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 &

Widespread changes in the distribution and abundance of plant functional types (PFTs) are occurring in Arctic and boreal ecosystems due to the intensification of disturbances, such as fire, and climate-driven vegetation dynamics, such as tundra shrub expansion. To understand how these changes affect boreal and tundra ecosystems, we need to first quantify change for multiple PFTs across recent years. While landscape patches are generally composed of a mixture of PFTs, most previous moderate resolution (30-m) remote sensing analyses have mapped vegetation distribution and change within land cover categories that are based on the dominant PFT; or else the continuous distribution of one or a few PFTs, but for a single point in time. Here we map a 35-year time-series (1985–2020) of top cover (TC) for seven PFTs across a 1.77 x 106 km² study area in northern and central Alaska and northwestern Canada. We improve on previous methods of detecting vegetation change by modeling TC, a continuous measure of plant abundance. The PFTs collectively include all vascular plants within the study area as well as light macrolichens, a nonvascular class of high importance to caribou management. We identified net increases in deciduous shrubs (66 x 103 km²), evergreen shrubs (20 x 103 km²), broadleaf trees (17 x 103 km²), and conifer trees (16 x 103 km²), and net decreases in graminoids (-40 x 103 km²) and light macrolichens (-13 x 103 km²) over the full map area, with similar patterns across Arctic, Oroarctic, and Boreal bioclimatic zones. Model performance was assessed using spatially blocked, nested 5-fold cross-validation with overall root mean square errors ranging from 8.3–19.0%. Most net change occurred as succession or plant expansion within areas undisturbed by recent fire, though PFT TC change also clearly resulted from fire disturbance. These maps have important applications for assessment of surface energy budgets, permafrost changes, nutrient cycling, and wildlife management and movement analysis.

Ecosystems in the North American Arctic-Boreal Zone (ABZ) experience a diverse set of disturbances associated with wildfire, permafrost dynamics, geomorphic processes, insect outbreaks and pathogens, extreme weather events, and human activity. Climate warming in the ABZ is occurring at over twice the rate of the global average, and as a result the extent, frequency, and severity of these disturbances are increasing rapidly. Disturbances in the ABZ span a wide gradient of spatiotemporal scales and have varying impacts on ecosystem properties and function. However, many ABZ disturbances are relatively understudied and have different sensitivities to climate and trajectories of recovery, resulting in considerable uncertainty in the impacts of climate warming and human land use on ABZ vegetation dynamics and in the interactions between disturbance types. Here we review the current knowledge of ABZ disturbances and their precursors, ecosystem impacts, temporal frequencies, spatial extents, and severity. We also summarize current knowledge of interactions and feedbacks among ABZ disturbances and characterize typical trajectories of vegetation loss and recovery in response to ecosystem disturbance using satellite time-series. We conclude with a summary of critical data and knowledge gaps and identify priorities for future study.

Rapid warming has occurred over the past 50 years in Arctic Alaska, where temperature strongly affects ecological patterns and processes. To document landscape change over a half century in the Arctic National Wildlife Refuge, Alaska, we visually interpreted geomorphic and vegetation changes on time series of coregistered high-resolution imagery. We used aerial photographs for two time periods, 1947-1955 and 1978-1988, and Quick Bird and IKONOS satellite images for a third period, [2000][2001][2002][2003][2004][2005][2006][2007]. The stratified random sample had five sites in each of seven ecoregions, with a systematic grid of 100 points per site. At each point in each time period, we recorded vegetation type, microtopography, and surface water. Change types were then assigned based on differences detected between the images. Overall, 23% of the points underwent some type of change over thẽ 50-year study period. Weighted by area of each ecoregion, we estimated that 18% of the Refuge had changed. The most common changes were wildfire and postfire succession, shrub and tree increase in the absence of fire, river erosion and deposition, and ice-wedge degradation. Ice-wedge degradation occurred mainly in the Tundra Biome, shrub increase and river changes in the Mountain Biome, and fire and postfire succession in the Boreal Biome. Changes in the Tundra Biome tended to be related to landscape wetting, mainly from increased wet troughs caused by ice-wedge degradation. The Boreal Biome tended to have changes associated with landscape drying, including recent wildfire, lake area decrease, and land surface drying. The second time interval, after~1982, coincided with accelerated climate warming and had slightly greater rates of change.

Arctic vegetation communities are rapidly changing with climate warming, which impacts wildlife, carbon cycling and climate feedbacks. Accurately monitoring vegetation change is thus crucial, but scale mismatches between field and satellite-based monitoring cause challenges. Remote sensing from unmanned aerial vehicles (UAVs) has emerged as a bridge between field data and satellite-based mapping. We assess the viability of using high resolution UAV imagery and UAV-derived Structure from Motion (SfM) to predict cover, height and aboveground biomass (henceforth biomass) of Arctic plant functional types (PFTs) across a range of vegetation community types. We classified imagery by PFT, estimated cover and height, and modeled biomass from UAV-derived volume estimates. Predicted values were compared to field estimates to assess results. Cover was estimated with root-mean-square error (RMSE) 6.29-14.2% and height was estimated with RMSE 3.29-10.5 cm, depending on the PFT. Total aboveground biomass was predicted with RMSE 220.5 g m^-2, and per-PFT RMSE ranged from 17.14-164.3 g m^-2. Deciduous and evergreen shrub biomass was predicted most accurately, followed by lichen, graminoid, and forb biomass. Our results demonstrate the effectiveness of using UAVs to map PFT biomass, which provides a link towards improved mapping of PFTs across large areas using earth observation satellite imagery.