We present a new 23-site network of white spruce ring-width chronologies near boreal treeline in Old

A number of contemporary dendroecological studies from northwestern North America have highlighted a divergence in growth trends during recent decades. These studies suggest that warmer temperatures are now exceeding the physiological threshold of some northern tree species, or perhaps are contributing to increased drought stress as current precipitation is insufficient to offset increasing water demands under warmer conditions. Here we document additional evidence of these diverging growth trends from the Mackenzie Delta region of Canada and show they are anomalous to the twentieth century. Using wavelet coherency analyses we demonstrate that our white spruce tree ring chronologies exhibit little divergence from one another during the past four centuries, but coherency of the data‐sets rapidly break down after the 1930s.

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We analyzed subfossil chironomids, sediment organic matter and sediment particle size data from a 1.11-m-long freeze core collected from Carleton Lake (unofficial name), located approximately 120 km north of the modern treeline. This well-dated core spans the last ca. 6,500 years. Two chironomid transfer functions were applied to infer mean July air temperatures. Our results indicated that the chironomid-inferred temperatures from this lake sediment record did not pass a significance test, suggesting that other factors in addition to temperature may have been important in structuring the chironomid community through time. Although not statistically significant, the chironomid-inferred temperatures from this site do follow a familiar pattern, with highest inferred temperatures occurring during the Holocene Thermal Maximum (*6-4 cal kyr BP), followed by a longterm cooling trend, which is reversed during the last 600 years. The largest change in the chironomid assemblage, which occurred between ca. 4,600 and 3,900 cal yr BP is possibly related to the welldocumented northward advance and subsequent retreat of treeline in this region.

One of the most ominous predictions related to recent climatic warming is that low-lying coastal environments will be inundated by higher sea levels. The threat is especially acute in polar regions because reductions in extent and duration of sea ice cover increase the risk of storm surge occurrence. The Mackenzie Delta of northwest Canada is an ecologically significant ecosystem adapted to freshwater flooding during spring breakup. Marine storm surges during the open-water season, which move saltwater into the delta, can have major impacts on terrestrial and aquatic systems. We examined growth rings of alder shrubs (Alnus viridis subsp. fruticosa) and diatoms preserved in dated lake sediment cores to show that a recent marine storm surge in 1999 caused widespread ecological changes across a broad extent of the outer Mackenzie Delta. For example, diatom assemblages record a striking shift from freshwater to brackish species following the inundation event.What is of particular significance is that the magnitude of this recent ecological impact is unmatched over the >1,000-year history of this lake ecosystem. We infer that no biological recovery has occurred in this lake, while large areas of terrestrial vegetation remain dramatically altered over a decade later, suggesting that these systems may be on a new ecological trajectory. As climate continues to warm and sea ice declines, similar changes will likely be repeated in other coastal areas of the circumpolar Arctic. Given the magnitude of ecological changes recorded in this study, such impacts may prove to be long lasting or possibly irreversible.paleoecology | paleolimnology | dendrochronology | limnology | salinization