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Winter is a critical season for land‐surface feedbacks and ecosystem processes; however, most high‐latitude paleo‐environmental reconstructions are blind to cold season conditions. Here we introduce a winter‐sensitive, paleo‐proxy record that is based on the relative frequency of tangential rows of traumatic resin ducts (TRDs) in the annual growth rings of mountain hemlocks (Tsuga mertensiana) growing near treeline in Southeast Alaska. Hemlocks produce a row of TRDs in the earlywood portion of their annual rings in response to cambial damage incurred during winter. Multidecadal bouts of TRD production were followed by growth‐leader replacement, reaction wood formation, and divergence in radial growth between storm‐damaged trees and less exposed mountain hemlock forests. These patterns are consistent with TRDs being a response to tree damage caused by ice and snowstorms, a conclusion supported by the krummholz tree architecture at these sites. This relationship is further corroborated by significant correlations between our TRD record and the strength of the wintertime Aleutian Low (AL) pressure system that is linked to tree‐damaging agents like wind, precipitation, and ice storm strength in Southeast Alaska. The combined TRD/krummholz architecture record indicates that abrupt shifts between strong and weak AL phases occurred every several decades since CE 1700 and that the 1800s had relatively long AL phases with heavy snowpacks. In addition to describing the magnitude and tempo of wintertime climate change in Northwestern North America, these results suggest that North Pacific Decadal Variability underlies the long‐term dynamics of treeline ecosystems along the northeast Pacific coast. Traumatic resin ducts in the cross‐dated tree rings of subalpine mountain hemlocks record tree damage during winter storms in Southeast AlaskaWarmer, windier winters that accompany strong Aleutian Low pressure systems result in more tree damageDecadal‐scale variability in North Pacific climate has impacted treeline ecosystems throughout this region

ABSTRACTCalendar dating of tree-ring series from 16 logs sampled near the margin of Casement Glacier combined with tree-ring dates on 36 detrital logs from Adams Inlet, Glacier Bay National Park and Preserve, Alaska, show killing of trees by ice and lake sediments from the mid-sixth through mid-seventh centuries c.e. The dates from the land-terminating Casement Glacier show ice advance into a forest between 560 and 570 c.e. within a few kilometers of the 2011 retreating margin. Advance of the tidewater glacier in Muir Inlet blocked off Adams Inlet forming Lake Adams between 540 and 640 c.e. This glacier and lake history for Glacier Bay is consistent with other land-terminating ice expansions across the Gulf of Alaska that similarly show advance centered on 600 c.e., as well as other proxy records from lakes all suggesting cooling during this interval. The cooling closely follows a series of eruptions in the mid to late sixth century, which may have contributed to the cooling. Radiocarbon ages in Adams Inlet suggest that Lake Adams persisted through 880 cal. yr c.e. and drained by 1170 cal. yr c.e. Ice retreat and this lake drainage are broadly coincident with Medieval warming recognized along the Gulf of Alaska in dendroclimatic reconstructions. Shortly after this retreat, Little Ice Age readvance occurred with Casement Glacier coalescing with glaciers in Adams Inlet and the West Arm, subsequently filling all of Glacier Bay to its Holocene maximum by 1750 c.e.

AbstractRecent observations of divergence between tree growth at high-latitude sites and temperature, as well as the decline of yellow cedar [Callitropsis nootkatensis(D. Don) Oerst. ex D.P. Little] in southeast Alaska due to warming, emphasize a need to investigate nonstationary climate response of Alaskan coastal forests to warming in other tree species. Comparison of annual tree growth in mountain hemlock [Tsuga mertensiana(Bong.) Carrière] to mean monthly temperature and precipitation data from Sitka, Alaska, from A.D. 1830s to 1990s along an elevational transect reveals nonstationarity in tree growth response to climate that suggests an ongoing biome shift. We observe a marked weakening in the positive relationship between annual growth and warmer growth season temperatures at low-elevation hemlock sites, and a concurrent increase in growth and sensitivity at higher elevations coupled with increased correlation between growth and April precipitation at all sites along our transects. As previously observed with yellow cedar, the mechanism of the hemlock biome shift may be due to an increased susceptibility of roots to damage from late frosts resulting from earlier seasonal loss of protective snowpack.

AbstractTidewater glaciers have coalesced to advance through Icy Bay, Alaska, three times during the past 3800 yr. Radiocarbon ages show that the first of these expansions was underway by 3750 cal yr B.P. and culminated at the outer coast between 3505 and 3245 cal yr B.P. Subsequent recession and readvance brought the ice margin back to the outer coast by 1525 cal yr B.P. (cal a.d. 425) where it remained for about 650 yr before retreating. Tree-ring cross-dates of glacially killed trees show that the most recent ice advance was underway through the inner bay by the a.d. 1640s and reached into the outer bay in the 1810s. Historical data support ice expansion through the outer bay in the early 19th century and show a late 19th century maximum prior to 20th century retreat. These results are a significant revision and extension of previous studies of the Holocene glacial history of Icy Bay. Average advance rates for the most recent expansion were typical of modern tidewater glaciers in the inner bay but much faster in the outer bay; shallow water here may have been important to this latter phase of unusually rapid advance.

AbstractThe late Holocene history of eolian activity in a parabolic dune complex in the northern Chugach Mountains, Alaska, is reconstructed using 80 tree-ring dates and 5 radiocarbon ages. A radiocarbon age on detrital organics shows mobilization of sands about cal yr A.D. 1260. General forest growth over the dune field area indicates that this active interval was followed by dune stabilization by A.D. 1500. The present interval of dune migration activity, reconstructed from calendar dates and radiocarbon ages on buried trees, began as early as the late A.D. 1600s, was well under way by the mid–A.D. 1700s, and continues today at a diminishing rate. Intervals of increased eolian activity correspond with pulses of the Little Ice Age as reconstructed from a well-established glacial history for the region over the last 1000 yr. Century-scale cooling in climate appears to have forced dune activity through general geomorphic instabilities of nearby glacial and fluvial-lacustrine systems. Geomorphic activity includes a combination of dynamic fluvial incision of cutbanks and release of dormant dune sands, increased wind intensity, as well as internal feedback mechanisms associated with parabolic and blowout dune dynamics. Over the past 200 yr, average rates of dune migration between 1 to 3 m yr−1are estimated using tree-ring dating. Observations and tree ages along the dune margins suggest that vegetative cover may now be increasing and dune stabilization may be under way.

Fluctuations of the piedmont lobe of Bering Glacier and its sublobe Steller Glacier over the past two millennia are reconstructed using 34 radiocarbon dates and tree-ring data from 16 sites across the glaciers' forelands. The general sequence of glacial activity is consistent with well-dated fluctuations of tidewater and land-terminating glaciers elsewhere along the Gulf of Alaska. Extensive forested areas along 25 km of the Bering ice margin were inundated by glacio-lacustrine and glacio-fluvial sediments during a probable ice advance shortly before 500 cal yr A.D. Regrowth of forests followed the retreating ice as early as the 7th century A.D., with frequent interruptions of tree growth due to outwash aggradation. Forests overrun by ice and buried in outwash indicate readvance about 1080 cal yr A.D. Retreat followed, with ice-free conditions maintained along the distal portions of the forefield until the early 17th century after which the ice advanced to within a few kilometers of its outer Neoglacial moraine. Ice reached this position after the mid-17th century and prior to 200 yr ago. Since the early 20th century, glacial retreat has been punctuated by periodic surges. The record from forests overrun by the nonsurging Steller Lobe shows that this western ice margin was advancing by 1250 A.D., reaching near its outer moraine after 1420 cal yr A.D. Since the late 19th century, the lobe has dominantly retreated.

Variations in annual growth rings can be utilized to assign precise calendar dates to the study of Quaternary geologic or geomorphic events such as earthquakes, mass movements, glaciations and flooding. Living trees may provide the necessary absolute dates of events or minimum ages for surfaces. Crossdating among living trees and with subfossil wood can identify missing rings and extend tree-ring chronologies further back in time. These methods can lead to the accurate dating of tree rings in both living and dead trees that may have been affected by a geomorphic process. The dating of partial to complete ring growth, compression and release, as well as dating externally apparent responses, such as tree scarring or root sprouting, can often lead to the identification of the season and the year an event occurred. Ring-width patterns when compared with control chronologies can reveal the growth anomalies in witness trees that may be related to geomorphic events. Examples of the application of tree rings to Quaternary geology and geomorphology are varied and numerous. Root shearing, topping of trees, and missing rings tie a rupture along the San Andreas fault to the year 1812 A.D. Narrow tree rings linked to defoliation and tephra fallout differentiated distinct eruptions of Mt. St. Helens in 1480 and in 1482 A.D. Ring analysis has helped unravel glacial chronologies, and tree-ring linkages with climate have also verified that late Holocene ice advances in Argentina correlated with intervals of cool, wet summers. In another application, an 1818 A.D. Que´bec landslide has been tied to the growing season, whereas slides dated to 1834 and 1846 A.D. occurred during the spring.