Expansion of Canopy-Forming Willows Over the Twentieth Century on Herschel Island, Yukon Territory, Canada

Canopy-forming shrubs are reported to be increasing at sites around the circumpolar Arctic. Our results indicate expansion in canopy cover and height of willows on Herschel Island located at 70° north on the western Arctic coast of the Yukon Territory. We examined historic photographs, repeated vegetation surveys, and conducted monitoring of long-term plots and found evidence of increases of each of the dominant canopy-forming willow species (Salix richardsonii, Salix glauca and Salix pulchra), during the twentieth century. A simple model of patch initiation indicates that the majority of willow patches for each of these species became established between 1910 and 1960, with stem ages and maximum growth rates indicating that some patches could have established as late as the 1980s. Collectively, these results suggest that willow species are increasing in canopy cover and height on Herschel Island. We did not find evidence that expansion of willow patches is currently limited by herbivory, disease, or growing conditions. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s13280-011-0168-y) contains supplementary material, which is available to authorized users

Stratigraphy and palaeoenvironments of Richards Island and the eastern Beaufort continental shelf during the last glacial-interglacial cycle

The Pleistocene lithostratigraphy exposed on northern Richards Island comprises seven units that correlate with the offshore seismostratigraphy of the eastern Beaufort Continental Shelf. Land-sea correlations, cryostratigraphic observations and proxy indicators of environmental change provide a record of palaeoenvironmental history that commences before the last glacial-interglacial cycle. After the high sea-level stand of the Last (Sangamonian) Interglaciation, marine regression exposed a large area of the eastern Beaufort Shelf to cold subaerial conditions and permafrost aggradation. Northward progradation of a braided river system was abruptly replaced by aeolian activity, probably as a result of diversion of the palaeo-Porcupine River by the advancing Laurentide Ice Sheet during the Late Wisconsinan. Glaciation of Richards Island by the Mackenzie palaeo-ice stream was brief (sometime between ~22 000 and 16 000 cal. yr BP), and deglaciation had certainly commenced by 14 300 cal. yr BP and perhaps by ~16 000 cal. yr BP or earlier. Major fluvial erosion during the Last Glacial-Interglacial Transition incised cross-shelf valleys and formed a regional erosion surface. The Holocene marine transgression trimmed some of the erosion surface and offshore, covered it with marine deposits