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1. Bottom-up drivers of future fire regimes in western boreal North America

   https://doi.org/10.1088/1748-9326/ac4c1e  

   Foster, Adrianna C ; Shuman, Jacquelyn K ; Rogers, Brendan M ; Walker, Xanthe J ; Mack, Michelle C ; Bourgeau-Chavez, Laura L ; Veraverbeke, Sander ; Goetz, Scott J ( January 2022 , Environmental Research Letters)

   Abstract Forest characteristics, structure, and dynamics within the North American boreal region are heavily influenced by wildfire intensity, severity, and frequency. Increasing temperatures are likely to result in drier conditions and longer fire seasons, potentially leading to more intense and frequent fires. However, an increase in deciduous forest cover is also predicted across the region, potentially decreasing flammability. In this study, we use an individual tree-based forest model to test bottom-up (i.e. fuels) vs top-down (i.e. climate) controls on fire activity and project future forest and wildfire dynamics. The University of Virginia Forest Model Enhanced is an individual tree-based forest model that has been successfully updated and validated within the North American boreal zone. We updated the model to better characterize fire ignition and behavior in relation to litter and fire weather conditions, allowing for further interactions between vegetation, soils, fire, and climate. Model output following updates showed good agreement with combustion observations at individual sites within boreal Alaska and western Canada. We then applied the updated model at sites within interior Alaska and the Northwest Territories to simulate wildfire and forest response to climate change under moderate (RCP 4.5) and extreme (RCP 8.5) scenarios. Results suggest that changing climate will act to decrease biomass and increase deciduous fraction in many regions of boreal North America. These changes are accompanied by decreases in fire probability and average fire intensity, despite fuel drying, indicating a negative feedback of fuel loading on wildfire. These simulations demonstrate the importance of dynamic fuels and dynamic vegetation in predicting future forest and wildfire conditions. The vegetation and wildfire changes predicted here have implications for large-scale changes in vegetation composition, biomass, and wildfire severity across boreal North America, potentially resulting in further feedbacks to regional and even global climate and carbon cycling. 

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2. Fuel Loads and Plant Traits as Community‐Level Predictors of Emergent Properties of Vulnerability and Resilience to a Changing Fire Regime in Black Spruce Forests of Boreal Alaska

   https://doi.org/10.1029/2021JG006696  

   Grzesik, E. J. ; Hollingsworth, T. N. ; Ruess, R. W. ; Turetsky, M. R. ( March 2022 , Journal of Geophysical Research: Biogeosciences)

   Black spruce forest communities in boreal Alaska have undergone self‐replacement succession following low‐to‐moderate severity fires for thousands of years. However, recent intensification of interior Alaska's fire regime, particularly deeper burning of the soil organic layer, is leading to shifts to deciduous‐dominated successional pathways, resulting in many socioecological consequences. Both fuel load quantity and quality (or “burnability”) influence black spruce plant communities' potential to burn. Even relatively low fuel loads, such as those seen in black spruce forest understory, can be highly influential drivers of fire behavior due to their high flammability. Additionally, black spruce community self‐replacement following fire can be largely attributed to the suite of functional and life history traits possessed by the species dominating these communities. We used fuel load (quantity and quality) and amount of within‐population plant trait variation (coefficient of variation; CV) as community‐level emergent properties to investigate black spruce forest vulnerability and resilience to a changing fire regime across the landscape. Our burn severity potential index (BSPI), calculated from fuel load quantity and quality measurements, indicates that drier, higher elevation stands with thicker active layers were the most vulnerable to fire‐induced vegetation shifts under a changing fire regime. Forest resilience to fire‐induced vegetation shift, represented by higher CV, was negatively associated with BSPI and greatest in ecoregions dominated by lowland black spruce forests. Together, these analyses provide critical information for determining the likelihood of stand‐replacing shifts in dominant vegetation following fire and for implementing appropriate ecosystem management practices.

    

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3. Effects of plant hydraulic traits on the flammability of live fine canopy fuels

   https://doi.org/10.1111/1365-2435.13771  

   Scarff, Fiona R. ; Lenz, Tanja ; Richards, Anna E. ; Zanne, Amy E. ; Wright, Ian J. ; Westoby, Mark ; McCulloh, ed., Katherine ( February 2021 , Functional Ecology)

   Plant species vary in how they regulate moisture and this has implications for their flammability during wildfires. We explored how fuel moisture is shaped by variation within five hydraulic traits: saturated moisture content, cell wall rigidity, cell solute potential, symplastic water fraction and tissue capacitance.

   Using pressure–volume curves, we measured these hydraulic traits in twigs and distal shoots (i.e. twigs + leaves) in 62 plant species across four wooded communities in south‐eastern Australia. Moisture content of fine fuels was then estimated for circumstances typical of fire weather. These projections were made assuming that under the hot, dry, windy conditions typical of large wildfires, leaves and fine twigs would function at internal water pressures close to wilting point (i.e. turgor loss point, TLP). The effect of different moisture contents at TLP on ignition time was then modelled using a fully mechanistic, finite element model of biomass ignition based on standard principles of physical chemistry.

   We also measured predawn water potential, an indication of plant access to soil water that is influenced by root architecture. These data were used to model how root traits influence fuel moisture and ignition time.

   Most variation among species in fuel moisture under fire weather conditions arose from differences in saturated moisture content (3.4‐ to 3.6‐fold variation). Twig capacitance was also an important driver of fuel moisture under these weather conditions (1.9‐ to 2.2‐fold variation in moisture content). A suite of other leaf and root traits influencing how much shoots dry out as they approach wilting point each contributed 1.0‐ to 1.6‐fold variation in projected fuel moisture during fire weather. Fuel moisture variation in turn drove variation in flammability by modifying predicted ignition time.

   Two main life‐history types in fire‐prone habitats are obligate seeders and resprouters. There were no significant differences between these species groups in estimated fuel moisture during fire weather, nor in any measured hydraulic traits.

   Live fuel moisture is an important determinant of wildfire activity. Our data show that variation in tissue saturated moisture content among co‐occurring species represents an important ecological store of variation in flammability in the study communities.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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4. Simulating dynamic fire regime and vegetation change in a warming Siberia

   https://doi.org/10.1186/s42408-023-00188-1  

   Williams, Neil G. ; Lucash, Melissa S. ; Ouellette, Marc R. ; Brussel, Thomas ; Gustafson, Eric J. ; Weiss, Shelby A. ; Sturtevant, Brian R. ; Schepaschenko, Dmitry G. ; Shvidenko, Anatoly Z. ( May 2023 , Fire Ecology)

   Climate change is expected to increase fire activity across the circumboreal zone, including central Siberia. However, few studies have quantitatively assessed potential changes in fire regime characteristics, or considered possible spatial variation in the magnitude of change. Moreover, while simulations indicate that changes in climate are likely to drive major shifts in Siberian vegetation, knowledge of future forest dynamics under the joint influence of changes in climate and fire regimes remains largely theoretical. We used the forest landscape model, LANDIS-II, with PnET-Succession and the BFOLDS fire extension to simulate changes in vegetation and fire regime characteristics under four alternative climate scenarios in three 10,000-km²study landscapes distributed across a large latitudinal gradient in lowland central Siberia. We evaluated vegetation change using the fire life history strategies adopted by forest tree species: fire resisters, fire avoiders, and fire endurers.

   Annual burned area, the number of fires per year, fire size, and fire intensity all increased under climate change. The relative increase in fire activity was greatest in the northernmost study landscape, leading to a reduction in the difference in fire rotation period between study landscapes. Although the number of fires per year increased progressively with the magnitude of climate change, mean fire size peaked under mild or moderate climate warming in each of our study landscapes, suggesting that fuel limitations and past fire perimeters will feed back to reduce individual fire extent under extreme warming, relative to less extreme warming scenarios. In the Southern and Mid-taiga landscapes, we observed a major shift from fire resister-dominated forests to forests dominated by broadleaved deciduous fire endurers (BetulaandPopulusgenera) under moderate and extreme climate warming scenarios, likely associated with the substantial increase in fire activity. These changes were accompanied by a major decrease in average cohort age and total vegetation biomass across the simulation landscapes.

   Our results imply that climate change will greatly increase fire activity and reduce spatial heterogeneity in fire regime characteristics across central Siberia. Potential ecological consequences include a widespread shift toward forests dominated by broadleaved deciduous species that employ a fire endurer strategy to persist in an increasingly fire-prone environment.

    

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5. Twenty‐five years of tree demography in a frequently burned oak woodland: implications for savanna restoration

   https://doi.org/10.1002/ecs2.3844  

   Davis, Mark A. ( December 2021 , Ecosphere)

   Due to decades of fire suppression, much of the Upper Midwest savanna habitat has converted to oak woodland. In efforts to restore oak savanna habitat, fire has been re‐introduced in many of these woodlands. A primary purpose of these burns is to kill the fire‐sensitive mesophytic tree species, which had established themselves during the decades of fire suppression, reduce the number of understory trees, and preserve the larger more widely spaced oaks. It is clear from ongoing efforts that restoring oak savannas will require frequent fires over decades, but frequent fires over the long term can also threaten the desirable oaks. Long‐term demographic studies at savanna restoration sites experiencing frequent fires are necessary to determine the extent to the frequent burns are supporting and/or confounding restoration goals. Results presented here are from a twenty‐five‐year demographic study of an Upper Midwest bur oak (Quercus macrocarpa) savanna/woodland experiencing frequent fire, during which both the survival and growth of more than 9000 trees were documented. Survival was assessed annually and growth every five years. In the face of frequent fires, stem survival was found to be strongly associated with tree species, stem size, and stem growth. In turn, stem growth was found to be related to tree species and stem size. Decades of frequent burning in this oak woodland have substantially reduced the abundance of unwanted trees, specifically mesophytic species andQuercus ellipsoidalis, the latter which outcompetesQ. macrocarpain the absence of fire. WhileQ. macrocarpamid‐sized (10–25 cm dbh) and large (≥25 cm dbh) trees are quite resistant to fire and now dominate the savanna landscape, they are not immune from fire‐induced mortality. It is recommended that the number and density of these trees should be re‐evaluated every few years to ensure that desirable numbers remain. If necessary, fires should be suspended for a period of time. This will give smallerQ. macrocarpatrees time to grow larger and become more fire‐resistant, thereby ensuring successive generations ofQ. macrocarpa.

    

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