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  1. Abstract

    Wildfire is an essential earth‐system process, impacting ecosystem processes and the carbon cycle. Forest fires are becoming more frequent and severe, yet gaps exist in the modeling of fire on vegetation and carbon dynamics. Strategies for reducing carbon dioxide (CO2) emissions from wildfires include increasing tree harvest, largely based on the public assumption that fires burn live forests to the ground, despite observations indicating that less than 5% of mature tree biomass is actually consumed. This misconception is also reflected though excessive combustion of live trees in models. Here, we show that regional emissions estimates using widely implemented combustion coefficients are 59%–83% higher than emissions based on field observations. Using unique field datasets from before and after wildfires and an improved ecosystem model, we provide strong evidence that these large overestimates can be reduced by using realistic biomass combustion factors and by accurately quantifying biomass in standing dead trees that decompose over decades to centuries after fire (“snags”). Most model development focuses on area burned; our results reveal that accurately representing combustion is also essential for quantifying fire impacts on ecosystems. Using our improvements, we find that western US forest fires have emitted 851 ± 228 Tg CO2(~half of alternative estimates) over the last 17 years, which is minor compared to 16,200 Tg CO2from fossil fuels across the region.

     
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  2. 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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    Free, publicly-accessible full text available September 12, 2024
  3. Climate change has intensified the scale of global wildfire impacts in recent decades. In order to reduce fire impacts, management policies are being proposed in the western United States to lower fire risk that focus on harvesting trees, including large-diameter trees. Many policies already do not include diameter limits and some recent policies have proposed diameter increases in fuel reduction strategies. While the primary goal is fire risk reduction, these policies have been interpreted as strategies that can be used to save trees from being killed by fire, thus preventing carbon emissions and feedbacks to climate warming. This interpretation has already resulted in cutting down trees that likely would have survived fire, resulting in forest carbon losses that are greater than if a wildfire had occurred. To help policymakers and managers avoid these unintended carbon consequences and to present carbon emission sources in the same context, we calculate western United States forest fire carbon emissions and compare them with harvest and fossil fuel emissions (FFE) over the same timeframe. We find that forest fire carbon emissions are on average only 6% of anthropogenic FFE over the past decade. While wildfire occurrence and area burned have increased over the last three decades, per area fire emissions for extreme fire events are relatively constant. In contrast, harvest of mature trees releases a higher density of carbon emissions (e.g., per unit area) relative to wildfire (150–800%) because harvest causes a higher rate of tree mortality than wildfire. Our results show that increasing harvest of mature trees to save them from fire increases emissions rather than preventing them. Shown in context, our results demonstrate that reducing FFEs will do more for climate mitigation potential (and subsequent reduction of fire) than increasing extractive harvest to prevent fire emissions. On public lands, management aimed at less-intensive fuels reduction (such as removal of “ladder” fuels, i.e., shrubs and small-diameter trees) will help to balance reducing catastrophic fire and leave live mature trees on the landscape to continue carbon uptake. 
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  4. Climate change has intensified the scale of global wildfire impacts in recent decades. To help policymakers and managers avoid these unintended carbon consequences and to present carbon emission sources in the same context, we calculate western US forest fire carbon emissions and compare them with harvest and fossil fuel emissions over the same timeframe. We find that forest fire carbon emissions are on average only 6% of anthropogenic fossil fuel emissions (FFE) over the past decade. While wildfire occurrence and area burned have increased over the last three decades, per area fire emissions for extreme fire events are relatively constant. In contrast, harvest of mature trees releases a higher density of carbon emissions (e.g., per unit area) relative to wildfire (150-800%) because harvest causes a higher rate of tree mortality than wildfire. Shown in context, our results demonstrate that reducing FFEs will do more for climate mitigation potential (and subsequent reduction of fire) than increasing extractive harvest to prevent fire emissions. 
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  6. Forests store a large amount of terrestrial carbon, but this storage capacity is vulnerable to wildfire. Combustion, and subsequent tree mortality and soil erosion, can lead to increased carbon release and decreased carbon uptake. Previous work has shown that non-constant fire return intervals over the past 4000 years strongly shaped subalpine forest carbon trajectories. The extent to which fire-regime variability has impacted carbon trajectories in other subalpine forest types is unknown. Here, we explored the interactions between fire and carbon dynamics of 14 subalpine watersheds in Colorado, USA. We tested the impact of varying fire frequency over a ~2000 year period on ecosystem productivity and carbon storage using an improved biogeochemical model. High fire frequency simulations had overall lower carbon stocks across all sites compared to scenarios with lower fire frequencies, highlighting the importance of fire-frequency in determining ecosystem carbon storage. Additionally, variability in fire-free periods strongly influenced carbon trajectories across all the sites. Biogeochemical trajectories (e.g., increasing or decreasing total ecosystem carbon and carbon-to-nitrogen (C:N) ratios) did not vary among forest types but there were trends that they may vary by elevation. Lower-elevations sites had lower overall soil C:N ratios, potentially because of higher fire frequencies reducing carbon inputs more than nitrogen losses over time. Additional measurements of ecosystem response to fire-regime variability will be essential for improving estimates of carbon dynamics from Earth system models. 
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  7. Abstract. Wildfire is a dominant disturbance agent in forest ecosystems, shaping important biogeochemical processes including net carbon (C) balance. Long-term monitoring and chronosequence studies highlight a resilience of biogeochemical properties to large, stand-replacing, high-severity fire events. In contrast, the consequences of repeated fires or temporal variability in a fire regime (e.g., the characteristic timing or severity of fire) are largely unknown, yet theory suggests that such variability could strongly influence forest C trajectories (i.e., future states or directions) for millennia. Here we combine a 4500-year paleoecological record of fire activity with ecosystem modeling to investigate how fire-regime variability impacts soil C and net ecosystem carbon balance. We found that C trajectories in a paleo-informed scenario differed significantly from an equilibrium scenario (with a constant fire return interval), largely due to variability in the timing and severity of past fires. Paleo-informed scenarios contained multi-century periods of positive and negative net ecosystem C balance, with magnitudes significantly larger than observed under the equilibrium scenario. Further, this variability created legacies in soil C trajectories that lasted for millennia. Our results imply that fire-regime variability is a major driver of C trajectories in stand-replacing fire regimes. Predicting carbon balance in these systems, therefore, will depend strongly on the ability of ecosystem models to represent a realistic range of fire-regime variability over the past several centuries to millennia.

     
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