Boreal forest and tundra ecosystems cover approximately 33% of Earth's terrestrial surface (McGuire et al. 1995) and are experiencing climatic warming at rates twice as fast as the global average (Serreze and Barry 2011). The ecosystem impacts of warming are well documented, including permafrost thawing (Schuur et al. 2008), shrub expansion (Myers‐Smith et al. 2011), altered forest productivity (Beck et al. 2011), and increased fire activity (Kelly et al. 2013). Northern high‐latitude ecosystems also play a key role in the global climate system, storing an estimated 50% of global soil carbon (McGuire et al. 2009). The fate of these massive carbon stocks is directly tied to wildfire (Bond‐Lamberty et al. 2007, Kelly et al. 2016), and thus to potential shifts in 21st‐century fire regimes (i.e. the expected pattern of burning over broad spatiotemporal scales; Baker 2009). For example, the 2007 Anaktuvuk River Fire in the Brooks Foothills ecoregion of Alaska, an event locally unprecedented in the past 6500 yr (Chipman et al. 2015), resulted in an estimated 2.1 Tg C emitted to the atmosphere, comparable to the annual net carbon sink of the tundra biome (Mack et al. 2011). Thus, increased fire activity in this tundra region would likely result in novel levels of burning, with important implications for ecosystem structure and function, including carbon storage.

Climate warming is expected to alter fire activity globally (Flannigan et al. 2009), but anticipating regional fire‐regime shifts requires understanding how potential changes may manifest across space and time. The direction and impacts of shifting fire regimes will vary among ecosystems due to regional variation in climate change, vegetation composition, disturbance histories, ecosystem productivity, and carbon storage. For example, there is a wide range of fire‐driven fuel consumption across boreal forests (0.6 to 12.9 kg C m–2) due to regional differences in fuel composition and combustion efficiency (van Leeuwen et al. 2014). Therefore, regional differences in fire‐regime changes could have important implications for wildfire emissions and carbon cycling. Spatial variability of northern high‐latitude fire regimes (Rocha et al. 2012, Boulanger et al. 2013) is ultimately a product of climate and landscape controls on fuel productivity and fuel drying (Kasischke et al. 2010, Parisien et al. 2011). Anticipating potential fire‐regime shifts and associated impacts of 21st‐century climate change thus requires understanding the controls of spatial variability in historical fire regimes.

Statistical models of fire–climate relationships at annual timescales across broad regions of boreal forest or tundra suggest strong links between annual area burned and summer moisture deficits, highlighting mechanisms related to low fuel moisture (Duffy et al. 2005, Hu et al. 2015). Consequently, under future scenarios with higher summer moisture deficits, models project increased annual area burned, in some cases by up to 200% by the end of the 21st century (Balshi et al. 2009, Hu et al. 2015). Annual‐scale models also have several important limitations for projecting potential fire‐regime shifts. First, annual‐scale models generally trade off spatial for temporal resolution, with fire and climate information aggregated over broad spatial regions (Duffy et al. 2005, Hu et al. 2015). These models thereby average across regional or sub‐regional variation in climate and landscape features that influence fire activity, masking regional variability in future fire activity. Second, these models are inherently sensitive to inter‐annual climatic variability, a feature not well captured in global climate models (Rupp et al. 2013).

Multi‐decadal scale statistical modeling offers a complementary approach to annual‐scale models, trading off temporal for spatial resolution (Parisien et al. 2014). Using spatially resolved long‐term (e.g. 30 yr) climatic averages and local landscape features, multi‐decadal scale models explain fire occurrence at spatial resolutions from 1 to 100 km2 (Krawchuk et al. 2009, Paritsis et al. 2013). These models help reveal mechanisms that drive spatial variation in modern fire activity (Parisien et al. 2014), and they may provide more robust scenarios of future fire activity because they are less sensitive to uncertainty in projections of inter‐annual climatic variability (Moritz et al. 2012). While in many ecosystems annual‐scale fire–climate relationships align with multi‐decadal scale relationships (i.e. warm, dry conditions facilitate burning at both scales), alignment between these two scales is not ubiquitous. For example, fire activity is low in the warmest and driest biomes of Earth, due to consistently high fuel moisture or limited burnable biomass, respectively (Krawchuk and Moritz 2011). It remains unclear where tundra ecosystems fall along this ‘resource gradient’ of burnable biomass. Global‐scale analyses suggest that tundra fire regimes may be primarily fuel limited (Moritz et al. 2012), making them fundamentally different from fire regimes in North American boreal forests. This contrasts with evidence from Alaskan tundra, which occupies some of the warmest, wettest regions of circumpolar tundra (Hu et al. 2015) and in some areas has burned as often as boreal forests (Higuera et al. 2011a).

Here we use multi‐decadal scale statistical modeling to elucidate the historical drivers of regional fire‐regime variability in boreal forest and tundra ecosystems, and then project potential fire‐regime changes under 21st‐century climate. To quantify historical and future fire regimes, we modeled the spatially explicit 30‐yr probability of fire occurrence in Alaska at 2‐km resolution using explanatory variables representing climate, vegetation, and topography. The 30‐yr probability of fire occurrence can be related to the annual percent area burned, thus allowing a direct comparison to other fire‐regime metrics from historical and paleo‐fire records (e.g. fire frequency, mean fire return interval; Baker 2009, Chipman et al. 2015). Alaska is ideal for studying fire–climate relationships in boreal forest and tundra ecosystems, because estimated fire frequencies span several orders of magnitude, from one fire per 50 yr in areas of boreal forests (Kelly et al. 2013) to less than one fire per 10 000 yr in areas of tundra (Chipman et al. 2015). Alaska also offers one of the longest, most continuous fire records available for both boreal forest and tundra (< http://fire.ak.blm.gov/ >), with high‐resolution downscaled climate data available for the region (Scenarios Network for Alaska and Arctic Planning 2015a, b2015b). We expect multi‐decadal climate to be an important control of Alaskan fire regimes, but we also expect the nature of fire–climate relationships to vary between boreal forest and tundra ecosystems across this vast region. Thus, two key questions we address in this work are: 1) what are the key climatic and landscape (e.g. vegetation, topography) factors controlling fire‐regime variability in Alaskan boreal forest and tundra ecosystems, and 2) how does vulnerability to climatically induced fire‐regime shifts vary across Alaska throughout the 21st century?