The (changing) ecology of snow

We are well into winter in the Northern Hemisphere--Thanksgiving holidays are just around the corner in the US--and for much of the area, this is time defined by cold, dark, and snow. So it seemed appropriate to write a post about snow.

Much of the Northern Hemisphere, the Antarctic, and alpine areas are historically snow covered for more than two months of the year. In parts of the Arctic, snow cover may last 9 months of the year.

From Marchand 2014.

Though only a few degrees different from rain, snow alters an ecosystem through its unique physical properties. It is a physical force, an ecological pressure, and an opportunity: snow alters movement, creates and destroys habitat, and places immense pressures on individuals and species. The ecological implications of snow are immense and wide-ranging.

Snow has unique physical properties that make it particularly important, compared to equivalent amounts of precipitation. It stores energy and water. It insulates the soil underneath it, buffering it from cold temperatures and slowing its eventual re-warming. Snow limits light to the plants underneath it, reducing photosynthesis, and drastically cutting primary productivity during its stay. In addition to these physical properties, the sheer weight of snow has to be considered. Plants beneath a pile of snow risk compression, breakage, and deformation. 

A heavy blanket of snow, variable in its depth and consistency, changes the matrix and thus significantly alters movement: snow can ease dispersal, make it much more costly, or even prevent it altogether. Ease of movement in snow is in turn is tied to foraging and predation success. For example, small, lightweight vertebrates such as shrews become active underneath the snow, tunnelling in search of food and constructing nests under deep cover. For them, snow cover may aid winter survival. On top of the snow, some animals (hares, fox, etc.) enjoy ease of movement. If individuals are light enough to travel over the top of the snow, snow can reduce landscape complexity, burying brambles and filling hollows. However, for larger species, snow may come at a cost. Moose or reindeer for example, with their large masses and long, slender legs are at risk when snow depths are too high or a hard crust covers the snow. In these conditions, they may sink, slowing their escape from lighter predators.

Snow is difficult for all life, but plants in particular cannot escape. Places with long winter seasons and late snow melt filter out all but the most adapted vegetation. The plants common to Arctic and alpine areas share many life-history traits. Similar groups of species - mosses, lichens, low-growing shrubs, and grasses - are found in all of these areas. Such species have developed strategies for the conditions, such as seed germination cued by freeze/thaw cycles, small leaf areas to reduce water loss. There are also opportunities. Snow insulates – plants may benefit from burial under drifts, or from collecting snow in dead tissue above ground. Adaptations may permit early growth under thinning snow in the spring, by allowing photosynthesis in cold conditions and low light. 

The type, amount and depth of snow-cover may differentiate plant communities on a fine scale, between wind-exposed ridges where drought tolerance is necessary, and snow-accumulating depressions where tolerance of short growing seasons is required. Early naturalist literature recognized these snow-driven micro-differences, describing them as “schneetälchen” or little snow valleys. The Front Range of Colorado has been used for a number of studies of snow gradients on vegetation, and while many environmental factors vary along snow-melt gradients, the timing of snow melt alone greatly affects species presence and abundance.

Diagram of micro-habitats in alpine areas in Colorado,
where snow affects vegetation dynamics.

Distribution of 2 species in relation to snow depth. Both from Walker et al (2001)

Snow can be a significant source of water, sometimes the majority of water necessary for the year. It is also a sink for nutrients (N, S) from the atmosphere, the canopy, and the soil – leading it to sometimes be called ‘poor man’s fertilizer’. This isn’t always for the best – high concentrations of N and S in snowmelt can damage plant tissues, and snowdrifts can be reservoirs for airborne pollutants. In samples from the Athabasca River (Alberta, Canada), upstream of oil sands facilities, dissolved polycyclic aromatic compound levels averaged between 0.025-0.03 ug/L. However, measures from melting snow around the river had concentrations up to 4.8 ug/L, suggesting that spring snowmelt could have large environmental impacts. 

Snow algae (most common species: Chlamydomonas nivalis)
growth colours snow various shades of pink or red.

These nutrients in snow support unique microbial life. Snow algae, bacteria, yeasts and snow fungi arise. Snow algae are adapted to a life history spent wholly within melting snow – these algae find homes in glaciers, alpine peaks, and the dry valley lakes of Antarctica. These species have various adaptations to a snow-bound life, including enzymes resistant to freezing, and special pigments. These unique populations help replace some of the lost winter primary productivity. Small invertebrates graze on snow microbes, and the energy flows into local food webs.

So snow is unique, with wide-ranging implications for ecology and evolution where it occurs. But now snow is changing. Warming temperatures are having widespread and disastrous effects on snow ecosystems around the world. Arctic and alpine systems are among the most vulnerable regions to climate change, and the effects are already showing. Changes in the snow cover, depth and the timing of snowmelt are altering plant phenology and fitness, and restructuring local communities. The effects of changes in snow may be incredibly complex, may interact or be independent from changes in temperatures, and thus are difficult to predict. Some effects of changes in snow (and ice) are already obvious – for example, Glacier National Park in the US is likely to be glacier-less in 30 years. Polar bears, so reliant on their frozen habitat, face a very difficult future. But other effects are very subtle, and may take years to be fully recognized. For example, changes in snow conditions may decrease dispersal and gene flow between Canada lynx (Lynx Canadensis) populations which occur at different ends of a winter climate gradient. With declines in gene flow, the lynx may separate into two increasingly (ecologically?) distinct groups. Shrub encroachment in the low Arctic is a prime example of the complexity of changes in snow. Data shows that shrubs are increasing in abundance in the Arctic over the last 50 years. The snow-shrub hypothesis suggests that this is - indirectly - an outcome of increased snowfall in these regions (though note that warming temperatures are also causing this snow to melt earlier). Shrubs in the region accumulate larger amounts of snow in their branches, resulting in greater insulation and warmer soil temperatures. These warming temperatures encourage greater microbial activity, and perhaps enhance mineralization. The species most able to take advantage of these altered soils are, in fact, more shrubs. All of these examples are reminders that snow - and the organisms adapted to places full of snow - is changing.

References:

Ernakovich, J. G., Hopping, K. A., Berdanier, A. B., Simpson, R. T., Kachergis, E. J., Steltzer, H. and Wallenstein, M. D. (2014). Predicted responses of arctic and alpine ecosystems to altered seasonality under climate change. Global Change Biology, 20: 3256–3269. doi: 10.1111/gcb.12568

Jones, H.G. and Pomeroy, J.W. The ecology of snow-covered systems: summary and relevance to Wolf Creek, Yukon. In Wolf Creek Research Basin: Hydrology, Ecology, Environment (1999), pp. 1-15.

Kelly, E.N. , et al. (2009) Oil sands development contributes polycyclic aromatic compounds to the Athabasca River and its tributaries. PNAS, 106 (52) 22346-2235.

Larose, C., Aurélien Dommergue, and Timothy M. Vogel. (2013). The Dynamic Arctic Snow Pack: An Unexplored Environment for Microbial Diversity and Activity. Biology, 2(1), 317-330.

Marchand, P.J. (2014). Life in the Cold: An Introduction to Winter Ecology, fourth edition. University Press of New England.

Parmesan, C. (2006) Ecological and Evolutionary Responses to Recent Climate Change. Annual Review of Ecology, Evolution, and Systematics, Vol. 37, pp. 637-669

Pomeroy, J. W., and Eric Brun. "Physical properties of snow." Snow ecology(2001): 45-126.

Row, J.R., et al. (2014). The subtle role of climate change on population genetic structure in Canada lynx. Global Change Biology, doi: 10.1111/gcb.12526.

Sturm, Matthew, et al. "Snow-shrub interactions in Arctic tundra: a hypothesis with climatic implications." Journal of Climate 14.3 (2001): 336-34

Walker, D. A., W. D. Billings, and J. G. De Molenaar. (2001) "Snow–vegetation interactions in tundra environments." Snow ecology: an interdisciplinary examination of snow-covered ecosystems  266-324.

http://www.nytimes.com/2014/11/23/us/climate-change-threatens-to-strip-the-identity-of-glacier-national-park.html