Tussocks Enduring or Shrubs Greening: Alternate Responses to Changing Fire Regimes in the Noatak River Valley, Alaska

Abstract

Introduction The Implications for Changing Tundra Fire RegimesWildland fire is an important ecological disturbance in the tundra biome (Rocha et al., 2012), and increased burning will likely accelerate ecosystem responses to ongoing climate warming across certain regions of the biome (Hu et al., 2010;Landhausser & Wein, 1993;Racine et al., 2004). Rapid warming in the Arctic has resulted in permafrost thaw and the expansion of upright shrub communities in many regions (Martin et al., 2017;Smith et al., 2010;Tape et al., 2006), but it may take decades to centuries for landscape responses to be fully realized because ecological responses to changes in climate can take longer in regions like the Arctic where plants are dormant for much of the year (Chapin & Starfield, 1997). In addition, even the Arctic's relatively simple ecosystems have properties that can buffer them from climate warming, with the result that ecological responses can be delayed or muted (Folke et al., 2004;Loranty et al., 2018). The primary negative feedback that currently buffers tundra in the Low Arctic (<70°N) from climate changes is the widespread presence of a surface organic soil horizon (peat), which insulates underlying permafrost from warming air temperatures (Figure 1a) (Baughman et al., 2015;Yi et al., 2007) and resists vegetation changes by virtue of its cold, water-saturated, and acidic growing medium (Tape et al., 2012). But the tundra's peat can burn during warm, dry summers (Jones et al., 2009;Mack et al., 2011), and shrub expansion and permafrost thaw can proceed rapidly after fires combust peat (Jones et al., 2013(Jones et al., , 2015. In short, when Arctic warming co-occurs with tundra fires, postfire ecosystem responses are more likely to equilibrate with "the new normal" relative to a more tempered response in the absence of fire (Jones et al., 2013;Landhausser &

Topography, Climate and Fire History Regulate Wildfire Activity in the Alaskan Tundra

“…When reburn fire trend was compared with non‐reburn fires, we identified an upward drift in annual reburn pattern since 1997, with a 61% increase in reburn areas over the past decade. This increase in reburn area suggests a potential fire‐induced increase in fuel production and flammability as recently identified in Noatak River Valley tundra (Gaglioti et al., 2021). However, further analysis would be needed to determine whether the likelihood of reburning has increased in Alaskan tundra, because reburn area tends to increase with the length and completeness of fire history records which likely accounts for at least some of the increasing trend in reburn area that we observed.…”

“…Vegetation types with longer FRI that exhibited self‐limiting fire behavior were mainly in the moist tundra and wetland complexes, primarily dominated by moss, tussock‐sedge, low shrubs (>40 cm tall), and dwarf shrubs (<40 cm tall). Tussock‐sedge vegetation is highly flammable and represents one of the highest percentage burned area in Noatak River Watershed of Alaska (Gaglioti et al., 2021).…”

Fire-vegetation interactions in Arctic tundra and their spatial variability

“…However, the expected increases in fire extent, frequency, and severity 25,52,53 , coupled with the crucial role of circumpolar tundra in global climate change, renders understanding tundra fires better an urgent and strategically important matter. Previous studies, notably Camac, et al 27 , Gaglioti, et al 28 , and Chen, et al 29 , have shown that a fire-shrub positive feedback loop exists at the local scales in the tundra.…”

“…A larger shrub fraction in vegetation composition is, therefore, likely to lead to more spatially extensive and deeper burns 23,27,32 , forming a positive feedback loop. The existence of this fire-shrub positive feedback loop has been shown in several local-scale studies [27][28][29] , however, whether this feedback loop operates widely across the Arctic tundra remains unclear. Thus, our understanding of the present-day ecosystem-wide fire regimes and our ability to develop future projections are strongly linked to our understanding of the tundra-wide patterns of fire-shrub relationship beyond local-scale observations.…”

“…Considering the substantial Arctic warming and the associated shrubification, the present-day tundra may be reaching a tipping point where fire regimes, which are already believed to be increasingly active 25,26 , may be further intensified. A key dynamic in this fire-shrub relationship has been speculated to be a positive feedback loop between shrubs and wildfires 23,[27][28][29] . On the one hand, fires may facilitate shrubification (i.e., leading to more shrub cover or higher shrub biomass compared to the unburned sites).…”