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1. Impacts of pre-fire conifer density and wildfire severity on ecosystem structure and function at the forest-tundra ecotone

   https://doi.org/10.1371/journal.pone.0258558  

   Walker, Xanthe J.; Howard, Brain K.; Jean, Mélanie; Johnstone, Jill F.; Roland, Carl; Rogers, Brendan M.; Schuur, Edward A.; Solvik, Kylen K.; Mack, Michelle C. (October 2021, PLOS ONE)

   Hui, Dafeng (Ed.)

   Wildfire frequency and extent is increasing throughout the boreal forest-tundra ecotone as climate warms. Understanding the impacts of wildfire throughout this ecotone is required to make predictions of the rate and magnitude of changes in boreal-tundra landcover, its future flammability, and associated feedbacks to the global carbon (C) cycle and climate. We studied 48 sites spanning a gradient from tundra to low-density spruce stands that were burned in an extensive 2013 wildfire on the north slope of the Alaska Range in Denali National Park and Preserve, central Alaska. We assessed wildfire severity and C emissions, and determined the impacts of severity on understory vegetation composition, conifer tree recruitment, and active layer thickness (ALT). We also assessed conifer seed rain and used a seeding experiment to determine factors controlling post-fire tree regeneration. We found that an average of 2.18 ± 1.13 Kg C m -2 was emitted from this fire, almost 95% of which came from burning of the organic soil. On average, burn depth of the organic soil was 10.6 ± 4.5 cm and both burn depth and total C combusted increased with pre-fire conifer density. Sites with higher pre-fire conifer density were also located at warmer and drier landscape positions and associated with increased ALT post-fire, greater changes in pre- and post-fire understory vegetation communities, and higher post-fire boreal tree recruitment. Our seed rain observations and seeding experiment indicate that the recruitment potential of conifer trees is limited by seed availability in this forest-tundra ecotone. We conclude that the expected climate-induced forest infilling (i.e. increased density) at the forest-tundra ecotone could increase fire severity, but this infilling is unlikely to occur without increases in the availability of viable seed. 

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2. Patterns, drivers, and implications of postfire delayed tree mortality in temperate conifer forests of the western United States

   https://doi.org/10.1002/ecs2.4805  

   Busby, Sebastian; Evers, Cody; Holz, Andrés (April 2024, Ecosphere)

   Abstract Conifer forest resilience may be threatened by increasing wildfire activity and compound disturbances in western North America. Fire refugia enhance forest resilience, yet may decline over time due to delayed mortality—a process that remains poorly understood at landscape and regional scales. To address this uncertainty, we used high‐resolution satellite imagery (5‐m pixel) to map and quantify delayed mortality of conifer tree cover between 1 and 5 years postfire, across 30 large wildfires that burned within three montane ecoregions in the western United States. We used statistical models to explore the influence of burn severity, topography, soils, and climate moisture deficit on delayed mortality. We estimate that delayed mortality reduced live conifer tree cover by 5%–25% at the fire perimeter scale and 12%–15% at the ecoregion scale. Remotely sensed burn severity (1‐year postfire) was the strongest predictor of delayed mortality, indicating patch‐level fire effects are a strong proxy for fire injury severity among surviving trees that eventually perish. Delayed mortality rates were further influenced by long‐term average and short‐term postfire climate moisture deficits, illustrating the impact of drought on fire‐injured tree survival. Our work demonstrates that delayed mortality in conifer forests of the western United States can be remotely quantified at a fine grain and landscape scale, is a spatially extensive phenomenon, is driven by fire–climate–environment interactions, and has important ecological implications. 

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3. Extreme fire spread events burn more severely and homogenize post-fire landscapes in the southwestern US

   McFarland, Jessika; Coop, Jonathan; Balik, Jared; Rodman, Kyle; Parks, Sean; Stevens-Rumann, Camille (August 2024, Ecological Society of America Annual Meeting)

   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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4. Fire refugia are robust across Western US forested ecoregions, 1986–2021

   https://doi.org/10.1088/1748-9326/ad11bf  

   Platt, Rutherford Vance; Chapman, Teresa B.; Balch, Jennifer K. (December 2023, Environmental Research Letters)

   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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5. Sparse subalpine forest recovery pathways, plant communities, and carbon stocks 34 years after stand‐replacing fire

   https://doi.org/10.1002/ecm.1644  

   Kiel, Nathan_G; Mavencamp, Eileen_F; Turner, Monica_G (January 2025, Ecological Monographs)

   Abstract Changing global climate and wildfire regimes are threatening forest resilience (i.e., the ability to recover from disturbance). Yet distinguishing areas of “no” versus “slow” postfire forest recovery is challenging, and consequences of sparse tree regeneration for plant communities and carbon dynamics are uncertain. We studied previously forested areas where tree regeneration remained sparse 34 years after the large, stand‐replacing 1988 Yellowstone fires (Wyoming, USA) to ask the following questions: (1) What are the recovery pathways in areas of sparse and reduced forest recovery and how are they distributed across the landscape? (2) What explains variation in postfire tree regeneration density (total and by species) among sparse recovery pathways? (3) What are the implications of sparse recovery for understory plant communities? (4) How diminished are aboveground carbon stocks in areas of sparse postfire forest recovery? Tree densities and species‐specific age distributions, understory plant communities, and carbon stocks were sampled in 55 plots during summer 2022. We detected three qualitatively distinct sparse recovery pathways (persistent sparse or non‐forest, continuous tree infilling, and recent seedling and sapling establishment). Nearly half of the plots appeared “locked in” as persistently sparse or non‐forest, while the remaining may be on a slow path to forest recovery. Plots with nearby upwind seed sources as well as in situ seed pressure from young postfire trees appear likely to recover to forest. Where trees were sparse or absent, plant communities resembled those found in meadows, capturing compositional changes expected to become more common with continued forest loss. However, forest‐affinity species persisted in mesic locations, indicating mismatches between some plant communities and future forest change. Aboveground carbon stocks were low owing to minimal tree reestablishment. Almost all (96%) carbon was stored in coarse wood, a sharp departure from C storage patterns where forests are recovering. If not offset by future tree regeneration, decomposition of dead biomass will protract postfire aboveground carbon stock recovery. As global disturbance regimes and climate continue to change, determining the drivers of ecosystem reorganization and understanding how such changes will cascade to influence ecosystem structure and function will be increasingly important. 

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