Notes from the Field: Ecological Adaptation to Wildfire in Oregon
This 2020 wildfire season was certainly severe in Oregon, with over a million acres of affected forest and likely well up into the 100s of millions of dollars in property damage. Tragically, lives were lost, and many families suffered profound impact. Some of our most beautifully forested river basins on the west slope of the Cascades, including parts of the Clackamas, Santiam, and McKenzie, will be scarred by fire and may well suffer from landslides, erosion, and stream sedimentation for many years.
Perhaps we can begin to find some solace by remembering the natural resistance to and resilience from wildfire in Pacific Northwest ecosystems. Wildfire has always had a place in the fire-adapted ecosystems of Central Oregon’s arid forest, scrub steppe, and grassland. A more simplistic picture of ecological succession proceeding in a predictable, stepwise fashion from pioneer to climax seral stages has been largely succeeded itself by a more complex and dynamic concept including periodic or continual renewal or rejuvenation by the disturbance from wildfires. And, most wildfires don’t uniformly burn or will burn with variable severity across affected areas, ultimately resulting in a patchy distribution, or mosaic pattern, of habitat types across the forest, lending much to its resilience, biodiversity, and aesthetic quality.
Wildflowers after a wildfire. (Monica Turner, CC BY-ND)
Many big, mature, overstory conifers have thick, fire resistant bark and self-prune lower branches. Some produce closed “serotinous” cones, dependent on the heat from fires to stimulate opening and seed release. The soil enrichment with mineral and organic material, a newly reopened understory, and thermal scarification result in seed bank activation and the typically exuberant bursts of wildflowers and saplings following most fires. Other seeds and nuts are brought into fire-scarred areas by a wide range of dispersal mechanisms, including those borne by wind or fur, deposited in droppings or scat, buried or hoarded by squirrels or rodents, or cached by jays or woodpeckers.
Lewis's Woodpecker, Melanerpes lewis. (sdakotabirds.com)
Though different types of wildlife might flee, fly away, or burrow as a fire approaches, many perish. Afterwards wildlife populations become gradually re-established as the community of animal species undergoes succession in parallel with changes in the composition and structure of the plant community. Human disturbance is particularly problematic right after a burn; it can further impact fragile soils, exacerbate erosion, or facilitate the establishment of invasive weeds.
The burnt, dead, and slowly decaying trees, both fallen and those remaining upright as snags, are themselves important habitat for many species of vertebrates and invertebrates, often for many years following a fire. Insects and other arthropods quickly recolonize burnt forests, the wood soon rich in deposited eggs and developing larvae, and are followed by insectivorous wildlife. Woodpeckers, especially Black-backed and Lewis’s, will move in, probing for those bugs and excavating nest cavities in the burnt trees. This provides homes for secondary cavity nesters, both mammalian (like tree and flying squirrels) and avian (like chickadees, swallows, bluebirds, flycatchers, and kestrels).
With a remarkably short passage of time, further recovery from wildfire and changes in the plant and animal community become evident. A typically mosaic pattern of burn severity combined with other soil and hydrological variables as well as simply chance dispersal promote heterogeneity in community type across the affected landscape. Understory burns can result in open, parklike Ponderosa pine forests. Intermixed, in patches with sufficient moisture, Quaking aspen can sprout from intact root systems and grow up rapidly into a clone grove; aspen has been in decline across much of its range with fire suppression. Newly establishing meadows of mixed wildflowers and grasses can be especially productive as hummingbirds are attracted to nectar-rich forbs and shrubs and mule deer increase with the proliferation of herbaceous browse.
While fires roar across the landscape, directly impacting some communities in their paths and indirectly affecting much of the state with diminished air quality, and immediately afterwards when the scars are fresh, it is understandably difficult to appreciate the place of wildfire in the ecosystem. Longer term, over the coming years, it will be heartening to watch the quiet and fascinating progression of plant and animal community replacement and the ongoing revitalization of the land set, and constantly reset, in motion by wildfire.