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1. Fire-regime variability impacts forest carbon dynamics for centuries to millennia

   https://doi.org/10.5194/bg-14-3873-2017  

   Hudiburg, Tara W.; Higuera, Philip E.; Hicke, Jeffrey A. (January 2017, Biogeosciences)

   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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2. Fire‐regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed

   https://doi.org/10.1111/1365-2745.14201  

   Clark‐Wolf, Kyra D.; Higuera, Philip E.; McLauchlan, Kendra K.; Shuman, Bryan N.; Parish, Meredith C. (December 2023, Journal of Ecology)

   1. Wildfires strongly influence forest ecosystem processes, including carbon and nutrient cycling, and vegetation dynamics. As fire activity increases under changing climate conditions, the ecological and biogeochemical resilience of many forest ecosystems remains unknown. 2. To investigate the resilience of forest ecosystems to changing climate and wildfire activity over decades to millennia, we developed a 4800‐year high‐resolution lake‐sediment record from Silver Lake, Montana, USA (47.360° N, 115.566° W). Charcoal particles, pollen grains, element concentrations and stable isotopes of C and N serve as proxies of past changes in fire, vegetation and ecosystem processes such as nitrogen cycling and soil erosion, within a small subalpine forest watershed. A published lake‐level history from Silver Lake provides a local record of palaeohydrology. 3. A trend towards increased effective moisture over the late Holocene coincided with a distinct shift in the pollen assemblage c. 1900 yr BP, resulting from increased subalpine conifer abundance. Fire activity, inferred from peaks in macroscopic charcoal, decreased significantly after 1900 yr BP, from one fire event every 126 yr (83–184 yr, 95% CI) from 4800 to 1900 yr BP, to one event every 223 yr (175–280 yr) from 1900 yr BP to present. 4. Across the record, individual fire events were followed by two distinct decadal‐scale biogeochemical responses, reflecting differences in ecosystem impacts of fires on watershed processes. These distinct biogeochemical responses were interpreted as reflecting fire severity, highlighting (i) erosion, likely from large or high‐severity fires, and (ii) nutrient transfers and enhanced within‐lake productivity, likely from lower severity or patchier fires. Biogeochemical and vegetation proxies returned to pre‐fire values within decades regardless of the nature of fire effects. 5. Palaeorecords of fire and ecosystem responses provide a novel view revealing past variability in fire effects, analogous to spatial variability in fire severity observed within contemporary wildfires. Overall, the palaeorecord highlights ecosystem resilience to fire across long‐term variability in climate and fire activity. Higher fire frequencies in past millennia relative to the 20th and 21st century suggest that northern Rocky Mountain subalpine ecosystems could remain resilient to future increases in fire activity, provided continued ecosystem recovery within decades. 

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3. A Meta‐Regression of 18 Wildfire Chronosequences Reveals Key Environmental Drivers and Knowledge Gaps in the Boreal Nitrogen Balance

   https://doi.org/10.1111/gcb.70398  

   Hupperts, Stefan F; Berninger, Frank; Chen, Han_Y H; Fenton, Nicole; Jean, Mélanie; Köster, Kajar; Larjavaara, Markku; Mack, Michelle C; Nilsson, Marie‐Charlotte; Palviainen, Marjo; et al (August 2025, Global Change Biology)

   ABSTRACT Climate change has increased the size and frequency of wildfires across the boreal biome. Severe wildfires in boreal forests have been found to trigger shifts from evergreen to deciduous canopies, which has cascading effects on carbon and nitrogen cycling. Ecosystem productivity and carbon uptake in boreal forests are strongly linked with nitrogen, and Earth system models increasingly depend on our understanding of the nitrogen balance to predict post‐fire carbon uptake. To investigate the post‐fire boreal nitrogen balance, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 boreal wildfire chronosequences that varied in both wildfire regime and post‐fire canopy type, comprising 527 forest stands. We found that deciduous‐ or mixed‐dominance boreal forests establishing after severe, stand‐replacing fires had the highest nitrogen accumulation rates (15.7 ± 3.8 kg ha⁻¹ year⁻¹), while evergreen‐dominated forests establishing after surface‐ or mixed‐severity fires had the lowest nitrogen accumulation rates (1.4 ± 1.1 kg ha⁻¹ year⁻¹). Annual known inputs from nitrogen deposition and biological nitrogen fixation combined, estimated from published data, largely failed to explain the rate of nitrogen accumulation, particularly in deciduous or mixed‐dominance forests establishing after stand‐replacing fires, suggesting that the origins of most nitrogen in these forest types remain poorly understood. As the frequency of severe wildfires increases across the boreal biome and shifts toward deciduous canopies become more common, our study reveals a large knowledge gap in the resulting nitrogen balance that needs to be resolved in order to improve predictions of forest carbon uptake. 

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4. North American boreal forests are a large carbon source due to wildfires from 1986 to 2016

   https://doi.org/10.1038/s41598-021-87343-3  

   Zhao, Bailu; Zhuang, Qianlai; Shurpali, Narasinha; Köster, Kajar; Berninger, Frank; Pumpanen, Jukka (December 2021, Scientific Reports)

   null (Ed.)

   Abstract Wildfires are a major disturbance to forest carbon (C) balance through both immediate combustion emissions and post-fire ecosystem dynamics. Here we used a process-based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to simulate C budget in Alaska and Canada during 1986–2016, as impacted by fire disturbances. We extracted the data of difference Normalized Burn Ratio (dNBR) for fires from Landsat TM/ETM imagery and estimated the proportion of vegetation and soil C combustion. We observed that the region was a C source of 2.74 Pg C during the 31-year period. The observed C loss, 57.1 Tg C year −1 , was attributed to fire emissions, overwhelming the net ecosystem production (1.9 Tg C year −1 ) in the region. Our simulated direct emissions for Alaska and Canada are within the range of field measurements and other model estimates. As burn severity increased, combustion emission tended to switch from vegetation origin towards soil origin. When dNBR is below 300, fires increase soil temperature and decrease soil moisture and thus, enhance soil respiration. However, the post-fire soil respiration decreases for moderate or high burn severity. The proportion of post-fire soil emission in total emissions increased with burn severity. Net nitrogen mineralization gradually recovered after fire, enhancing net primary production. Net ecosystem production recovered fast under higher burn severities. The impact of fire disturbance on the C balance of northern ecosystems and the associated uncertainties can be better characterized with long-term, prior-, during- and post-disturbance data across the geospatial spectrum. Our findings suggest that the regional source of carbon to the atmosphere will persist if the observed forest wildfire occurrence and severity continues into the future. 

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5. Post-Fire Carbon Dynamics in Subalpine Forests of the Rocky Mountains

   https://doi.org/10.3390/fire2040058  

   Bartowitz, Kristina J.; Higuera, Philip E.; Shuman, Bryan N.; McLauchlan, Kendra K.; Hudiburg, Tara W. (December 2019, Fire)

   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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