The Wallow Fire began with an abandoned campfire on the Apache-Sitgreaves National Forests in Arizona’s White Mountains on May 29, 2011. By the time it was controlled 40 days later, it had become the largest wildfire in the state’s history. Flames blazed across 538,000 acres that range from high-country grasslands to the giant pine forests favored by bats. And bats, like most other wildlife, will likely face more and more charred habitat in the years to come. Thanks to decades of fire suppression and livestock grazing, plus the stirrings of climate change, wildfires are becoming bigger and more frequent throughout the American West.
Our field crew, a half-dozen biologists – plus 50 volunteers from Virginia to California who stepped up to help for a week – spent an intense and arduous summer within the boundaries of that immense fire last summer as part of a study into how bats adapt to a burned-over landscape. We captured bats in mist nets over ponds, attached tiny radio transmitters to reproductive females and tracked them back to often-surprising maternity roosts. We call our research project, a collaboration of Northern Arizona University and the National Forests, “Bats in the Burns,” and we hope to expand into other wildfire-burned forests in the Southwest.
Our preliminary evidence suggests that, not surprisingly, bats prefer unburned areas for travel, foraging and drinking. Roost selection was a different story: bats of some species chose roosts in completely charred tree trunks, including some surrounded by burned-over forests.
The forests of the White Mountains range from short-statured piñon pine and juniper woodlands around 5,000 feet (1,500 meters) elevation to subalpine meadows above 9,000 feet (2,750 meters). In between are forests of tall ponderosa pine, quaking aspen, and Douglas-fir trees. During summers, the White Mountains are green, cool and lush with scattered ponds, lakes and streams. At least 10 bat species spend their summers here, roosting in live trees and the dead trees known as snags. Many of them gather by species into maternity colonies to give birth and raise pups.
Previous research has found that bats typically use snags of more than two feet (60 centimeters) in diameter. They roost in vertical cracks in the snags, but will also wedge themselves under patches of loose bark that can house anywhere from one bat to hundreds, depending on the species of bat and the size of the sheltering bark. More than 900 Arizona myotis (Myotis occultus) were once counted as they emerged from a single snag.
Wildfires, meanwhile, have been part of forest ecosystems of the southwestern United States for centuries. Until the mid-1800s, lightning-caused fires burned through the ponderosa pine forests every 2 to 20 years. The low flames of those fires burned grasses and shrubs, but moved too fast to kill large pine trees with their thick, fire-resistant bark. That changed when Euro-Americans arrived. Livestock grazing eliminated much of the understory vegetation that had maintained low-intensity fires in the past. Plus, these new settlers considered such fires destructive and eventually began to extinguish them quickly.
Then, in the early twentieth century following a bumper seed crop and a wet year, millions of pine seedlings germinated and, without low-intensity fires to kill many of the tiny seedlings, tree densities increased from tens to thousands per acre. And these now-dense forests are facing yet another stressor in the form of changing climate. The unusually dry summers and winters that the Southwest is now experiencing have changed the way fires burn in forests. Tall flames now reach forest canopies and incinerate whole trees and snags. The decades of accumulated needles and forest litter smolder on the ground, killing old pine trees that would usually survive the fast-moving, pre-settlement fires. Today’s forest fires can be so hot they create their own weather and wind patterns: a virtual firestorm. In addition, humans are now one of the leading causes of fires.
The Wallow Fire scorched or incinerated many existing bat-friendly snags. Although new snags were created from trees killed by fire, many were smaller than the size preferred by bats. So the question becomes: would bats accept or reject these blackened snags?
To find out, we captured bats at 20 livestock ponds. Not all the area burned, so we split our efforts among ponds in areas of high severity (at least 75 percent of surrounding landscape burned) or low (25 percent or less). Despite some rainy nights, between mid-June and the end of July, we captured more than 650 bats of 13 species, including the uncommon Allen’s big-eared bat (Idionycteris phyllotis). The long-legged myotis (Myotis volans) was the most common capture, accounting for 25 percent of the total. Arizona myotis, long-eared myotis (M. evotis), silver-haired bats (Lasionycteris noctivagans) and big brown bats (Eptesicus fuscus) rounded out the top five, which represented 83 percent of our captures for the summer.
With long days of driving over rough, rocky and muddy roads, plus rugged hikes into forested ravines, we tracked our radiotagged females back to their roosts. We also occasionally resorted to telemetry flights to locate roosts from the air. In all, we found 19 roosts, including one snag that was shared by an Arizona myotis and a long-legged myotis colony, each of which used a different part of the snag.
More than half the roosts (58 percent) were large ponderosa pine snags, while 21 percent were Douglas-fir, 16 percent quaking aspen and 5 percent white fir. The pine snags averaged 24 inches (62 centimeters) in diameter and the Douglas-fir snags were 17 inches (43 centimeters). The average height of the roost snags was 80 feet (24 meters).
Most of the bats roosted in unburned snags, and bats were mostly captured while foraging and drinking at ponds in habitat relatively untouched by fire. The Arizona myotis and long-legged myotis roosted in unburned snags surrounded by unburned forest. However, four individuals of three species (long-eared myotis, fringed myotis [M. thysanodes] and Allen’s big-eared bat) used snags that were completely charred – picture a huge, black toothpick. And big brown bats, long-eared myotis, fringed myotis and the single Allen’s big-eared bat roosted in the midst of burned-out forest. What causes these species to choose burned or unburned areas for roosting? Perhaps thermal properties of roosts at these high elevations are important. We hope to find out more next summer, when we will be back in the White Mountains to hunt down still more roosts.
This project has been full of surprises, not the least of which is that so many people are willing to volunteer to work at night in remote and challenging terrain. And we were amazed at how bats choose and use roosts in this wildfire-burned area. We were astonished when 70 bats emerged from a completely charred pine snag. We found species segregating the use of snags based on the severity of fire damage in the surrounding landscape. That bats can bear and raise pups at elevations above 8,000 feet (2,400 meters) in such cold temperatures shows how unique and tough these little animals can be.
We will continue our investigation next summer to expand our initial results into how bats are using the Wallow Fire zone. And we hope in the future to explore the remnants of large fires in Arizona and New Mexico. Given the certainty of climate change, it is imperative that we learn how this complex assemblage of bats in the Southwest responds to this transformed habitat.
CAROL CHAMBERS is a Professor of Wildlife Ecology and ERIN SAUNDERS is a Master of Science Candidate in the School of Forestry at Northern Arizona University in Flagstaff.