Snow-shrub interactions in Arctic tundra: A hypothesis with climatic implications

In the Arctic, where wind transport of snow is common, the depth and insula tive properties of the snow cover can be determined as much by the wind as by spatial variations in precipitation. Where shrubs are more abundant and larger, greater amounts of drifting snow are trapped and suffer less loss d ue to sublimation. The snow in shrub patches is both thicker and a better t hermal insulator per unit thickness than the snow outside of shrub patches. As a consequence, winter soil surface temperatures are substantially highe r, a condition that can promote greater winter decomposition and nutrient r elease, thereby providing a positive feedback that could enhance shrub grow th. If the abundance, size, and coverage of arctic shrubs increases in resp onse to climate warming, as is expected, snow-shrub interactions could caus e a widespread increase (estimated 10%-25%) in the winter snow depth. This would increase spring runoff, winter soil temperatures, and probably winter CO2 emissions. The balance between these winter effects and changes in the summer energy balance associated with the increase in shrubs probably depe nds on shrub density, with the threshold for winter snow trapping occurring at lower densities than the threshold for summer effects such as shading. It is suggested that snow-shrub interactions warrant further investigation as a possible factor contributing to the transition of the arctic land surf ace from moist graminoid tundra to shrub tundra in response to climatic war ming.