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With climate warming, shrubs have been observed to grow on Arctic tundra.Their presence is known to increase snow height and is expected to increasethe thermal insulating effect of the snowpack. An important consequence wouldbe the warming of the ground, which will accelerate permafrost thaw,providing an important positive feedback to warming. At Bylot Island(73° N, 80° W) in the Canadian high Arctic where bushes ofwillows (<i>Salix richardsonii</i> Hook) are growing, we have observed thesnow stratigraphy and measured the vertical profiles of snow density, thermalconductivity and specific surface area (SSA) in over 20 sites of high Arctictundra and in willow bushes 20 to 40 cm high. We find that shrubs increasesnow height, but only up to their own height. In shrubs, snow density,thermal conductivity and SSA are all significantly lower than on herb tundra.In shrubs, depth hoar which has a low thermal conductivity was observed togrow up to shrub height, while on herb tundra, depth hoar only developed to 5to 10 cm high. The thermal resistance of the snowpack was in general higherin shrubs than on herb tundra. More signs of melting were observed in shrubs,presumably because stems absorb radiation and provide hotspots that initiatemelting. When melting was extensive, thermal conductivity was increased andthermal resistance was reduced, counteracting the observed effect of shrubsin the absence of melting. Simulations of the effect of shrubs on snowproperties and on the ground thermal regime were made with the Crocus snowphysics model and the ISBA (Interactions between Soil–Biosphere–Atmosphere)land surface scheme, driven by in situ and reanalysis meteorological data.These simulations did not take into account the summer impact of shrubs. Theypredict that the ground at 5 cm depth at Bylot Island during the 2014–2015winter would be up to 13 °C warmer in the presence of shrubs. Suchwarming may however be mitigated by summer effects

The growth of shrubs on high Arctic tundra at Bylot Island: impact on snow physical properties and permafrost thermal regime