Wildfires have memories

wildfire spread mosaic behavior — Photo from the research described below.

Researchers studying how wildfires have burned at a particular location found that subsequent fires have a “memory” that helped to self-regulate fire sizes and fire severity. When historical fires burned unabated, landscape patterns of surface and canopy fuels developed that provided barriers to future fire spread. Those same barriers can continue to influence the spread of additional fires.

Susan Prichard and Paul Hessburg were the principal investigators on the “Reburn Project” for the Joint Fire Science Program. They developed a paper, “Evaluating the influence of prior burn mosaics on subsequent wildfire behavior, severity, and fire management options”.

Here are some additional highlights from their findings:

The Reburn Project was motivated by a need to better understand wildfires as fuel reduction treatments and to assess the impacts of decades of wildland fire suppression activities on forested landscapes. The study examined three areas, located in the inland Pacific Northwest, central Idaho and interior British Columbia. Each area had experienced a recent large wildfire event in montane forests.

Reburn Highlights

Past wildfires generally mitigate burn severity for a time, even under extreme fire weather conditions that are associated with large fires.
Since around 1900-1934, fire suppression and not wildfire has been a primary influence on forest and fuel succession. Quantifying the effects of fire suppression on particular landscapes is difficult given the long history and its prevalence across the region. Results from simulation modeling have the potential to illustrate in compelling ways the combined effects of removing fires from landscapes that experienced variable fire severity and spatial extent.
The researchers developed a state-transition model that allowed them to simulate the growth and potential severity 20th century ignitions that were suppressed. Fire growth simulations were modeled using the daily meteorology available at the time of the ignitions and the FSPro model. The researchers found that the simulated landscapes were reburn landscapes; i.e., the complexity of forest seral stage conditions and fuelbeds was an emergent property of successive reburning over the centuries, and fuel succession explained most of the severity patterns observed.
Modern-day fire suppression scenarios led to “boom and bust” landscapes, where continuous mature forests developed, that were capable of supporting large fire spread, and were eventually burned with mostly high severity. However, using a variety of historical landscape conditions as an initial basis, in scenarios where most or all fires were allowed to burn, fine- to meso-grained patchworks resulted, and they provided a highly diverse range of habitats and values over time, and landscapes were much less susceptible to large, high severity events. Instead, more typical fire size distributions and more characteristic variation in fire severity were restored.

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