Snow interacts with plants in many ways, and it can either harm or them plants depending on the timing, amount, and characteristics of the snow. Some of the main processes through which snow affects plants are:
- Interception (the accumulation and/or shedding of snow by plants)
- Abrasion (wearing away of plant parts from being pelted with hard, wind-blown snow)
- Insulation (snow’s ability to help keep plants warm when the air is cold)
Catching and Shedding Snow
Different types of trees and shrubs have different adaptations for responding to accumulation of snow on their branches (snow interception).
Evergreen conifer trees such as spruce, which are the predominant type of tree in boreal (northern) forests, catch and hold snow, but their branches can flex and bend under its weight, allowing them to shed the snow before too much has piled up. Their branches also slope downward, helping snow to fall off.
Deciduous trees drop their leaves before winter and therefore do not usually collect heavy enough snow loads to cause damage. When snow arrives early in the season before the leaves have fallen, however, deciduous tree limbs often break rather than bend under the added weight of snow collected by their leaves.
Shrubs are flexible, able to bend under a snow load and spring back up when the snow melts without being significantly harmed.
In general, snow interception by trees and shrubs is not overly harmful, unless the snow is extremely heavy, wet, and sticky, in which case it can cause branches to break off.
Abrasion and Flagging
Blowing snow is like a natural sandblasting system. It is a powerful abrasive that can kill branches at the snow surface, wear away tree bark, and even kill trees and shrubs. Snow grains (clusters of snow crystals bonded together) moving in the wind are essentially tiny ice rocks, and when they are driven across the surface of the snow by strong winds, these grains can wear away at branches of shrubs and trees. Wind can also cause plants to lose water, resulting in desiccation (drying out), because when temperatures are below freezing, water exists primarily in the form of snow and ice, which plants aren’t able to use.
Wind-driven snow can kill branches, resulting in trees and shrubs with few or no branches near the snow surface. Further above the surface of the snow, blowing wind carries much less snow. At this height, steady wind from one direction reduces the number and length of branches on the upwind side of trees and shrubs. “Flag” trees result when wind-blown snow damages the upwind side of a tree, preventing branches from growing on that side. The result is asymmetrical form that resembles a flag blowing in the wind.
Even in the summertime, the shape and structure of trees in a certain area provides clues to what the winter conditions are like there. The height on a tree trunk at which there aren’t many branches likely indicates the average level of the snow surface, where the tree is “sandblasted” by blowing snow. The direction in which the branches face on flag trees is a clue to the prevailing winter wind direction in that area.
Insulation and Moisture
Although plants that inhabit cold climates have evolved adaptations for surviving winter’s challenges, they still risk freezing and drying out, especially without a protective covering of snow to provide warmth and moisture. Snow can protect low-growing plants (like grasses) from cold temperatures and desiccation (drying out), and snow cover is especially important to plant survival during winters that are very cold for extended periods of time.
Because it contains a lot of air spaces, snow is a great insulator, helping to keep plants from freezing. Snow also provides moisture to plants growing underneath it, which is critical during winter, when the water that plants need to survive is frozen and therefore not available for them to use.
In the cases of grasses and forbs (non-woody, usually low-growing, flowering plants) inhabiting cold climates, the above-ground parts of the plant die in the fall, and all of the water and nutrients they contain are taken up by the roots. One reason that it is beneficial for a plant to concentrate all of its resources in its roots is that soil temperatures are generally warmer and more stable than air temperatures during the winter. Many northern environments are cold enough that plant roots, soil, and soil bacteria necessary for plant growth can still freeze, however. By providing a layer of insulation, snow reduces the risk of freezing. The soil beneath a healthy snow cover remains considerably warmer than in places where the snow has been swept away.
Death of plants (and animals) due to the challenges of winter is called “winterkill.” Two major causes of winterkill in plants are dessication and freezing, which damages cells when freezing water within them causes cells to burst. Winterkill is more likely during winters with cold temperatures and little snow to provide insulation and during winters during which temperatures fluctuate drastically.
Although snow is generally beneficial to plants, it presents challenges as well. In places where snow is deep enough to cover low-growing plants for many months, they can’t start growing, flowering, and producing seeds until the snow melts in late spring or even early summer in some parts of the Arctic. Damp conditions created by a long-lasting snow cover can also lead to the growth of snow mold, a fungus that is especially damaging to grasses.
Plants affect snow, too!
Plants aren’t only affected by snow; they also affect snow patterns and processes by influencing the location of snow drifts, absorbing heat, and even influencing the occurrence of avalanches in some areas.
Trees and shrubs cause windblown snow to be deposited on their downwind side. The trees and shrubs create a barrier that causes wind to slow down as it goes around them, causing snow crystals fall to the ground on the downwind side. In turn, low-growing plants can benefit from the accumulation of protective snow cover that results.
If you take a walk in the forest when the ground is covered in deep snow, you might notice the presence of bowl-shaped depressions in the snow around tree trunks. Because dark-colored objects (like brown or gray tree bark) absorb heat, the area around a tree’s trunk is warmer than the surroundings, causing snow to melt there. These places where snow melts around tree trunks are called “tree wells.”
The term “tree well” can also refer to much larger depressions caused when tree branches prevent snow from accumulating underneath them, resulting in the formation of a deep pit of loose, powdery snow around tree trunks. In areas like California’s Sierra Nevada Mountains, which have been known to receive up to sixty-five feet of snowfall during the winter, tree wells can be more than ten feet deep. This type of tree well poses a hazard to skiers and snowboarders, who can sustain injuries and suffocate if they fall into a hidden tree well from which they can’t rescue themselves.
Snow tends to slide downhill on a smooth surface, like smooth rock, ice, or bare ground, more readily than a rough surface, such as one covered with trees or large rocks. Although avalanches can destroy trees, under some conditions trees can act like “anchors,” helping the snow to stay in place rather than to slide. Under other conditions, however, trees can actually increase avalanche risk by weakening the connectivity of the snowpack.
Note: There are a lot of different factors that influence avalanche conditions at any one time and place. Just because an area has trees, it does not mean it is safe.