Climatic thresholds shape northern high‐latitude fire regimes and imply vulnerability to future climate change

Abstract

Boreal forests and arctic tundra cover 33% of global land area and store an estimated 50% of total soil carbon. Because wildfire is a key driver of terrestrial carbon cycling, increasing fire activity in these ecosystems would likely have global implications. To anticipate potential spatiotemporal variability in fire‐regime shifts, we modeled the spatially explicit 30‐yr probability of fire occurrence as a function of climate and landscape features (i.e. vegetation and topography) across Alaska. Boosted regression tree (BRT) models captured the spatial distribution of fire across boreal forest and tundra ecoregions (AUC from 0.63–0.78 and Pearson correlations between predicted and observed data from 0.54–0.71), highlighting summer temperature and annual moisture availability as the most influential controls of historical fire regimes. Modeled fire–climate relationships revealed distinct thresholds to fire occurrence, with a nonlinear increase in the probability of fire above an average July temperature of 13.4°C and below an annual moisture availability (i.e. P‐PET) of approximately 150 mm. To anticipate potential fire‐regime responses to 21st‐century climate change, we informed our BRTs with Coupled Model Intercomparison Project Phase 5 climate projections under the RCP 6.0 scenario. Based on these projected climatic changes alone (i.e. not accounting for potential changes in vegetation), our results suggest an increasing probability of wildfire in Alaskan boreal forest and tundra ecosystems, but of varying magnitude across space and throughout the 21st century. Regions with historically low flammability, including tundra and the forest–tundra boundary, are particularly vulnerable to climatically induced changes in fire activity, with up to a fourfold increase in the 30‐yr probability of fire occurrence by 2100. Our results underscore the climatic potential for novel fire regimes to develop in these ecosystems, relative to the past 6000–35 000 yr, and spatial variability in the vulnerability of wildfire regimes and associated ecological processes to 21st‐century climate change.

Hydrologic Regime Changes in a High-Latitude Glacierized Watershed under Future Climate Conditions

“…A regime shift with regard to forest health is projected to occur mid-century, when most of the forested river valley will experience hot and dry conditions for three months, versus one month now. Young et al (2017) found that wildfire occurrence in the CRW escalates when forests experience both air temperature of 13.4 • C or greater and a positive moisture deficit. Wildfire is not the only risk to forests under stress; heat stress and moisture deficit also make forests vulnerable to insect and microbial infestations.…”

“…Recent work by Young et al (2017) [12] determined environmental factors linked to wildfire occurrence; one of the factors identified for the CRW Forested River Valley was temperature in excess of 13.4 °C. In the Forested River Valley, temperatures will be above 13.4 °C for three months by the end of the century (RCP 8.5 model mean) compared to only one month during the historical period .…”

“…The ET deficit (PET-ET) increases by 37% in the Forested River Valley. Forest vulnerability is identified here as conditions with both high temperature (>13.4 • C) and a positive moisture deficit [12].Historically, these hot and dry forest conditions were prevalent only during July, but, starting mid-century, most of the forested area will be hot and dry for three months (June, July and August), tripling the duration of forest stress. All water budget fluxes (precipitation, transfers to the atmosphere, and streamflow) increase at statistically significant rates, as well as contributions to streamflow from melting glacier ice (Figure 8).…”

Differences in Human versus Lightning Fires between Urban and Rural Areas of the Boreal Forest in Interior Alaska

“…One major disturbance is fire which has increased recently in frequency, extent, and severity [3][4][5][6][7] accompanied by potentially substantial losses of stored carbon [8][9][10]. Models predict continued increases in area burned and carbon emissions in boreal North America in the future [11][12][13][14] in response to a strengthening of arctic amplification of global climate change [15]. However, there are indications of an ecosystem shift from highly flammable mature spruce to less flammable early successional deciduous vegetation [16][17][18][19].…”

“…Wildfire burn area is closely associated with a moist enough climate to produce vegetative fuel and temporary high temperature and dry conditions [14,20,21]. It can be modulated by vegetation [22][23][24], land use [25], and suppression [26][27][28] after being ignited by either lightning strikes [29] or people [30][31][32].…”