Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Abstract

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire-climate-vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.Canada | drought code | global circulation model | paleoclimate R ecent increases in wildfire frequency and biomass burning in boreal regions in response to ongoing climate warming threaten the carbon sink strength of native ecosystems and, by extension, further contribute to global warming (1). Up-to-date model-based fire predictions indicate that these trends will persist in the coming decades as atmospheric greenhouse gas concentrations will attain unprecedented levels by the end of this century (2, 3). However, model-based fire predictions depend on data collected over short periods-usually less than 100 y-that do not cover a wide range of fire-climate interactions and feedback processes arising from changes in vegetation features. This reduces the robustness of fire predictions, which must therefore be supplemented by paleoecological investigations (4). These investigations often integrate several scientific disciplines, datasets, approaches, and methodologies, thereby providing a robust assessment of how recent trends in fire activity fit into the long-term perspective.Until now, paleofire reconstructions based on charcoal lacustrine deposits have mostly focused on describing past fire activity in terms of frequency and biomass burning (5). Here we address an additional aspect of fire history and fire-climate relationships, namely, the change in fire size over periods of substantial climate change. To do this, we use sedimentary charcoal records extracted from nine kettle lakes located in the eastern North American boreal forest and model simulations of past climate. We introduce a new metric, developed from the combination of the fire frequency and biomass burning components, which allows us to assess the mean biomass burned per fi...

Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years

“…Charcoal analysis of lake sediments is widely used to quantify fire frequency and the relative amount of biomass burning through time (14,15). Although this approach is generally supported theoretically (16) and empirically (17,18), data remain limited for direct verification. Our study is particularly well-suited to address this limitation, as our sampling design provides unusually high spatiotemporal resolution (14 sites within an ∼1,000-km 2 region and median near-surface resolution of ∼5 y/sample).…”

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…Advection of humid air masses over eastern Canada between 1940 and 1970 contributed to the creation of moister conditions, which can lessen the capacity of a fire to spread after a lightning-induced fire ignition (Macias Fauria and Johnson, 2008). Both interannual variation and unsynchronized trends in climatic variables may have brought about changes in fire activity and could have affected the fire season, as it is proposed to have occurred over millennial timescales during the Holocene (Ali et al, 2012). For example, during the years 1977 and 1980, an increase in spring temperatures was observed, whereas spring precipitation decreased, which resulted in the total areas that were burned in spring being 50 % greater than in summer (Fig.…”

“…Warmer springs and winters that lead to an earlier start of the fire season are anticipated, together with an increase in the frequency of extreme drought years due to more frequent and persistent high-pressure blocking systems (Girardin and Mudelsee, 2008). These phenomena are expected to lead to an increase in the frequency and size of fires in eastern boreal Canada in response to the on-going global warming (Ali et al, 2012). Effects of these changes in seasonal onset and dryness are such that the average size of spring wildfires could be multiplied by a factor of 3 for each additional 1 • C of warming (Ali et al, 2012;Girardin et al, 2013a;Price et al, 2013).…”

“…These phenomena are expected to lead to an increase in the frequency and size of fires in eastern boreal Canada in response to the on-going global warming (Ali et al, 2012). Effects of these changes in seasonal onset and dryness are such that the average size of spring wildfires could be multiplied by a factor of 3 for each additional 1 • C of warming (Ali et al, 2012;Girardin et al, 2013a;Price et al, 2013). An increase in area burned would affect both forest management plans and fire suppression strategies.…”

Resistance of the boreal forest to high burn rates

“…Models suggest this biome will experience rapid temperature increases during the 21st century, with a potential 30-500% increase in burn rates (6)(7)(8)(9). Both fire size and the frequency of large fire years are expected to increase (8,10,11), with a cascading effect on ecosystem dynamics (12)(13)(14)(15)(16) and carbon storage (17)(18)(19)(20)(21).…”