Patterns of Ecosystem Structure and Wildfire Carbon Combustion Across Six Ecoregions of the North American Boreal Forest

Abstract

Increases in fire frequency, extent, and severity are expected to strongly impact the structure and function of boreal forest ecosystems. An important function of the boreal forest is its ability to sequester and store carbon (C). Increasing disturbance from wildfires, emitting large amounts of C to the atmosphere, may create a positive feedback to climate warming. Variation in ecosystem structure and function throughout the boreal forest is important for predicting the effects of climate warming and changing fire regimes on C dynamics. In this study, we compiled data on soil characteristics, stand structure, pre-fire C pools, C loss from fire, and the potential drivers of these C metrics from 527 sites distributed across six ecoregions of North America's western boreal forests. We assessed structural and functional differences between these fire-prone ecoregions using data from 417 recently burned sites (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) and estimated ecoregion-specific relationships between soil characteristics and depth from 167 of these sites plus an additional 110 sites (27 burned, 83 unburned). We found that northern boreal ecoregions were generally older, stored and emitted proportionally more belowground than aboveground C, and exhibited lower rates of C accumulation over time than southern ecoregions. We present ecoregion-specific estimates of depth-wise soil characteristics that are important for predicting C combustion from fire. As climate continues to warm and disturbance from wildfires increases, the C dynamics of these fire-prone ecoregions are likely to change with significant implications for the global C cycle and its feedbacks to climate change.

Interactions among wildfire, forest type and landscape position are key determinants of boreal forest carbon stocks

“…Thinner organic layers tend to be warmer following fire (Harden et al, 2006) and can cause permafrost thaw for decades (Pastick et al, 2017; Yoshikawa et al, 2002) and accelerate decomposition of deep, old organic matter, resulting in substantial CO 2 losses (Estop‐Aragonés et al, 2018; Gibson et al, 2019). Moreover, as fire return intervals shorten (Buma et al, 2022), the threat to deeper soil C pools is much greater in dry sites (Walker et al, 2020) or areas with thinner organic layers (Hoy et al, 2016), even in response to low severity fires, which could result in significant atmospheric CO 2 emissions.…”

“…Baltzer et al (2021) found that the majority of black spruce stands surveyed in boreal North America (62%) were able to regenerate following recent fire, but concluded that this resilience may be challenged by future moisture deficits. Fine‐scale factors such as pre‐fire stand density and fuels (Walker et al, 2020, 2021), soil drainage (Whitman et al, 2018), and the timing of fire ignition (Kasischke & Hoy, 2012; Turetsky et al, 2011) affect organic layer combustion and fire severity in boreal forests. Remote sensing tools capable of capturing drivers of within‐fire variation in fire severity (e.g.…”

“…Fire severity, frequency and size have increased in recent decades (Coops et al, 2018;Kasischke et al, 2010) which could result in substantial direct transfer of long-held "legacy C" from deep organic layers to the atmosphere (Walker et al, 2019) and may lead to the redistribution of C to aboveground pools in severely burned sites that transition to deciduous dominance (Mack et al, 2021). Fire severity is often mediated by forest type, stand density, topography, weather and the timing of fire (Kasischke & Hoy, 2012;Turetsky et al, 2011;Veraverbeke et al, 2015;Walker et al, 2020Walker et al, , 2021Whitman et al, 2018), creating a mosaic of altered ecosystem properties that can vary substantially within a single fire perimeter (Duffy et al, 2007;Ferster et al, 2016;Kasischke & Hoy, 2012;Kolden et al, 2012;Lavoie & Mack, 2012) and complicating predictions of biome-wide responses to wildfire.…”

Climatic variation drives loss and restructuring of carbon and nitrogen in boreal forest wildfire

“…TEM was given as integer values ranging from 0 to 240 (in order of increasing moisture potential) and was based on the soil topographic wetness index (Buchanan et al, 2014) in areas where soil type information was available and on the two topographic indices depth to water (Murphy et al, 2007) and the topographic wetness index (Beven and Kirkby, 1979) where soil information was unavailable. These measures gave an estimate of soil drainage, which can be predictive of long-term soil moisture patterns (Walker et al, 2020b). Elevation data were provided by the Swedish Mapping, Cadastral and Land Registration Authority from a 50 m resolution digital elevation model (Lantmäteriet, 2021).…”

Impacts of pre-fire conifer density and wildfire severity on ecosystem structure and function at the forest-tundra ecotone

“…In boreal black spruce forests C emissions are highest in intermediately drained landscape positions because these areas accumulate deep SOLs and are also sufficiently dry for combustion [7,17]. Field based estimates of tundra fire severity are rare, but pre-fire C pools and C emissions per unit area reported for a tundra wildfire in Alaska [16] are at the high and low range, respectively of those reported for boreal black spruce forests [23,24]. As such, the severity of wildfires and C emissions might decrease in association with decreasing tree cover and increased SOL across the forest-tundra ecotone.…”