distributed over the landscape. Snow melt provides an fall, thereby indirectly moderating the amount of snow
important source of nitrogen for tundra ecosystems and precipitation. The presence of a forest canopy generally
can result in a flush of moss growth in spring. However, slows the rate of snow melt (up to three fold) because it
the accumulation of chemicals by snow can have nega- reduces net radiation and wind speeds
62
, while a shrub
tive effects on vegetation. Although accumulations of canopy slightly increases the rate of snow melt. Snow
nitrate can be potentially assimilated by mosses and re- within shrub canopies is deeper and less dense, which
lated plants under the snow pack, at high concentrations reduces heat transfer through the snow pack and in-
characteristic of areas south of the Arctic, both nitrate creases winter soil temperatures by 2 ºC relative to adja-
and sulphate can cause physiological damage to plants cent shrub-free tundra
63
. In spring when the snow starts
under the snow
54
. to melt, the contrasting albedos of the vegetation and
the snow enable the vegetation to transfer heat to the
Just as snow has numerous effects on vegetation, veg- ground, resulting in local melt which creates holes in
etation in turn exerts major effects on snow-cover dy- the snow around vegetation.
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 Impacts of projected snow-cover changes
speeds and thereby affect the distribution of snow on the on ecosystems
ground
58
. The forest canopy can trap snow, especially in
mountain regions with coniferous vegetation, resulting The effect of future snow regimes on vegetation will in-
in increased snow depth underneath the vegetation
59
. volve complex interactions between changes in the du-
Depending on the canopy characteristics of the vegeta- ration of snow cover and changes in snow depth. The
tion, the opposite scenario can also occur. In dense co- timing of snow cover has effects on the productivity of
niferous forests, up to 60 per cent of snow fall can be ecosystems. For areas of seasonal snow cover, the snow-
intercepted
60
by the canopy and stored on the branches of free period in summer determines the length of the po-
the trees. This results in a decrease in snow depth under- tential growing season for plants and thus ecosystem
neath the vegetation, as much of the snow changes to gas and net primary productivity
64
(Figure 4.8). The timing
(sublimates) or blows away before it falls to the ground
61
. of the spring melt has a great impact on productivity as,
Snow that reaches the ground has been “filtered” by the in the Arctic, leaf production occurs relatively late in the
canopy and is less dense than that in open areas. season following thaw when the amount of solar radia-
tion received is already at its maximum or declining. At
Vegetation also affects the amount of snow precipita- an alpine site, productivity was decreased by 3 per cent
tion and the rate of snow melt. Trees and shrubs affect for each day that snow melt was delayed
65
. In contrast,
surface albedo – for example, black spruce can inter- the timing of onset of winter snow has less influence on
cept up to 95 per cent of incoming radiation. Trees and productivity as it comes at a time when solar angles are
shrubs thus increase local temperatures that affect snow low and potential plant production is also low.
50 GLOBAL OUTLOOK FOR ICE AND SNOW
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