Rather than hibernating in caves, some bat species in the southeastern U.S. get through the coldest parts of winter by roosting under fallen leaves, twigs, and other dead plant material on the forest floor. Although this leaf litter protects bats from the cold, it could also put them at risk of being injured or killed by prescribed fires.
“Prescribed fires are often conducted in winter, which minimizes the risk to ground-nesting birds, reptiles, and other animals,” says Roger Perry, a U.S. Forest Service Southern Research Station (SRS) scientist. “However, burn crews frequently observe bats being flushed from the leaf litter during winter burning, and these observations led us to ponder the potential effects of winter burning on litter-roosting bats.”
A study led by Perry, a research wildlife biologist at the SRS Southern Pine Ecology unit, and published in the International Journal of Wildland Fire, offers some recommendations for managers conducting winter prescribed fires in areas where bat species roost in forest floor litter.
Litter-roosting bats enter torpor, which means that their body temperatures drop and their metabolism slows. Bats may be in shallow or deep torpor, and the colder it is, the deeper the torpor. “Bats in deep torpor take longer to arouse and respond to potential dangers,” says Perry. “However, bats in shallow torpor are able to escape more quickly from approaching flames.”
Perry and his colleagues established 64 study plots in the Ouachita Mountains of Arkansas. Personnel from the Ouachita National Forest conducted the prescribed fires, which were generally low intensity. Researchers measured temperatures below the leaf litter just before and during the prescribed fires. Across all the plots, the average maximum temperature during burning was 558o F and ranged from 50o to 1323o F.
Although scientists are not sure exactly what temperatures are lethal to bats, Perry and his colleagues estimated that bats could survive temperatures as high as 140o F as long as these temperatures lasted less than a minute. Based on this estimate, if bats were unable to escape approaching flames, they would have survived in only 5 percent of the study plots.
Just before the arrival of fire, the average temperature below leaf litter was 67o F. “Apparently, leaf litter warms up pretty well on clear afternoons during winter, and these warmer temperatures keep the bats in shallow torpor during this time,” says Perry. “Studies suggest that fire cues, such as smoke, may cause these bats to arouse from torpor, and these warmer temperatures allow bats to arouse quickly and escape.”
Because bats in deep torpor would take longer to arouse, they could potentially be killed in the fire. Therefore, conducting prescribed fires during the warmer periods of winter and during afternoons, when bats are likely to be in shallow torpor, could promote bat survival while still improving wildlife habitat, consuming hazardous fuels, and providing the many other benefits of prescribed fire.
For more information, email Roger Perry at firstname.lastname@example.org.