Impacts of projected snowcover changes on ecosystems

The effect of future snow regimes on vegetation will involve complex interactions between changes in the duration 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 potential growing season for plants and thus ecosystem and net primary productivity64 (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 radiation 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 delayed65. 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.

The increased snow cover that is predicted in some northern areas as temperatures rise will affect both ecosystem structure and function. Long-term experimental increases in snow cover affected species abundance, height of the vegetation, and diversity of plant functional types in the Alaskan tundra40. The increase in snow cover had a greater impact on vegetation than experimental summer warming, partly because insulation by increased snow in winter caused higher soil warming than increased air temperatures. In the subarctic, an experimental doubling of winter snow cover on a peat moss bog increased air and soil temperatures and strongly increased moss growth41. This increase in moss growth could increase the carbon sink effectiveness of northern peat lands in areas where snow depth increases.

Figure 4.8: The duration of snow cover is a major determinant of the length of the growing season for plants. Shown here is a persistent snow patch in western Greenland. With increasing distance from the centre of the snow patch, the growing season becomes longer, and thus plant communities become more developed and productivity increases. Photo: Terry Callaghan

More frequent winter thaws can also affect ecosystems. Thawing changes the mechanical properties of snow dramatically. This can reduce the insulating properties of the snow cover, with increased potential for frost penetration into the soil and root damage to certain plant species. During the brief thaw, soil microbial activity may also release greenhouse gases. This occurs at a time when plant uptake of carbon, which could offset the increase in atmospheric carbon, is not possible, and adds to atmospheric concentrations of greenhouse gases. In addition, re-freezing occurs after thawing, which forms ice layers that can be on the surface, throughout the snow cover or, if snow falls after a thawing event, under the snow in contact with the ground. Ice layers can act as a mechanical barrier, preventing herbivores such as musk oxen66 and reindeer from digging through the snow to reach critical lichens and other forage (see box on the snow-loving deer of the Arctic). This greatly affects their health in winter and can determine their survival67. Ice layers may also inhibit the diffusion of organic compounds that reindeer possibly use to detect food68. Presence of ice layers affects the survival of other animals such as voles as well69. Ice layers can act as a barrier to small mammals accessing shelter, food, nests and protection from predators.

Snow cover in mountain regions is a critical source of freshwater; changes in snow cover could have indirect effects on ecosystems due to changes in availability of these water resources. One potential effect is increased intensity and size of wildfires because of moisture stress on mountain forests. There could also be impacts on anadramous fish, which require high stream flow for their migration to the ocean after hatching in fresh water.

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