Global Outlook for Ice & Snow

distributed over the landscape. Snow melt provides an important source of nitrogen for tundra ecosystems and can result in a flush of moss growth in spring. However, the accumulation of chemicals by snow can have nega- tive effects on vegetation. Although accumulations of nitrate can be potentially assimilated by mosses and re- lated plants under the snow pack, at high concentrations characteristic of areas south of the Arctic, both nitrate and sulphate can cause physiological damage to plants under the snow 54 . Just as snow has numerous effects on vegetation, veg- etation in turn exerts major effects on snow-cover dy- namics 20,55,56 . Wind can remove up to 70 per cent of the snow cover in alpine areas, as well as in polar regions and on the prairies 57 . Trees and tall shrubs reduce wind speeds and thereby affect the distribution of snow on the ground 58 . The forest canopy can trap snow, especially in mountain regions with coniferous vegetation, resulting in increased snow depth underneath the vegetation 59 . Depending on the canopy characteristics of the vegeta- tion, the opposite scenario can also occur. In dense co- niferous forests, up to 60 per cent of snow fall can be intercepted 60 by the canopy and stored on the branches of the trees. This results in a decrease in snow depth under- neath the vegetation, as much of the snow changes to gas (sublimates) or blows away before it falls to the ground 61 . Snow that reaches the ground has been “filtered” by the canopy and is less dense than that in open areas. Vegetation also affects the amount of snow precipita- tion and the rate of snow melt. Trees and shrubs affect surface albedo – for example, black spruce can inter- cept up to 95 per cent of incoming radiation. Trees and shrubs thus increase local temperatures that affect snow

fall, thereby indirectly moderating the amount of snow precipitation. The presence of a forest canopy generally slows the rate of snow melt (up to three fold) because it reduces net radiation and wind speeds 62 , while a shrub canopy slightly increases the rate of snow melt. Snow within shrub canopies is deeper and less dense, which reduces heat transfer through the snow pack and in- creases winter soil temperatures by 2 ºC relative to adja- cent shrub-free tundra 63 . In spring when the snow starts to melt, the contrasting albedos of the vegetation and the snow enable the vegetation to transfer heat to the ground, resulting in local melt which creates holes in the snow around vegetation. The effect of future snow regimes on vegetation will in- volve complex interactions between changes in the du- ration of snow cover and changes in snow depth. The timing of snow cover has effects on the productivity of ecosystems. For areas of seasonal snow cover, the snow- free period in summer determines the length of the po- tential growing season for plants and thus ecosystem and net primary productivity 64 (Figure 4.8). The timing of the spring melt has a great impact on productivity as, in the Arctic, leaf production occurs relatively late in the season following thaw when the amount of solar radia- tion received is already at its maximum or declining. At an alpine site, productivity was decreased by 3 per cent for each day that snow melt was delayed 65 . In contrast, the timing of onset of winter snow has less influence on productivity as it comes at a time when solar angles are low and potential plant production is also low. Impacts of projected snow-cover changes on ecosystems

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GLOBAL OUTLOOK FOR ICE AND SNOW