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Abstract Ecosystem engineering by beavers is a nascent disturbance in the Arctic tundra, appearing in the 1970s in western Alaska and since expanding deeper into tundra regions. Evidence from modeling and observations indicates that beaver ponds act as biophysical oases, and we anticipate myriad changes as these disturbances are constructed along tundra streams, sloughs, and lake outlets. We used over 11 000 mapped beaver pond locations in Arctic Alaska and their climatic, geographic, and environmental attributes to understand (1) which of those attributes control the distribution of beaver ponds, and, if temperature is a factor, (2) how beaver pond distribution will change under future climate scenarios. Of the variables used in the ensemble modeling approach, mean annual temperature was the most important variable in determining beaver pond locations, with pond occurrences more likely in warmer locales (>−2 °C). The distance to water was also important in determining beaver pond locations, as expected, with higher likelihood of ponds closer to water features. Lowland topographic variables were also relevant in determining the distribution of beaver ponds. Under the current climate, beaver ponds are widespread in most of western Alaska, matching the predicted extent of potential occupancy, with the exception of areas furthest from treeline, implying possible dispersal lags or other factors. By 2050, under future climate scenarios (RCP8.5; 2090 for RCP6.0), the entire North Slope of Alaska, which currently has no beaver ponds, is predicted to be suitable for beaver ponds, comparable to western Alaska in 2016. The vast extent of future beaver engineering in tundra regions will require reenvisioning the typical tundra stream ecosystems of northern Alaska, northern Canada, northern Europe, and northern Asia to include more extensive wetlands, routine disturbances, permafrost thaw, and other features of these nascent oases that are not fully understood. 

Abstract Beavers (Castor canadensis) are rapidly colonizing the North American Arctic, transforming aquatic and riparian tundra ecosystems. Arctic tundra may respond differently than temperate regions to beaver engineering due to the presence of permafrost and the paucity of unfrozen water during winter. Here, we provide a detailed investigation of 11 beaver pond complexes across a climatic gradient in Arctic Alaska, addressing questions about the permafrost setting surrounding ponds, the influence of groundwater inputs on beaver colonization and resulting ponds, and the change in surface water and aquatic overwintering habitat. Using field measurements, in situ dataloggers, and remote sensing, we evaluate permafrost, water quality, pond ice phenology, and physical characteristics of impoundments, and place our findings in the context of pond age, local climate, permafrost setting, and the presence of perennial groundwater inputs. We show beavers are accelerating the effects of climate change by thawing permafrost adjacent to ponds and increasing liquid water during winter. Beavers often exploited perennial springs in discontinuous permafrost, and summertime water temperatures at spring‐fed (SF) beaver ponds were roughly 5°C lower than sites lacking springs (NS). Late winter liquid water was generally present at pond complexes, although liquid water below seasonal ice cover was shallow (5–82 cm at SF and 5–15 cm at NS ponds) and ice was thick (median: 85 cm). Water was less acidic at SF than NS sites and had higher specific conductance and more dissolved oxygen. We estimated 2.4 dams/km of stream at sites on the recently colonized (last ~10 years) Baldwin Peninsula and 7.4 dams/km on the Seward Peninsula, where beavers have been present longer (~20+ years) and groundwater‐surface water connectivity is more common. Our study highlights the importance of climatic and physiographic context, especially permafrost presence and groundwater inputs, in determining the characteristics of the Arctic beaver pond environment. As beavers continue their expansion into tundra regions, these characteristics will increasingly represent the future of aquatic and riparian Arctic ecosystems. 

Abstract In recent decades the habitat of North American beaver (Castor canadensis) has expanded from boreal forests into Arctic tundra ecosystems. Beaver ponds in Arctic watersheds are known to alter stream biogeochemistry, which is likely coupled with changes in the activity and composition of microbial communities inhabiting beaver pond sediments. We investigated bacterial, archaeal, and fungal communities in beaver pond sediments along tundra streams in northwestern Alaska (AK), USA and compared them to those of tundra lakes and streams in north‐central Alaska that are unimpacted by beavers.β‐glucosidase activity assays indicated higher cellulose degradation potential in beaver ponds than in unimpacted streams and lakes within a watershed absent of beavers. Beta diversity analyses showed that dominant lineages of bacteria and archaea in beaver ponds differed from those in tundra lakes and streams, but dominant fungal lineages did not differ between these sample types. Beaver pond sediments displayed lower relative abundances of Crenarchaeota and Euryarchaeota archaea and of bacteria from typically anaerobic taxonomic groups, suggesting differences in rates of fermentative organic matter (OM) breakdown, syntrophy, and methane generation. Beaver ponds also displayed low relative abundances of Chytridiomycota (putative non‐symbiotic) fungi and high relative abundances of ectomycorrhizal (plant symbionts) Basidiomycota fungi, suggesting differences in the occurrence of plant and fungi mutualistic interactions. Beaver ponds also featured microbes with taxonomic identities typically associated with the cycling of nitrogen and sulfur compounds in higher relative abundances than tundra lakes and streams. These findings help clarify the microbiological implications of beavers expanding into high latitude regions. 

Abstract Beavers were not previously recognized as an Arctic species, and their engineering in the tundra is considered negligible. Recent findings suggest that beavers have moved into Arctic tundra regions and are controlling surface water dynamics, which strongly influence permafrost and landscape stability. Here we use 70 years of satellite images and aerial photography to show the scale and magnitude of northwestward beaver expansion in Alaska, indicated by the construction of over 10,000 beaver ponds in the Arctic tundra. The number of beaver ponds doubled in most areas between ~ 2003 and ~ 2017. Earlier stages of beaver engineering are evident in ~ 1980 imagery, and there is no evidence of beaver engineering in ~ 1952 imagery, consistent with observations from Indigenous communities describing the influx of beavers over the period. Rapidly expanding beaver engineering has created a tundra disturbance regime that appears to be thawing permafrost and exacerbating the effects of climate change. 

Abstract Beaver engineering in the Arctic tundra induces hydrologic and geomorphic changes that are favorable to methane (CH₄) production. Beaver-mediated methane emissions are driven by inundation of existing vegetation, conversion from lotic to lentic systems, accumulation of organic rich sediments, elevated water tables, anaerobic conditions, and thawing permafrost. Ground-based measurements of CH₄emissions from beaver ponds in permafrost landscapes are scarce, but hyperspectral remote sensing data (AVIRIS-NG) permit mapping of ‘hotspots’ thought to represent locations of high CH₄emission. We surveyed a 429.5 km²area in Northwestern Alaska using hyperspectral airborne imaging spectroscopy at ∼5 m pixel resolution (14.7 million observations) to examine spatial relationships between CH₄hotspots and 118 beaver ponds. AVIRIS-NG CH₄hotspots covered 0.539% (2.3 km²) of the study area, and were concentrated within 30 m of waterbodies. Comparing beaver ponds to all non-beaver waterbodies (including waterbodies >450 m from beaver-affected water), we found significantly greater CH₄hotspot occurrences around beaver ponds, extending to a distance of 60 m. We found a 51% greater CH₄hotspot occurrence ratio around beaver ponds relative to nearby non-beaver waterbodies. Dammed lake outlets showed no significant differences in CH₄hotspot ratios compared to non-beaver lakes, likely due to little change in inundation extent. The enhancement in AVIRIS-NG CH₄hotspots adjacent to beaver ponds is an example of a new disturbance regime, wrought by an ecosystem engineer, accelerating the effects of climate change in the Arctic. As beavers continue to expand into the Arctic and reshape lowland ecosystems, we expect continued wetland creation, permafrost thaw and alteration of the Arctic carbon cycle, as well as myriad physical and biological changes. 

Beavers build dams that change the way water moves between streams, lakes, and the land. In Alaska, beavers are moving north from the forests into the Arctic tundra. When beavers build dams in the Arctic, they cause frozen soil, called permafrost, to thaw. Scientists are studying how beavers and the thawing of permafrost are impacting streams and rivers in Alaska’s national parks. For example, permafrost thaw from beavers can add harmful substances like mercury to streams. Mercury can be taken up by stream food webs, including fish, which then become unhealthy to eat. Permafrost thaw can also move carbon (from dead plants) to beaver ponds. When this carbon decomposes, it can be released from beaver ponds into the air as greenhouse gases, which cause Earth’s climate to warm. Scientists are trying to keep up with these busy beavers to better understand how they are changing Arctic landscapes and Earth’s climate. 

Abstract. Lakes in the Arctic are important reservoirs of heat withmuch lower albedo in summer and greater absorption of solar radiation thansurrounding tundra vegetation. In the winter, lakes that do not freeze totheir bed have a mean annual bed temperature >0 ∘C inan otherwise frozen landscape. Under climate warming scenarios, we expectArctic lakes to accelerate thawing of underlying permafrost due to warmingwater temperatures in the summer and winter. Previous studies of Arcticlakes have focused on ice cover and thickness, the ice decay process,catchment hydrology, lake water balance, and eddy covariance measurements,but little work has been done in the Arctic to model lake heat balance. Weapplied the LAKE 2.0 model to simulate water temperatures in three Arcticlakes in northern Alaska over several years and tested the sensitivity ofthe model to several perturbations of input meteorological variables(precipitation, shortwave radiation, and air temperature) and several modelparameters (water vertical resolution, sediment vertical resolution, depthof soil column, and temporal resolution). The LAKE 2.0 model is aone-dimensional model that explicitly solves vertical profiles of waterstate variables on a grid. We used a combination of meteorological data fromlocal and remote weather stations, as well as data derived from remotesensing, to drive the model. We validated modeled water temperatures withdata of observed lake water temperatures at several depths over severalyears for each lake. Our validation of the LAKE 2.0 model is a necessarystep toward modeling changes in Arctic lake ice regimes, lake heat balance,and thermal interactions with permafrost. The sensitivity analysis shows usthat lake water temperature is not highly sensitive to small changes in airtemperature or precipitation, while changes in shortwave radiation and largechanges in precipitation produced larger effects. Snow depth and lake icestrongly affect water temperatures during the frozen season, which dominatesthe annual thermal regime of Arctic lakes. These findings suggest thatreductions in lake ice thickness and duration could lead to more heatstorage by lakes and enhanced permafrost degradation. 

Deciduous shrubs are expanding across the graminoid-dominated nutrient-poor arctic tundra. Absorptive root traits of shrubs are key determinants of nutrient acquisition strategy from tundra soils, but the variations of shrub root traits within and among common shrub genera across the arctic climatic gradient are not well resolved. Consequently, the impacts of arctic shrub expansion on belowground nutrient cycling remain largely unclear. Here, we collected roots from 170 plots of three commonly distributed shrub genera ( Alnus , Betula , and Salix ) and a widespread sedge ( Eriophorum vaginatum ) along a climatic gradient in northern Alaska. Absorptive root traits that are relevant to the strategy of plant nutrient acquisition were determined. The influence of aboveground dominant vegetation cover on the standing root biomass, root productivity, vertical rooting profile, as well as the soil nitrogen (N) pool in the active soil layer was examined. We found consistent root trait variation among arctic plant genera along the sampling transect. Alnus and Betula had relatively thicker and less branched, but more frequently ectomycorrhizal colonized absorptive roots than Salix , suggesting complementarity between root efficiency and ectomycorrhizal dependence among the co-existing shrubs. Shrub-dominated plots tended to have more productive absorptive roots than sedge-dominated plots. At the northern sites, deep absorptive roots (>20 cm depth) were more frequent in birch-dominated plots. We also found shrub roots extensively proliferated into the adjacent sedge-dominated plots. The soil N pool in the active layer generally decreased from south to north but did not vary among plots dominated by different shrub or sedge genera. Our results reveal diverse nutrient acquisition strategies and belowground impacts among different arctic shrubs, suggesting that further identifying the specific shrub genera in the tundra landscape will ultimately provide better predictions of belowground dynamics across the changing arctic.