Permafrost thaw, tundra shrubification, and changes in snow cover properties are documented impacts of climate warming, particularly in subarctic regions where discontinuous permafrost is disappearing. To obtain some insight into those changes, permafrost, active layer thickness, vegetation, snow cover, ground temperature, soil profiles, and carbon content were surveyed in an integrated approach in six field plots along a chronosequence of permafrost thaw on an ice-rich silty soil. Historical air photographs and dendrochronology provided the chronological context. Comparison of the plots reveals a positive feedback effect between thaw settlement, increased snow cover thickness, shrub growth, increase in soil temperature, and the process of permafrost decay. By the end of the sequence permafrost was no longer sustainable. Along the estimated 90 year duration of the chronosequence, the originally centimeter-thin pedogenic horizons under mosses and lichens increased to a thickness of nearly 65 cm under shrubs and trees. Snow cover increased from negligible to over 2 m. The thickness of soil organic layers and soil organic matter content increased manyfold, likely a result of the increased productivity in the shrub-dominated landscape. The results of this study strongly suggest that permafrost ecosystems in the subarctic are being replaced under climate warming by shrub and forest ecosystems enriched in carbon on more evolved soils.

<p><span>Climate warming induces shrub expansion on Arctic herb tundra, with effects on snow trapping and hence snow depth. We have used UAV-borne LiDAR and Terrestrial Laser Scanning (TLS) to investigate the impact of shrub height on snow depth at two close sites near Umiujaq, eastern Canadian low Arctic, where dwarf birch and willow shrubs are expanding on lichen tundra. The first site features lichen and high shrubs (50-100 cm), a moderate relief, and a snowpack averaging 95 cm in spring. The second site consists of lichen and low shrubs (20-60 cm), more pronounced topography, and a deeper snowpack (101 cm). Digital Terrain and Surface Models were acquired in early fall to obtain topography and vegetation height. A Digital Surface Model obtained in spring produced snow depth maps at peak depth. TLS over a 400 m<sup>2</sup> area produced time series of snow depth throughout the winter. TLS data show preferential snow accumulation in shrubs, but also preferential melting in shrubs during fall warm spells and in spring. UAV data at the first site show a strong correlation between vegetation height and snow depth, even after snow depth has exceeded vegetation height. This correlation is not observed at the second site, probably because snow depth there is much greater than vegetation height. These data show the need to reconsider some paradigms on snow-vegetation interactions, for example that vegetation does not affect snow accumulation beyond its height. </span></p>