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1. Arctic and boreal paleofire records reveal drivers of fire activity and departures from Holocene variability

   https://doi.org/10.1002/ecy.3096  

   Hoecker, Tyler J.; Higuera, Philip E.; Kelly, Ryan; Hu, Feng Sheng (June 2020, Ecology)

   Abstract Boreal forest and tundra biomes are key components of the Earth system because the mobilization of large carbon stocks and changes in energy balance could act as positive feedbacks to ongoing climate change. In Alaska, wildfire is a primary driver of ecosystem structure and function, and a key mechanism coupling high‐latitude ecosystems to global climate. Paleoecological records reveal sensitivity of fire regimes to climatic and vegetation change over centennial–millennial time scales, highlighting increased burning concurrent with warming or elevated landscape flammability. To quantify spatiotemporal patterns in fire‐regime variability, we synthesized 27 published sediment‐charcoal records from four Alaskan ecoregions, and compared patterns to paleoclimate and paleovegetation records. Biomass burning and fire frequency increased significantly in boreal forest ecoregions with the expansion of black spruce, ca. 6,000–4,000 years before present (yr BP). Biomass burning also increased during warm periods, particularly in the Yukon Flats ecoregion from ca. 1,000 to 500 yr BP. Increases in biomass burning concurrent with constant fire return intervals suggest increases in average fire severity (i.e., more biomass burning per fire) during warm periods. Results also indicate increases in biomass burning over the last century across much of Alaska that exceed Holocene maxima, providing important context for ongoing change. Our analysis documents the sensitivity of fire activity to broad‐scale environmental change, including climate warming and biome‐scale shifts in vegetation. The lack of widespread, prolonged fire synchrony suggests regional heterogeneity limited simultaneous fire‐regime change across our study areas during the Holocene. This finding implies broad‐scale resilience of the boreal forest to extensive fire activity, but does not preclude novel responses to 21st‐century changes. If projected increases in fire activity over the 21st century are realized, they would be unprecedented in the context of the last 8,000 yr or more. 

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2. Global response of fire activity to late Quaternary grazer extinctions

   https://doi.org/10.1126/science.abj1580  

   Karp, Allison T.; Faith, J. Tyler; Marlon, Jennifer R.; Staver, A. Carla (November 2021, Science)

   Fire activity varies substantially at global scales because of the influence of climate, but at broad spatiotemporal scales, the possible effects of herbivory on fire activity are unknown. Here, we used late Quaternary large-bodied herbivore extinctions as a global exclusion experiment to examine the responses of grassy ecosystem paleofire activity (through charcoal proxies) to continental differences in extinction severity. Grassy ecosystem fire activity increased in response to herbivore extinction, with larger increases on continents that suffered the largest losses of grazers; browser declines had no such effect. These shifts suggest that herbivory can have Earth system–scale effects on fire and that herbivore impacts should be explicitly considered when predicting changes in past and future global fire activity. 

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3. Emerging palaeoecological frameworks for elucidating plant dynamics in response to fire and other disturbance

   https://doi.org/10.1111/geb.13416  

   Napier, Joseph D.; Chipman, Melissa L. (October 2021, Global Ecology and Biogeography)

   Abstract MotivationRapid climate change is altering plant communities around the globe fundamentally. Despite progress in understanding how plants respond to these climate shifts, accumulating evidence suggests that disturbance could not only modify expected plant responses but, in some cases, have larger impacts on compositional shifts than climate change. Climate‐driven disturbances are becoming increasingly common in many biomes and are key drivers of vegetation dynamics at both species and community levels. Palaeoecological records provide valuable observational windows for elucidating the long‐term impacts of these disturbances on plant dynamics; however, sparse resolution and difficulty in disentangling drivers of change limit our ability to understand the impact of disturbance on plant communities. In this targeted review, we highlight emerging opportunities in palaeoecology to advance our understanding about how disturbance, especially fire, impacts the ecological and evolutionary dynamics of terrestrial plant communities. LocationGlobal examples, with many from North America. ConclusionsWe propose a set of palaeoecological and integrative approaches that could greatly enhance our understanding of how disturbance regimes influence global plant dynamics. Specifically, we identify four future study areas: (1) focus on palaeoecological disturbance proxies beyond fire and leverage multi proxy research to examine the influence of interacting disturbances on plant community dynamics; (2) use advances in disturbance and vegetation reconstructions, including ancient sedimentary DNA, to provide the spatial, temporal and taxonomic resolution needed to resolve the relationship between changing disturbance regimes and corresponding shifts in plant community composition; (3) integrate palaeoecological, archaeological and Indigenous knowledge to disentangle the complex interplay between climate, human land use, fire and vegetation structure; and (4) apply “functional palaeoecology” and the synergy between palaeoecology and genetics to understand how fire disturbance has served as a long‐standing selective agent on plants. These frameworks could increase the resolution of disturbance‐driven plant dynamics, potentially providing valuable information for future management. 

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4. Century-Scale Fire Dynamics in a Savanna Ecosystem.

   Leys, Bérangère A.; Griffin, Daniel; Larson, Evan R.; McLauchlan, Kendra K. (September 2019, Fire)

   (1) Background: Frequent fire, climate variability, and human activities collectively influence savanna ecosystems. The relative role of these three factors likely varies on interannual, decadal, and centennial timescales. Here, we tested if Euro-American activities uncoupled drought and fire frequencies relative to previous centuries in a temperate savanna site. (2) Methods: We combined records of fire frequency from tree ring fire scars and sediment charcoal abundance, and a record of fuel type based on charcoal particle morphometry to reconstruct centennial scale shifts in fire frequency and fuel sources in a savanna ecosystem. We also tested the climate influence on fire occurrence with an independently derived tree-ring reconstruction of drought. We contextualized these data with historical records of human activity. (3) Results: Tree fire scars revealed eight fire events from 1822–1924 CE, followed by localized suppression. Charcoal signals highlight 13 fire episodes from 1696–2001. Fire–climate coupling was not clearly evident both before and after Euro American settlement The dominant fuel source shifted from herbaceous to woody fuel during the early-mid 20th century. (4) Conclusions: Euro-American settlement and landscape fragmentation disrupted the pre-settlement fire regime (fire frequency and fuel sources). Our results highlight the potential for improved insight by synthesizing interpretation of multiple paleofire proxies, especially in fire regimes with mixed fuel sources. 

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5. 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. (October 2023, Journal of Ecology)

   Abstract 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.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.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.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.Synthesis. 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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