Abstract
BackgroundIn the Inland Pacific Northwest of the United States, fire is a dominant driver of ecological change. Within wildfire perimeters, fire effects often vary considerably and typically include remnant patches of unburned islands. As fires reburn the landscape, some unburned islands remain persistently unburned. These persistent unburned islands can serve an important ecological function as fire refugia; however, their characteristics have not been quantified. The objective of this study was to assess the characteristics of persistent unburned islands and compare them to the burned areas that surround them. Using an existing database of unburned islands from 1984 to 2014, overlapping unburned islands were delineated. We sampled points in both persistent unburned islands and in areas burned by wildfire. At these sample points, we derived several topographical and other geospatial metrics, and we compared the characteristics of these groups. Because the study area covers many ecosystems, we stratified the analysis by different fire regime groups.ResultsOur analysis revealed that persistent unburned islands are not randomly distributed across the landscape. While the topography and vegetation fuel type that underlie persistent unburned islands differ from burned areas, these differences are dependent upon fire regime group and are less pronounced than what other studies have found. The topographic features that differed the most between persistent unburned islands and burned areas were terrain ruggedness, slope, and transformed aspect. We also found that, as unburned islands increased in persistence (i.e., remained unburned for an increasing number of overlapping fires), they decreased in size and shape complexity.ConclusionsOur research shows that the biophysical setting underlying persistent unburned islands differs between forests and rangelands, and also differs from burned areas, which has potential applications for fire refugia prediction and management. Characterizing fire refugia and understanding the processes that contribute to their creation and maintenance will be important for land management as climate changes and increasingly large areas are affected by wildfire.
Highlights
The northwestern United States has experienced a considerable increase in fire activity due to anthropogenic climate change, largely due to summertime drying and warming conditions (Abatzoglou and Williams 2016)
The study area was covered by 35% forest, 42% rangeland, with the remaining 23% including water, Datasets The datasets used for this study included a recently developed unburned island database (Meddens et al 2018a), a Landsat-based land cover type dataset (US Geological Survey 2011), topographical indices derived from a digital elevation model, and fire regime groups (FRG; Barrett et al 2010; LANDFIRE 2011a; see Fig. 1b) and fuel models (Anderson 1982; LANDFIRE 2011b) acquired from LANDFIRE
We presented the proportion of area of both unburned islands and persistent unburned islands
Summary
The northwestern United States has experienced a considerable increase in fire activity due to anthropogenic climate change, largely due to summertime drying and warming conditions (Abatzoglou and Williams 2016) This trend is expected to continue, yielding increased frequency of megafires (Barbero et al 2015). There is great concern for a loss of forest resilience associated with these fires leading to land cover transitions and loss of biodiversity and ecosystem services (Vaillant et al 2016; Stevens-Rumann et al 2018); this is magnified for areas where repeat wildfires alter forest recovery trajectories (Stevens-Rumann and Morgan 2016) Within these repeat fire scars, there are unburned islands that, through multiple fires, have escaped or perhaps resisted burning. Because the study area covers many ecosystems, we stratified the analysis by different fire regime groups
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