In the western United States, warmer and drier conditions have contributed to increases in large wildfire events in recent decades, a trend that’s expected to continue as the climate changes. A new, NW CASC-supported study led by Research Fellowship alum Michele Buonanduci, with University of Washington Associate Professor Brian Harvey and colleagues, describes an approach for anticipating the relationships between future fire sizes and burn severity patterns on a regional scale. The findings from this study can help scientists and managers better anticipate the ecological effects of future fire and plan management responses that promote fire-adapted landscapes.
Within individual wildfires, there is typically a range of burn severity. Areas of low- or moderate-severity fire burn less intensely and often leave behind surviving vegetation, while areas of high-severity fire burn more intensely and kill most or all the vegetation in an area.
The amount of forest experiencing high-severity fire is often strongly tied to the size of a given fire. Larger wildfires tend to contain bigger patches of high-severity fire, which leave behind burned areas that may be too far away from living trees for effective seed dispersal after fire. Because of this, larger wildfires tend to have a greater impact on forest structure than smaller wildfires, in which the patches of severely burned forest are smaller and closer to live seed sources, making forest recovery more rapid or likely.
These relationships between fire size and expected patterns of burn severity — referred to as spatial scaling relationships — prompt the question of how, at a regional scale, burn severity patterns will change as fire sizes shift under climate change. To answer this question, NW CASC-supported researchers set out to quantify the potential range of burn severity patterns that might be expected in a region, depending on the distribution of sizes of future fire events.
The research team first analyzed a satellite dataset of 1,615 fire events in the forested ecoregions of Wyoming, Montana, Idaho, Washington, Oregon and northern California between 1985-2020. Within this area, they focused on a case study in northwestern Cascadia, a forested region west of the Cascade Crest in Washington and northern Oregon, characterized by its wet and cool conditions and rich biomass. While historically, this area experienced long time periods between large and severe wildfires, increasing fire activity is expected to shape this area in the future as the climate warms.
Researchers first wanted to see if the spatial scaling relationships they modeled in northwestern Cascadia, an area with relatively limited wildfire data in contemporary satellite records, were consistent with the relationships they saw in the more data-rich parts of their study area. After comparing northwestern Cascadia and the broader study area, they found that spatial scaling relationships in northwestern Cascadia were comparable to other infrequent, high-severity fire regimes. This important finding means that fire records from data-limited regions could be supplemented by data from comparable fire regimes to understand scaling relationships and evaluate future fire effects.
The next step in this study was to use these spatial scaling relationships of burn severity, in combination with scenarios representing a range of future fire sizes, to simulate how regional burn severity patterns may vary depending on the number and size of individual fire events. Simply put, the researchers asked: If a cumulative total of one million hectares of forest were to burn, how would the patterns of burn severity likely differ if those million hectares burned in many small fires or few, large fires?
The simulations in this study found that, even if total area burned were to remain the same, a shift toward larger fire events will likely result in larger, high-severity patches with inside areas far from unburned seed sources. The increasing size and uniformity of high-severity fire patches may affect seed dispersal (depending on region-specific seed dispersal parameters), tree regeneration and young forest habitat, and the possible conversion to non-forest ecosystems. These findings have implications for both real-time fire management and post-fire management.
In areas such as the northwestern Cascadia example used in this study, one strategy for increasing forest resilience to fire and climate change would be to manage fire in a way that allows wildfires to reach a range of sizes, thus diversifying regional forest structure. Though this practice of modified fire suppression has the potential to provide ecological benefits, the high density of human populations and development in northwestern Cascadia limit the opportunities for this type of managed wildfire.
Another consideration for increasing forest resilience as the climate changes is to prioritize areas to replant following fire. In northwestern Cascadia, since the dominant species of trees depend on wind-driven seed dispersal for regeneration, planning post-fire replanting in burned areas that are likely to exceed seed dispersal distances will be important when maintaining forest cover is the management goal. The prospect of replanting huge swaths of severely burned forests sounds daunting, but this study also found some reassuring information. Based on the case study of northwestern Cascadia, this study found that even in the largest fire-size scenarios, around 85% of the total burned area is unlikely to require post-fire replanting. Only about 15% of the total burned area was outside of the likely dispersal distance of unburned, live trees.
While larger, high-severity fires have the potential to slow tree regeneration rates, they may also help restore ecologically and culturally important early-seral habitat. Early-seral ecosystems emerge after major disturbance events and are initially dominated by grasses and shrubs. They are high in biodiversity and structurally complex, providing habitat for animal and plant species, including huckleberry, a culturally important plant species for Tribes in the Northwest. Depending on the context, managers may decide not to intervene in severely-burned patches in an effort to promote early-seral habitat.
This study provides an approach for exploring and preparing for the range of possible ecological impacts of future fires. Since this approach is generalizable across different fire regimes, it can be applied across different regions to support decisions around stewarding our forests before and after wildfires.
Additional coauthors on this study include Dan Donato and Josh Halofsky of the Washington Department of Natural Resources and Maureen Kennedy, Associate Professor at the University of Washington – Tacoma.