion of the Coastal Plain in northern Florida and southern Georgia. Coastal plain pine forests are adapted to high fire frequency, where the fire return interval is typically between one and three years. Tall Timbers has a long history of fire research and is sometimes referred to as the birthplace of prescribed fire science.

“Tall Timbers was an ideal place for the study, because each burn block had a detailed burn history. Knowing the burn history is half the battle with studies like these. That and the guidance of Kevin Hiers, a fire scientist at Tall Timbers, made the study possible,” says Ulyshen.

As pyrodiversity increased, so did the number of bee and butterfly species. Pollinator abundance also increased. The team collected 3,735 bees in 70 families and 371 butterflies from 30 species. Areas that were burned most frequently, however, had lower counts of bee and butterfly species.

“Overall, burn diversity does benefit the diversity of pollinators across the landscape. But burning frequently over large areas can be hard on some species. Butterflies seem especially sensitive to burn frequency, probably because many species are exposed to fire on their host plants,” says Ulyshen. “Having a patchwork of different burn histories may help maintain populations of the most sensitive species.”

In addition to burning small blocks at different times, conditions on the landscape, like soil moisture or fuel type, naturally result in a variety of burn severities including patches of unburned fuel. “It’s not typical for huge areas to burn uniformly. There is always some heterogeneity, and these small patches of unburned area can help bees feed themselves and their young. It is important to recognize these refuge areas for both bees and butterflies.”

Annually burned areas (left side of photo) at Tall Timbers are more open than areas that burn every three years (right side). USDA Forest Service photo by Michael Ulyshen.

Another study published in Science of the Total Environment showed bee diversity was higher in fire-managed longleaf pine savannas than in pine plantations or unburned hardwood stands.

Ulyshen also led a separate study published in Insect Conservation and Diversity. That study showed that ground nesting bee abundance and diversity were higher in burned plots than unburned plots. Fire appears to improve nesting habitat for ground nesting bees. Specific species responses, however, differed between plots burned every year, every other year, and every three years. No bee species favored unburned plots.

“Different bee species responded differently to burn frequency. Coastal Plain pine forests are fire adapted, so we would expect the bee community to also be fire adapted. But it is a relatively unexplored area of science, and we learn a little more with each study,” says Ulyshen.

Pollinator populations, and especially bees, are in decline globally. This loss has serious implications for our agricultural systems and natural areas. Understanding how forest management affects pollinator populations will help us conserve species and, hopefully, increase the abundance of pollinators in the future.

For more information, email Michael Ulyshen at michael.d.ulyshen@usda.gov.

Find the latest publications by SRS scientists.

Subscribe to our newsletter!

Receive weekly updates