In forested landscapes across the western United States, the combined effects of fire suppression, logging, and grazing have altered forest structure and composition. These effects and recent climate-induced changes in fire regimes are resulting in uncharacteristically severe fire effects, dramatically affecting forested landscapes. In some cases, large fires have led to large-scale conversion of vegetation type (e.g., forest to non-forest), with potentially dire implications for existing biota and fundamental ecosystem processes [see previous Spotlight “Living on the edge: Trailing edge forests are at risk”].
Research suggests that fire refugia—places within high-severity burns that are unburned or that burn less severely—may help buffer forests from change and serve a key role in forest resilience. Not only do they provide a place for individual plants and animals to survive, but they harbor legacies (e.g., canopy shade and seed sources) that allow repopulation of surrounding areas and ecosystem recovery. Managers need to know how to identify fire refugia, understand their ecological functions, and make the best use of this knowledge to promote ecosystems that are resilient to fire.
Some fire refugia endure on the landscape for long time periods, remaining unburned even through multiple successive fires. These persistent fire refugia occur in places that are protected from fire such as topographically defined microclimates or in locations that are surrounded by nonflammable landscape features (e.g., lakes or exposed rock). Persistent refugia may be particularly important in retaining old-growth dependent communities, as well as rare species associated with microclimates not found in the surrounding burn mosaic. Other, more transient fire refugia are formed by chance coincidence with stochastic factors, such as variation in fuel continuity or unpredictable processes such as wind shifts during burning. Although less enduring, they nevertheless serve important ecosystem functions.
We developed methods to detect and map fire refugia and to predict their occurrence from topographic and weather information. To learn where fire refugia are most likely to occur, we used satellite-derived imagery for a set of large burns to develop predictive models based on a suite of topographic variables. We also developed methods using high-resolution (1-meter) aerial imagery so we could map smaller refugia.
To investigate the role of fire refugia in promoting tree regeneration, we conducted field studies of thirteen large burns in Oregon, Idaho, Colorado, Arizona, and New Mexico. We collected data on tree species composition and tree seedling abundances within fire refugia and in areas that burned at high severity. We asked how patterns of seed sources within fire refugia influence post-fire tree regeneration and developed a spatially explicit simulation model to explore how refugia patch size affects the rate and extent of forest recovery.
We expected distance to nearest surviving seed source would be important for post-fire conifer regeneration, but we also developed a continuous metric to describe the density of refugia within a 9-ha neighborhood. This density metric allowed us to consider the additive effect of isolated or scattered seed sources on regeneration.
Walker, RB; Coop, JD; Downing, WM; Krawchuk, MA; Malone, SL; Meigs, GW. 2019. How much forest persists through fire? High-resolution mapping of tree cover to characterize the abundance and spatial pattern of fire refugia across mosaics of burn severity. Forests 2019, 10, 782.