DOI: 10.5061/dryad.0gb5mkkxv
title: Data from: Arctic and boreal paleofire records reveal drivers of fire activity and departures from Holocene variability

description: 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. 6-4 thousand years before present (yr BP). Biomass burning
 also increased during warm periods, particularly in the Yukon Flats
 ecoregion from ca. 1000-500 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 21-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
 years or more.
description: All 27 lake-sediment charcoal records were developed with
 virtually identical methods, originally published by: Higuera et al. 2007,
 2009, 2011 a/b, Kelly et al. 2013 and Barrett et al. 2013. Sediment cores
 were collected from small (&lt; 10 ha), deep (&gt; 5 m) lakes with
 simple basin shapes and minimal inlets or outlets, using a polycarbonate
 tube fitted with a piston and/or a modified Livingstone coring device.
 Core tops were sampled in the field at continuous 0.5-1.0 cm intervals,
 and in the laboratory, cores were sampled in contiguous 0.25-0.5 cm
 intervals, with 0.5-3.0 cm<sup>3</sup> samples taken for
 charcoal analysis. Samples were treated with sodium metaphosphate,
 oxidized with sodium hypochlorite or hydrogen peroxide, and sieved to
 isolate macroscopic charcoal (&gt; 150-180 um). Charcoal particles
 were counted at 10-40x with a stereomicroscope, and CHAR (pieces
 cm<sup>-2</sup> year<sup>-1</sup>) was derived as
 the product of charcoal concentrations (pieces
 cm<sup>-3</sup>) and sediment accumulation rates (cm
 yr<sup>-1</sup>). Sediment accumulation rates were based on
 age-depth models, developed from <sup>210</sup>Pb (in 26 of 27
 records) and AMS <sup>14</sup>C (in all records) dating of
 terrestrial macrofossils or concentrated charcoal particles. All records
 underwent a decomposition and peak analysis procedure by their original
 authors using the CharAnalysis software
 (github.com/phiguera/CharAnalysis), which separates background (“noise”)
 from foreground (“signal”) patterns in CHAR to identify statistically
 significant peaks. We made no changes to the chronologies or peak
 identification analyses from the original publications. We excluded
 (portions of) records with a signal-to-noise index &lt; 3 from metrics
 that rely on peak identification (Kelly et al. 2011).
 To interpret changes in Holocene fire activity in the context of
 millennial-scale vegetation change, we rely on four previously published
 pollen records from each ecoregion (Tinner et al. 2006, Higuera et al.
 2009, 2011a, Kelly et al. 2013). To evaluate potential
 relationships between fire activity and millennial-scale climatic change,
 we rely on a recent summary of paleoclimate in northwestern North America
 (Kaufman et al. 2016) spanning the entire Holocene, and a tree-ring based
 air temperature reconstruction from 1200 yr BP to present (Wiles et al.
 2014). The composite temperature reconstructions from Kaufman <i>et
 al. </i>(2016), includes midge-inferred July air temperature (Clegg
 et al. 2011, Irvine et al. 2012), pollen-inferred air temperature (Szeicz
 et al. 1995, Bunbury and Gajewski 2009, Viau and Gajewski 2009), and
 temperature anomalies from other proxies (McKay et al. 2008).
description: See detailed description of data and analyses here:
 https://doi.org/10.5281/zenodo.3736520.