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

Abstract

Wildfire frequency and extent is increasing throughout the boreal forest-tundra ecotone as climate warms. Understanding the impacts of wildfire throughout this ecotone is required to make predictions of the rate and magnitude of changes in boreal-tundra landcover, its future flammability, and associated feedbacks to the global carbon (C) cycle and climate. We studied 48 sites spanning a gradient from tundra to low-density spruce stands that were burned in an extensive 2013 wildfire on the north slope of the Alaska Range in Denali National Park and Preserve, central Alaska. We assessed wildfire severity and C emissions, and determined the impacts of severity on understory vegetation composition, conifer tree recruitment, and active layer thickness (ALT). We also assessed conifer seed rain and used a seeding experiment to determine factors controlling post-fire tree regeneration. We found that an average of 2.18 ± 1.13 Kg C m-2 was emitted from this fire, almost 95% of which came from burning of the organic soil. On average, burn depth of the organic soil was 10.6 ± 4.5 cm and both burn depth and total C combusted increased with pre-fire conifer density. Sites with higher pre-fire conifer density were also located at warmer and drier landscape positions and associated with increased ALT post-fire, greater changes in pre- and post-fire understory vegetation communities, and higher post-fire boreal tree recruitment. Our seed rain observations and seeding experiment indicate that the recruitment potential of conifer trees is limited by seed availability in this forest-tundra ecotone. We conclude that the expected climate-induced forest infilling (i.e. increased density) at the forest-tundra ecotone could increase fire severity, but this infilling is unlikely to occur without increases in the availability of viable seed.

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

“…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.…”

“…This analysis relies on the assumptions that (i) pre‐fire stands were conifer‐dominated, as pure deciduous stands rarely burn (Krawchuk et al, 2006), (ii) severe fires consume the soil organic layer and result in replacement by deciduous species and (iii) mild to moderate fires allow for conifer regeneration in a relatively intact soil organic layer. These assumptions are based on a broad set of previous studies that have illustrated the importance of fire severity on post‐fire stand composition (Baltzer et al, 2021; Boby et al, 2010; Johnstone et al, 2020; Johnstone, Chapin, et al, 2010; Johnstone, Hollingsworth, et al, 2010; Walker et al, 2021). Thus, plots classified as deciduous at the time of the inventory that intersected with fire polygons were assumed to have experienced severe fire, whereas plots classified as coniferous that intersected with fire polygons were assumed to have experienced low severity fire.…”

“…While black spruce forests cover approximately 39–44% of forested land in interior Alaska (Calef et al, 2005; van Cleve et al, 1983), estimates of soil C from the remaining forests are scarce. Given the potential for biome‐wide changes in community composition in response to severe fire and a warming climate (Johnstone, Hollingsworth, et al, 2010; Walker et al, 2021), quantifying live tree and soil C pools across forest types is an important step toward refining C stock estimates.…”

“…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.…”

Dynamic disequilibrium: Recent widespread increases in vegetation cover on subarctic floodplains of Interior Alaska

“…In northwestern North America, there has been particular interest in detecting changes to tree and tall shrub extent near forest–tundra boundaries along gradients of climate, latitude, and elevation (Brodie et al, 2019; Danby & Hik, 2007; Dial et al, 2007; Roland et al, 2016; Terskaia et al, 2020). Boreal forest wildfire and successional processes have also been the subject of numerous investigations in the subarctic (Baltzer et al, 2021; Kasischke et al, 2010; Mack et al, 2021; Walker et al, 2021). However, comparatively little is known about vegetation dynamics in subarctic riparian zones, despite their disproportionately high importance as “hotspots” of hydrological processes (Ploum et al, 2021), biogeochemical cycling (Blackburn et al, 2017), species diversity (Andersson et al, 2000; Johansson et al, 1996; Johnson & Almlöf, 2016; Nilsson & Svedmark, 2002), wildlife habitat use (Cooke & Tauzer, 2020), and ecological disturbance (Scrimgeour et al, 1994).…”