US forest fires are getting too hot, even for fire-adapted animals
Imagine the coniferous forests of the American West and you probably picture tall trees, forest floors littered with pine needles and pinecones, black bears, mountain lions, croaking frogs and mountain blue birds. Natural forest fires should also be a part of that picture. Over millions of years, the forests of western North America have become adapted to routine burning, caused by lightning strikes, allowing forests to retain a variety of old, middle-aged, and young plants that support myriad species of animals.
Many of the animals that make their homes in those forests depend on or at least can tolerate the disturbance that comes from the blazes. Take the black-backed woodpecker. For decades, it has been held up as a prime example of a species that requires burned areas to survive. It mainly feeds on the larvae of beetles that colonise dying and dead trees after wildfires, and it excavates cavities in dead trees for its nests.
And while this species does indeed rely heavily on burned areas, it turns out what it really needs is pyrodiversity, a mosaic of burned and unburned areas. In a study published on 6 August 2019 in the journal The Condor, researchers studied how habitats affected the woodpeckers’ choice of nest sites. They observed that black-backed woodpeckers prefer nesting along edges of burned patches, within about 0,3 miles of areas with living, unburned trees.
"We're finding that pyrodiversity is a really important component of wildlife habitat after a fire," says the study’s lead author, Andrew Stillman, a graduate student in ecology and evolutionary biology at the University of Connecticut. Although forest fires are becoming more common, pyrodiversity is declining because fires are burning hotter and larger than they used to, resulting in a more homogenised burn zone. Stillman says that understanding the ecological impacts of these new types of forest fires is important for better supporting animals' recovery after fires and helping land managers balance competing demands from wildlife and humans.
Hotter, faster fires
Patterns of forest burning have changed considerably in recent years. In addition to natural fires, forests also must contend with fires started by humans, both intentionally and accidentally. And thanks to over a century of fire suppression practices, many forests now contain massive reserves of unburned fuel. That means that when fires do occur, they tend to burn hotter and longer and they spread faster.
"You'll be hard-pressed to find any scientist who says fire suppression, the way we've done it, is a good thing," says University of Florida wildlife ecologist Gavin Jones, who was not involved in the study. Letting forest fires to burn allows for natural forest regeneration, providing habitat for species like black-backed woodpeckers and mule deer, which like to forage on new growth following a fire.
Add in prolonged drought and longer, hotter summers, both consequences of climate change and it’s a recipe for a very different sort of fire than the one to which the ecosystem has adapted. Even for a fire-adapted species like the black-backed woodpecker, these new fires may become too frequent and too severe, though for now, at least, nestling survival doesn't seem to be severely impacted, according to the study.
The researchers hypothesise that by nesting near the edges of burned forest, black-backed woodpeckers may be ensuring that they can keep tabs on their offspring after they fledge.
The parents stick to burned areas because they can find the beetle larvae they love to eat on dead, burned wood. "But the fledglings, as soon as they leave the nest, move straight into areas of low [burn] severity," Stillman says. He suspects the extra cover offered by dense forest canopies helps them avoid predation by raptors during one of the most vulnerable periods in their lives.
Jones says that these findings "add some critical nuance to the understanding of what black-backed woodpecker habitat is and [offer] the potential for some resolution about what we should be doing in post-fire landscapes to achieve multiple, sometimes competing objectives."
Indeed, Stillman points out that land managers can use this information to balance the needs of fire-adapted species, including black-backed woodpeckers, with post-fire activities like salvage logging.
Source: National Geographic