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Extreme single-day fire spread events are associated with warmer and drier fire seasons and are expected to increase in the future. However, our understanding of the post-fire landscape outcomes of such events is limited. Here, we ask whether extreme events burn more severely or produce landscape patterns that are less conducive to forest recovery. To identify extreme single-day fire spread events, we used satellite fire detections to create day of burning (DOB) maps for 633 fire events >400 ha within forested ecoregions of the southwestern United States. We categorized daily rates of spread as extreme (top 2.5% of events; >4901 ha/day) and non-extreme (bottom 97.5%, <4901 ha/day). We contrasted satellite-measured burn severity and a suite of high severity patch landscape metrics between extreme and non-extreme spread events. We found that extreme single-day fire spread events were associated with increased severity, a greater proportion of area burned severely, and a higher percentage of like adjacencies between high severity pixels. Average distances from high severity patch interiors to edges were also greater in extreme single-day spread events. Additionally, area-weighted mean patch size and total core area of high severity patches were higher for fires containing one or more extreme single-day spread events. Larger and more homogenous high-severity patches produced during extreme events have been shown to limit tree seedling establishment, inhibiting forest recovery and facilitating vegetation type conversion. These landscape outcomes are expected to be magnified under future climate as extreme fire spread becomes more prevalent, accelerating fire-driven forest loss across the southwestern US.
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Fire refugia are robust across Western US forested ecoregions, 1986–2021
Abstract In the Western US, area burned and fire size have increased due to the influences of climate change, long-term fire suppression leading to higher fuel loads, and increased ignitions. However, evidence is less conclusive about increases in fire severity within these growing wildfire extents. Fires burn unevenly across landscapes, leaving islands of unburned or less impacted areas, known as fire refugia. Fire refugia may enhance post-fire ecosystem function and biodiversity by providing refuge to species and functioning as seed sources after fires. In this study, we evaluated whether the proportion and pattern of fire refugia within fire events have changed over time and across ecoregions. To do so, we processed all available Landsat 4–9 satellite imagery to identify fire refugia within the boundaries of large wildfires (405 ha+) in 16 forested ecoregions of the Western US. We found a significant change in % refugia from 1986–2021 only in one ecoregion—% refugia increased within fires in the Arizona/New Mexico Mountain ecoregion (AZ/NM). Excluding AZ/NM, we found no significant change in % refugia across the study area. Furthermore, we found no significant change in mean refugia patch size, patch density, or mean distance to refugia. As fire size increased, the amount of refugia increased proportionally. Evidence suggests that fires in AZ/NM had a higher proportion of reburns and, unlike the 15 other ecoregions, fires did not occur at higher elevation or within greener areas. We suggest several possibilities for why, with the exception of AZ/NM, ecoregions did not experience a significant change in the proportion and pattern of refugia. In summary, while area burned has increased over the past four decades, there are substantial and consistent patterns of refugia that could support post-fire recovery dependent on their spatial patterns and ability to function as seeds sources for neighboring burned patches.
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- Award ID(s):
- 2017889
- PAR ID:
- 10479554
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Environmental Research Letters
- Volume:
- 19
- Issue:
- 1
- ISSN:
- 1748-9326
- Format(s):
- Medium: X Size: Article No. 014044
- Size(s):
- Article No. 014044
- Sponsoring Org:
- National Science Foundation
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