Climate warming can cause arthropods to express plastic and/or evolved changes in morphology. Previous studies have demonstrated that body sizes of Arctic butterflies are influenced by the temperatures experienced as larvae. To investigate whether this was occurring among Alaskan butterflies, we analyzed temporal trends in the wing sizes of three Holarctic species, Colias hecla, Boloria chariclea and Boloria freija, using museum specimens collected in Arctic tundra regions of Alaska between 1971 and 1995. Wing length was compared to accumulated growing degree days (GDD) during both the spring of the year collected and the previous year's summer during the normal period of larval development. We used mixed‐effects models to test if spring and summer temperatures affected adult morphology. Results show that for every 1°C increase in average seasonal temperature, wingspans decreased between 0.7 and 5 mm, with B. freija the most strongly affected. Our results suggest that the morphological sensitivity of Arctic butterflies to warming is the outcome of interactions between life‐history traits and regional climate, with all species sensitive to warming the summer before the flight year as well as warming the spring of the flight year. Boloria freija, which overwinters as late instar larvae that do not feed before pupation the following spring, was particularly strongly affected by summer warming.

Deposits of surge-type glaciers are widespread in the glacial geologic record; however, it is unclear how climate changes occurring at time scales of decades to centuries affect surge-type glaciers. Here we reconstruct the history of the Black Rapids Glacier (BRG) in the eastern Alaska Range since AD 1400 using a combination of geomorphology, stratigraphy, lichenometry, radiocarbon dating, and dendrochronology. Moraines in the glacier's foreland record four advances, all of which left deposits typical of surging glaciers. A surge in the AD 1600s dammed a lake which drained in an outburst flood ca. AD 1703-04. Another outburst flood from a larger glacierdammed lake occurred in the AD 1400s. Based on the BRG's observed glaciology and its history over the last several centuries, its surge cycles have varied between 80 and 120 years. Between AD 1400 and 1900, the most extensive surges of the BRG coincided with minima in the Seuss / de Vries solar cycle when non-surging glaciers in the region also advanced. Synchroneity between the BRG, solar minima, and non-surging glaciers is surprising given that the terminus of the BRG was largely unresponsive to climate for 80-120 years between surges. One explanation is that the BRG's surge cycle shortened during the Little Ice Age (LIA, ca. AD 1300-1900) to the point that its climate-response lag resembled that of neighboring, non-surging glaciers. Although the reconstructed chronology of the BRG shows no indication of the surge cycle decreasing during the LIA, fading of the record with time makes it difficult to exclude this possibility. Another explanation is that the BRG's 80- to 120-year cycle is the result of tuning by the solar cycle over the course of millennia. Tuning occurred when quiescent phases that coincided with solar minima were shortened because of faster replenishment of the glacier's reservoir zone. The opposite occurred when the glacier's quiescent phases coincided with solar maxima. The net result was to align the surge cycle of the BRG with solar minima. Some combination of shortened surge cycles during the LIA and tuning by the solar cycle may be why the glacial-geologic record of this particular surge-type glacier provides a surprisingly dependable record of regional climate over the past 600 years.

How forests respond to accelerated climate change will affect the terrestrial carbon cycle. To better understand these responses, more examples are needed to assess how tree growth rates react to abrupt changes in growing-season temperatures. Here we use a natural experiment in which a glacier's fluctuations exposed a temperate rainforest to changes in summer temperatures of similar magnitude to those predicted to occur by 2050. We hypothesized that the onset of glacier-accentuated temperature trends would act to increase the variance in stand-level tree growth rates, a proxy for forest net primary productivity. Instead, dendrochronological records reveal that the growth rates of five, co-occurring conifer species became less synchronous, and this diversification of species responses acted to reduce the variance and to increase the stability of community-wide growth rates. These results warrant further inquiry into how climate-induced changes in tree-growth diversity may help stabilize future ecosystem services like forest carbon storage. Knowing how ecosystems responded to rapid climate changes in the past can help society prepare for the unprecedented rates of change expected in the future. Here, we take advantage of a natural experiment in which a fluctuating glacier caused a temperate rainforest to experience accentuated climate trends similar to those predicted to occur globally over the coming century. As climate changes became accentuated, tree species that once grew in unison shifted to more diversified growth patterns, which unexpectedly caused less variance and greater stability in community-wide growth rates. Similar diversified growth responses may become important in stabilizing rates of forest carbon sequestration elsewhere. A glacier-adjacent forest in Southeast Alaska serves as a natural climate change experiment Dendrochronology reveals that asynchronous species growth rates enhanced the forest-wide growth stability during accelerated climate trends

The Northwest Coast of North America stretches 4000 km from Bering Strait to Washington State. Here we review the history of glaciation, sea level, oceanography, and climate along the Northwest Coast and in the subarctic Pacific Ocean during the Last Glacial Maximum and deglaciation. The period of interest is Marine Isotope Stage 2 between ca. 29,000 calendar years ago (29 ka) and 11,700 calendar years ago (11.7 ka). The glacial history of the Northwest Coast involved multiple glacial systems responding independently to latitudinal variations in climate caused by changes in the North American ice sheets and in the tropical ocean-atmosphere system. Glaciers reached their maximum extents 1 - 5 kyrs later along the Northwest Coast than did large sectors of the Laurentide and Fennoscandian Ice Sheets. Local, Last Glacial Maxima were reached in a time-transgressive, north to south sequence between southwestern Alaska and Puget Sound. The history of relative sea level along the Northwest Coast during Marine Isotope Stage 2 was complex because of rapid isostatic adjustments by a thin lithosphere to these time-transgressive glacial fluctuations. Multiple lines of evidence suggest Bering Strait was first flooded by the sea after 11 ka and that it probably did not assume its present-day oceanographic functions until after 9 ka. The coldest intervals occurred during Heinrich Event 2 (ca. 26 - 23.5 ka), again between ca. 23 and 21.5 ka, and during Heinrich Event 1 (ca. 18 - 15 ka). During these times, mean annual sea surface temperatures cooled by 5 o to 8 o C in the Gulf of Alaska, and glacial equilibrium-line altitudes fell below present sea level in southern Alaska and along the Aleutian Island chain. Sea ice episodically expanded across the subarctic Pacific in winter. Oceanographic changes in the Gulf of Alaska tracked variations in the vigor of the Asian Summer Monsoon. The deglaciation of the Northwest Coast may have served as the trigger for global climate changes during deglaciation. Starting ca. 21 ka, marine-based glaciers there were increasingly destabilized by rising eustatic sea level and influxes of freshwater and heat associated with the rejuvenation of the Asian Summer Monsoon. Rapid retreat of marine-based glaciers began ca. 19 ka and released large numbers of ice bergs and vast amounts of freshwater into the Northeast Pacific. Resultant cooling of the North Pacific may have been teleconnected to the North Atlantic through the atmosphere, where it slowed Atlantic Meridional Overturning Circulation and initiated the global effects of Heinrich Event 1, ca. 18 - 15 ka. During the Younger Dryas, ca. 12.8 - 11.7 ka, mean annual sea surface temperatures were 4 o to 6 o C cooler than today in the Gulf of Alaska, and sea ice again expanded across the subarctic Pacific in winter. Conditions of extreme seasonality characterized by cold, dry winters and warm, steadily ameliorating summers caused by the southward diversion of the Aleutian Low in winter may explain the previously enigmatic records of Younger Dryas climate along the Northwest Coast.

Widespread glacial retreat is now occurring in many arctic mountain ranges, yet little is known about primary succession following deglaciation in these settings. Newly created habitats could provide refugia for flora and fauna whose ranges are threatened elsewhere by rapid warming. To assess vegetation responses to glacial retreat in an arctic–alpine setting, we first describe plant community development on two recently deglaciated moraines in the Brooks Range. We then compare these recent communities with communities developed along a moraine chronosequence that spans >125,000 years and ranges in altitude between 800 and 1,700 m.a.s.l. Results show that (1) within twenty-two to thirty-six years following deglaciation, primary succession begins with the assembly of small communities of eight to thirteen vascular and nonvascular plant species; (2) species turnover is low, with many pioneer taxa, particularly lichens, persisting at the oldest sites and across all altitudes; and (3) overall, succession is directional and slow, with species richness increasing for up to 25,000 years, and percentage vegetation cover reaching >100 percent on the oldest glacial deposits. This is the first vegetation study on primary succession in the high central Brooks Range, and it supplies a previously missing alpine element within a vegetation transect across northern Alaska’s bioclimatic gradient.

Reconstructing how biota have responded to fast‐paced warming events in the past can help predict their responses to rapid climate changes in the future. Here we suggest that natural communities located near glaciers are useful laboratories for this purpose as they experienced climate changes accentuated by past ice‐margin fluctuations. By reconstructing an Alaskan glacier's position over a 166‐year period and measuring the periglacial air temperatures over the last 3 years, we estimate that the adjacent temperate rainforest episodically cooled and warmed by 0.5–0.7°C/decade. These rates of change exceed most historical warming trends measured elsewhere on Earth and are comparable to the rates of climate warming predicted for the next century. The ring‐width responses of yellow‐cedar trees growing at varying distances from the ice edge illustrate the potential for using periglacial ecosystems to predict how forests may respond to rapid warming in the future.

Rapidly changing climate at high latitudes has triggered a search for bellwethers of ecological change there. If the initial signs of change can be identified, perhaps we can predict where these changes will lead. Large-bodied, terrestrial herbivores are potential candidates for bellwether taxa because of the key roles they play in some ecological communities. Here, we assembled historical, archaeological, and paleontological records of moose ( Alces alces (Linnaeus, 1758)) from the western Arctic and subarctic. The results showed that rather than having recently invaded tundra regions in response to post Little Ice Age warming, moose have inhabited river corridors several hundred kilometres north of the closed, boreal forest since they first colonized North America across the Bering Land Bridge ca. 14 000 years ago. The combination of high mobility, fluctuation-prone metapopulations, and reliance on early successional vegetation makes changes in the northern range limits of moose undependable bellwethers for other biotic responses to changing climate. The history of moose at high latitudes illustrates how understanding what happened in prehistory is useful for correctly assigning significance and cause to present-day ecological changes.

Polar bears (Ursus maritimus) and brown bears (Ursus arctos) are sister species possessing distinct physiological and behavioural adaptations that evolved over the last 500,000 years. However, comparative and population genomics analyses have revealed that several extant and extinct brown bear populations have relatively recent polar bear ancestry, probably as the result of geographically localized instances of gene flow from polar bears into brown bears. Here, we generate and analyse an approximate 20X paleogenome from an approximately 100,000-year-old polar bear that reveals a massive prehistoric admixture event, which is evident in the genomes of all living brown bears. This ancient admixture event was not visible from genomic data derived from living polar bears. Like more recent events, this massive admixture event mainly involved unidirectional gene flow from polar bears into brown bears and occurred as climate changes caused overlap in the ranges of the two species. These findings highlight the complex reticulate paths that evolution can take within a regime of radically shifting climate.

As the Arctic warms, tundra wildfires are expected to become more frequent and severe. Assessing how the most flammable regions of the tundra respond to burning can inform us about how the rest of the Arctic may be affected by climate change. Here we describe ecosystem responses to tundra fires in the Noatak River watershed of northwestern Alaska using shrub dendrochronology, active‐layer depth monitoring, and remotely sensed vegetation productivity. Results show that relatively productive tundra is more likely to experience fires and to burn more severely, suggesting that fuel loads currently limit tundra fire distribution in the Noatak Valley. Within three years of burning, most alder shrubs sampled had either germinated or resprouted, and vegetation productivity inside 60 burn perimeters had recovered to prefire values. Tundra fires resulted in two phases of increased primary productivity as manifested by increased landscape greening. Phase one occurred in most burned areas 3–10 years after fires, and phase two occurred 16–44 years after fire at sites where tundra fires triggered near‐surface permafrost thaw resulting in shrub proliferation. A fire‐shrub‐greening positive feedback is currently operating in the Noatak Valley and this feedback could expand northward as air temperatures, fire frequencies, and permafrost degradation increase. This feedback will not occur at all locations. In the Noatak Valley, the fire‐shrub‐greening process is relatively limited in tussock tundra communities, where low‐severity fires and shallow active layers exclude shrub proliferation. Climate warming and enhanced fire occurrence will likely shift fire‐poor landscapes into either the tussock tundra or erect‐shrub‐tundra ecological attractor states that now dominate the fire‐rich Noatak Valley.

The Bering Land Bridge (BLB) last connected Eurasia and North America during the Late Pleistocene. Although the BLB would have enabled transfers of terrestrial biota in both directions, it also acted as an ecological filter whose permeability varied considerably over time. Here we explore the possible impacts of this ecological corridor on genetic diversity within, and connectivity among, populations of a once wide-ranging group, the caballine horses (Equus spp.). Using a panel of 187 mitochondrial and eight nuclear genomes recovered from present-day and extinct caballine horses sampled across the Holarctic, we found that Eurasian horse populations initially diverged from those in North America, their ancestral continent, around 1.0-0.8 million years ago. Subsequent to this split our mitochondrial DNA analysis identified two bidirectional long-range dispersals across the BLB ~875-625 and ~200-50 thousand years ago, during the Middle and Late Pleistocene. Whole genome analysis indicated low levels of gene flow between North American and Eurasian horse populations, which probably occurred as a result of these inferred dispersals. Nonetheless, mitochondrial and nuclear diversity of caballine horse populations retained strong phylogeographical structuring. Our results suggest that barriers to gene flow, currently unidentified but possibly related to habitat distribution across Beringia or ongoing evolutionary divergence, played an important role in shaping the early genetic history of caballine horses, including the ancestors of living horses within Equus ferus.

In Southeast Alaska, many stands of yellow-cedar (Callitropsis nootkatensis (D. Don) Oerst. ex D.P. Little; hereinafter “YC”) contain numerous standing, dead snags. Snag-age estimates based on tree morphology have been used to support the interpretation that a warming climate after ca. 1880 has triggered unprecedented YC dieback. Here, we present new estimates of YC snag longevity by cross-dating 61 snags with morphologies that suggest they stood dead for extended periods. All but four of these snags have lost their outermost rings to decay, so we estimate when they died using a new method based on wood-ablation rates measured in six living trees that display partial cambial dieback. The results indicate that ∼59% of YC snags that lost their branches to decay (Class 5 snags) have remained standing for >200 years, and some for as long as 450 years (snag longevity mean ± SD: 233 ± 92 years). These findings, along with supporting evidence from historical photos, dendrochronology, and snag-morphology surveys in the published literature suggest that episodes of YC dieback also occurred before 1880 and before significant anthropogenic warming began. The roles played by climate change in these earlier dieback events remain to be further explored.

Little is known about woolly mammoth (Mammuthus primigenius) mobility and range. Here we use high temporal resolution sequential analyses of strontium isotope ratios along an entire 1.7-meter-long tusk to reconstruct the movements of an Arctic woolly mammoth that lived 17,100 years ago, during the last ice age. We use an isotope-guided random walk approach to compare the tusk's strontium and oxygen isotope profiles to isotopic maps. Our modeling reveals patterns of movement across a geographically extensive range during the animal's ~28-year life span that varied with life stages. Maintenance of this level of mobility by megafaunal species such as mammoth would have been increasingly difficult as the ice age ended and the environment changed at high latitudes.

Three new tephras have been identified in Southeast Alaska. An 8-cm-thick black basaltic tephra with nine discrete normally graded beds is present in cores from a lake on Baker Island. The estimated age of the tephra is 13,492 +/- 237 cal yr BP. Although similar in age to the MEd tephra from the adjacent Mt. Edgecumbe volcanic field, this tephra is geochemically distinct. Black basaltic tephras recovered from two additional sites in Southeast Alaska, Heceta Island and the Gulf of Esquibel, are also geochemically distinct from the MEd tephra. The age of the tephra from Heceta Island is 14,609 +/- 343 cal yr BP. Whereas the tephras recovered from Baker Island/Heceta Island/Gulf of Esquibel are geochemically distinct from each other, similarities in the ages of these tephras and the MEd tephra suggest a shared eruptive trigger, possibly crustal unloading caused by retreat of the Cordilleran Ice Sheet. The submerged Addington volcanic field on the continental shelf, which may have been subaerially exposed during the late Pleistocene, is a possible source for the Southeast Alaska tephras.

The Little Ice Age (LIA), ca. CE 1250-1850, was a cold period of global extent, with the nature and timing of reduced temperatures varying by region. The Gulf of Alaska (GOA) is a key location to study the climatic drivers of glacier fluctuations during the LIA because dendrochronologogical techniques can provide precise ages of ice advances and retreats. Here, we use dendrochronology to date the most recent advance of La Perouse Glacier in the Fairweather Range of Southeast Alaska. After maintaining a relatively contracted state since at least CE 1200, La Perouse advanced to its maximum LIA position between CE 1850 and 1895. Like many other glaciers bordering the GOA, the La Perouse Glacier reached this maximum position relatively late in the LIA compared with glaciers in other regions. This is curious because reconstructions of paleoclimate in the GOA region indicate the 19th century was not the coldest period of the LIA. Using newly available paleoclimate data, we hypothesize that a combination of moderately cool summers accompanying the Dalton Solar Minimum and exceptionally snowy winters associated with a strengthened Aleutian Low could have caused these relatively late LIA advances. Such a scenario implies that winter climate processes, which are heavily influenced by ocean-atmospheric variability in the North Pacific region, have modulated these coastal glaciers’ sensitivity to shifts in summer temperatures.