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

Abstract

Wildfires in forest ecosystems produce landscape mosaics that include relatively unaffected areas, termed fire refugia. These patches of persistent forest cover can support fire-sensitive species and the biotic legacies important for post-fire forest recovery, yet little is known about their abundance and distribution within fire perimeters. Readily accessible 30-m resolution satellite imagery and derived burn severity products are commonly employed to characterize post-fire landscapes; however, coarse image resolution, generalized burn severity thresholds, and other limitations can constrain accurate representation of fire refugia. This study quantifies the abundance and pattern of fire refugia within 10 fires occurring in ponderosa pine and dry mixed-conifer forests between 2000 and 2003. We developed high-resolution maps of post-fire landscapes using semi-automated, object-based classification of 1-m aerial imagery, conducted imagery- and field-based accuracy assessments, and contrasted these with Landsat-derived burn severity metrics. Fire refugia area within burn perimeters ranged from 20% to 57%. Refugia proportion generally decreased with increasing Landsat-derived burn severity, but still accounted for 3–12% of areas classified as high severity. Patch size ranged from 1-m2 isolated trees to nearly 8000 ha, and median patch size was 0.01 ha—substantially smaller than a 30-m Landsat pixel. Patch size was negatively related to burn severity; distance to fire refugia from open areas was positively related to burn severity. Finally, optimized thresholds of 30-m post-fire normalized burn ratio (NBR) and relative differenced normalized burn ratio (RdNBR) delineated fire refugia with an accuracy of 77% when validated against the 1-m resolution maps. Estimations of fire refugia abundance based on Landsat-derived burn severity metrics are unlikely to detect small, isolated fire refugia patches. Finer-resolution maps can improve understanding of the distribution of forest legacies and inform post-fire management activities including reforestation and treatments.

Still standing: Recent patterns of post-fire conifer refugia in ponderosa pine-dominated forests of the Colorado Front Range

“…RdNBR is more consistent in classifying burn severity in areas with low forest cover and has identified fire refugia in other regions [34,78]. Our analysis corroborates similar recent research in ponderosa pine and dry-mixed conifer forests of the western US demonstrating the high predictive power of RdNBR and fine scale imagery to detect fire refugia [84]. Overall, our findings support the use of RdNBR products to identify presence of post-fire conifer refugia in ponderosa pine dominated forests.…”

Influence of topography and fuels on fire refugia probability under varying fire weather conditions in forests of the Pacific Northwest, USA

“…Nevertheless, Landsat-based RdNBR mapping is valuable as a relative indicator of fire-induced change across numerous fire events spanning heterogeneous conditions, particularly when interpreted in the context of field-measured fire effects such as tree mortality (Reilly et al 2017;Chapman et al 2020). We recognize that the fire refugia threshold of 10% BA mortality is subjective, and future studies could test other refugia thresholds or leverage additional spectral information in Landsat imagery (Meddens et al 2016;Collins et al 2019), as well as finer-resolution satellite and aerial imagery (Walker et al 2019;Chapman et al 2020). Future studies could also integrate field observations to distinguish low-severity from truly unburned refugia (Meddens et al 2016) and quantify the distinctive composition and structure in old forests, particularly in the West Cascades where large, fireresistant Douglas-fir trees are prevalent.…”

“…Recent studies have mapped fire refugia patterns and quantified drivers with large geospatial databases and innovative quantitative approaches. Mapping studies have typically employed Landsat imagery to identify fire refugia within recent fire events as locations including both unburned and low-severity fire effects where fire resulted in low mortality to dominant trees (e.g., Krawchuk et al 2016;Meddens et al 2016;Haire et al 2017;Meigs and Krawchuk 2018;Collins et al 2019;Walker et al 2019;Chapman et al 2020). Functionally, these remote sensing approaches identify fire refugia as locations exhibiting minimal spectral change relative to the broader burn mosaic.…”

Natural disturbance regimes for implementation of ecological forestry: a review and case study from Nova Scotia, Canada

“…Depending on severity of the disturbance, different age class structures will result (MacLean and Ostaff 1989;Kneeshaw et al 2011;Taylor et al 2013;Waldron et al 2013). A single, high severity disturbance event (e.g., intense crown fire or strong wind) will usually kill most trees in a stand (Foster and Boose 1992;Haeussler and Bergeron 2004;Bouchard et al 2009;Taylor et al 2017a;Walker et al 2019), therefore initiating a predominantly 'single-aged' stand (only one age class present) or 'multi-aged' (two to three distinct age classes) stand structure (Ashton and Kelty 2018). In contrast, repeated episodes of less severe disturbances (e.g., low-to-moderate severity windstorms or insect outbreaks affecting only selected species, and/or having less impact), that kill fewer trees per disturbance event, will leave behind a greater abundance of standing live trees, and contribute to gap dynamics and/or development of more complex 'all-aged' stand structures in which four or more age classes of trees may coexist (Runkle 1991;Lorimer and Frelich 1994;McCarthy 2001).…”