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1. Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration

   https://doi.org/10.1073/pnas.1815107116  

   Davis, Kimberley T.; Dobrowski, Solomon Z.; Higuera, Philip E.; Holden, Zachary A.; Veblen, Thomas T.; Rother, Monica T.; Parks, Sean A.; Sala, Anna; Maneta, Marco P. (March 2019, Proceedings of the National Academy of Sciences)

   Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant, low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States. 

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2. Interactions Between Fire Refugia and Climate-Environment Conditions Determine Mesic Subalpine Forest Recovery After Large and Severe Wildfires

   https://doi.org/10.3389/ffgc.2022.890893  

   Busby, Sebastian U.; Holz, Andrés (May 2022, Frontiers in Forests and Global Change)

   Infrequent stand-replacing wildfires are characteristic of mesic and/or cool conifer forests in western North America, where forest recovery within high-severity burn patch interiors can be slow, yet successful over long temporal periods (decades to centuries). Increasing fire frequency and high-severity burn patch size, under a warming climate, however, may challenge post-fire forest recovery, promoting landscape-level shifts in forest structure, composition, and distribution of non-forest patches. Crucial to a delay and/or impediment to this shift, fire refugia (i.e., remnant seed sources) may determine forest recovery trajectories and potential forest state-transitions. To examine how fire refugia attributes (i.e. extent, composition, and structure) interact with local climate and environmental conditions to determine post-fire forest recovery responses, we developed fine-grain maps of fire refugia via remote sensing and conducted field-based assessment of post-fire conifer tree establishment largely originating (i.e., dispersed) from fire refugium in the Central Cascade Range of the Pacific Northwest United States. We found that limitations on seed availability, represented by the distance 2 -weighted density (D 2 WD) of fine-grain refugia extent, largely explained post-fire tree establishment responses within our relatively mesic and cool subalpine study sites. Interactions between seed availability, climate, and environmental conditions indicated that the structural attributes of refugia (e.g., tree height) and site abiotic/biotic environmental controls (e.g., climate water deficit, canopy cover, and coarse woody debris cover) interplayed to constrain or enhance species-specific tree establishment responses. Importantly, these interactions illustrate that when seed availability is critically low for a given area, climate-environment conditions may strongly determine whether forests recover following fire(s). Toward modelling and predicting tree establishment responses and potential forest state-transitions after large stand-replacing fires(s), our study demonstrates the importance of accurately quantifying seed availability via the fine-grain extent, configuration, and attributes of remnant seed source legacies. 

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3. 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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4. Wildfire impacts on forest microclimate vary with biophysical context

   https://doi.org/10.1002/ecs2.3467  

   Wolf, K. D.; Higuera, P.E; Davis, K.T.; Dobrowski, S.Z. (January 2021, Ecosphere)

   Increasing wildfire activity in western North America has the potential to remove forest canopy cover over large areas, increasing the vulnerability of understory plants and juvenile trees to micro- climatic extremes. To understand the impacts of wildfire on forest microclimatic buffering, we monitored daily temperature and vapor pressure deficit (VPD) in 33 plots over the first two growing seasons follow- ing two wildfires from 2017. The Lolo Peak and Sunrise fires occurred during a regionally extensive fire season, burning mixed-conifer and subalpine forests across complex mountainous topography in western Montana. Sensors were deployed from June to September in 2018 and 2019 in sites stratified by aspect, ele- vation, and fire severity (unburned, moderate, high) to capture a range of forest types, biophysical con- texts, and fire effects. Loss of canopy and understory vegetation had marked effects on microclimate: On average, sites burned at high severity had 3.7°C higher daily maximum temperatures and 0.81 kPa higher daily maximum VPD relative to paired unburned sites. Differences between burned and unburned sites were most pronounced when ambient temperatures were high, across diurnal and seasonal time scales. Differences were also more pronounced at sites with less canopy cover, more bare ground postfire, and greater long-term water availability (i.e., low climatic water deficit). Our results reveal fire-caused changes in microclimate extremes that are biologically meaningful for the postfire establishment of tree seedlings and understory vegetation. These effects depend strongly on biophysical context, with cool-wet forests more vulnerable to fire-caused changes in microclimate compared with warm-dry settings. Our results fur- ther highlight the functional importance of standing dead trees for moderating surface temperature in postfire environments. Anticipating forest ecosystem responses to increased warming and wildfire activity, and the potential for fire to catalyze vegetation changes, thus requires considering the substantial impacts of fire on microclimate. 

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5. Vulnerability of northern rocky mountain forests under future drought, fire, and harvest

   https://doi.org/10.3389/ffgc.2023.1146033  

   Stenzel, Jeffrey E.; Kolden, Crystal A.; Buotte, Polly C.; Bartowitz, Kristina J.; Walsh, Eric W.; Hudiburg, Tara W. (September 2023, Frontiers in Forests and Global Change)

   Novel climate and disturbance regimes in the 21st century threaten to increase the vulnerability of some western U.S. forests to loss of biomass and function. However, the timing and magnitude of forest vulnerabilities are uncertain and will be highly variable across the complex biophysical landscape of the region. Assessing future forest trajectories and potential management impacts under novel conditions requires place-specific and mechanistic model projections. Stakeholders in the high-carbon density forests of the northern U.S. Rocky Mountains (NRM) currently seek to understand and mitigate climate risks to these diverse conifer forests, which experienced profound 20th century disturbance from the 1910 “Big Burn” and timber harvest. Present forest management plan revisions consider approaches including increases in timber harvest that are intended to shift species compositions and increase forest stress tolerance. We utilize CLM-FATES, a dynamic vegetation model (DVM) coupled to an Earth Systems Model (ESM), to model shifting NRM forest carbon stocks and cover, production, and disturbance through 2100 under unprecedented climate and management. Across all 21st century scenarios, domain forest C-stocks and canopy cover face decline after 2090 due to the interaction of intermittent drought and fire mortality with declining Net Primary Production (NPP) and post-disturbance recovery. However, mid-century increases in forest vulnerability to fire and drought impacts are not consistently projected across climate models due to increases in precipitation that buffer warming impacts. Under all climate scenarios, increased harvest regimes diminish forest carbon stocks and increase period mortality over business-as-usual, despite some late-century reductions in forest stress. Results indicate that existing forest carbon stocks and functions are moderately persistent and that increased near-term removals may be mistimed for effectively increasing resilience. 

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