Recent fire activity in the boreal eastern interior of North America is below that of the past 2000 yr

Abstract

Abstract. The North American boreal forest has been developing since the end of the last glaciation approximately 10,000 yr ago. With climate warming and human occupation, it is anticipated that fire danger, ignition, and activity will be increasing, compromising forests' benefits for generations to come. In this study, we show, however, that a century of rapid climate changes and human densification has had the opposite effect in the boreal eastern interior of the North American continent, reducing biomass burning to values below two millennia of historical levels. A multi-millennial fire history was reconstructed for eight forested landscapes from the Lake of the Woods Ecoregion (LWE) located at the boreal-prairie ecotone. Fire history was reconstructed using a combination of archival (period 1920-2010), tree-ring (stand initiations and fire scars: period 1690-2010), and lake sediment charcoal (2500 BP to present) records. The archival record revealed recent large fires (>200 ha) in 1948, 1980, and 1988. An additional 19 fires were identified by the fire-scar record. Fire events in 1805, 1840, 1863, and the 1890s were identified in numerous locations around multiple lakes suggesting that they were of large extents. In accordance with the tree-ring record, the charcoal accumulation rate (CHAR) peak record generally identified the major fires but tended to lag from the tree-ring records by several decades. Within LWE, the long-term charcoal record revealed that CHAR was higher for each lake in the earlier portion of the record including the warm Medieval Climate Anomaly (AD À900 to AD 1000), followed by a progressive decrease toward the cool Little Ice Age period. This decline was abruptly interrupted in the mid-to late 19th century with large synchronized fires, also reported over western and central North America, and resumed approximately four decades later. Fire disturbance level is today below the historical range, despite the accentuated climate warming. Aging of the forest landscape may create biodiversity loss notably in fire-adapted species while at the same time setting the tone for major fires in upcoming decades if no action is taken for managing fuels.

Guidelines for the use and interpretation of palaeofire reconstructions based on various archives and proxies

“…S1). Once again, tree-ring analyses are shown to be a robust proxy to reconstruct fire histories at decadal to centennial time scales (Drobyshev et al, 2017;Swetnam et al, 1999;Waito et al, 2018).…”

“…Due to their age, tree-ring analyses, together with written archives, satellite imagery and aerial photographs, depending on availability, remain the most accurate methods to reconstruct local fire histories in the recent past (generally < 1000 years) (Higuera et al, 2011;Oris et al, 2014a;Swetnam et al, 1999;Waito et al, 2018). When fire dynamics must be investigated at longer, multimillennial time scales, the choice between archives and proxies depends on the study objectives and on the spatiotemporal scale considered (Fig.…”

“…In line with previous studies (Ali et al, 2009;Kelly et al, 2013;Lynch et al, 2004;Oris et al, 2014a), we recommend to use charcoal records from lake deposits processed with CharAnalysis (Table S4) for regional-scale fire history reconstructions. Indeed, lake-sediment charcoal analyses allow to reconstruct the longest fire histories and a large number of studies have validated their use to reconstruct long-term variations of fire regimes associated with past climate, vegetation dynamics or anthropogenic activities (Blarquez et al, 2015;Markgraf et al, 2007;Marlon et al, 2008;Millspaugh et al, 2000;Tinner et al, 2001;Waito et al, 2018). However, several lakes need to be analyzed to optimize the reconstructed regional trends in fire histories by minimizing the taphonomic biases specific to each lake (Ali et al, 2012(Ali et al, , 2009Blarquez et al, 2015;Calder et al, 2015;Clark and Royall, 1996 ., 2013;Mustaphi et al, 2015;Whitlock and Millspaugh, 1996).…”

Wildfire history of the boreal forest of southwestern Yakutia (Siberia) over the last two millennia documented by a lake-sedimentary charcoal record

“…More studies have been conducted in North America (e.g. Frégeau et al, 2015;Hély et al, 2010;Hoecker et al, 2020;Waito et al, 2018) and boreal Europe (e.g. Aakala et al, 2018;Feurdean et al, 2017;Molinari et al, 2020;Wallenius, 2011).…”

Coherent signature of warming-induced extreme sub-continental boreal wildfire activity 4800 and 1100 years BP

“…Lake-sediment charcoal records are used to reconstruct long-term fire trends and analyze the sensitivity of fire activity to various ecosystem and climatic processes, and can ultimately improve our ability to project future fire impacts (Heyerdahl et al 2008, Girardin et al 2013b, Waito et al 2018. Charcoal particles that are produced during fire are transported by wind and runoff across landscapes and deposited in lake sediments.…”