How Snow Falling on Pines Changes the Forest

Snow is a frequent visitor where I live now, and for the most part, I am delighted by it. Since I enjoy getting my work done, I am less enthusiastic when heavy snowfall turns into a snow day. Still, there is something wonderfully delicate about snow as it drifts quietly from the sky. I love its clean, crisp appearance, its brightness and the remarkable silence that comes with it. I also appreciate how freshly fallen snow reveals the movements of deer, turkeys, bobcats, coyotes and mountain lions passing around my home, and soon enough, the bears as well.

Snow on the Conifers

What I especially enjoy is seeing snow settle on the conifer trees. A tiny spruce made plump beneath a blanket of snow or a towering ponderosa carrying a heavy white load are among the sights I missed most during the years when I was away from Colorado.

One of the pleasures of living among pine trees comes the morning after a snowfall, particularly when sunlight arrives. That is when you can watch the snow disappear from the branches. At times, the effect resembles steam, which is essentially what it is: intense mountain sunlight causes sublimation, a process in which snow changes directly from solid ice into water vapor.

At other times, the trees seem to shake themselves free. Snow accumulated on higher branches can suddenly burst outward, forming miniature avalanches triggered by a breeze or the movement of a bird. The resulting cloud of powder drifts downward, settling onto lower branches, the earth, my dog and me.

The Science Behind Snow Interception

I wanted to discover whether there was a scientific name for this process or a particular explanation for why it occurs. Instead, I found something even more interesting. Those little clouds of vapor and snow powder deserve our attention because snow caught in trees can have an important influence on mountain snowpack.

Snow that becomes lodged among conifer needles and branches is an important part of the mountain water system, and the Forest Service has been investigating this phenomenon since at least the 1960s.

According to a 1966 Forest Service research paper, snow can disappear from the canopy before reaching the ground for five primary reasons: vapor released from melting water, vapor originating from snow itself, meltwater flowing or dripping from stems, snow sliding from branches and wind removing and carrying away intercepted snow. In simpler terms, that means sublimation, dripping, sliding and wind.

What Interception Means

This process is known as interception, referring to the ability of conifer trees to catch and temporarily retain falling snow. One study found that mature conifer forests could intercept as much as 60 percent of snowfall.

A significant portion of that intercepted snow can sublimate rapidly. In other words, instead of becoming meltwater and entering the soil or feeding streams, the water goes directly into the atmosphere. This matters because mountain snow is an important source of water for much of the American West.

Even when the snow does not disappear through sublimation, the powdery clouds that I enjoy watching can account for the removal of another 30 percent of the snowfall. The remaining portion eventually leaves the tree canopy as meltwater.

Snow Beneath the Forest Canopy

It makes sense that the amount of “snow water input” is generally lower directly beneath a tree canopy. Naturally, a clearing or meadow should receive more falling snow than an area covered by branches.

However, a 2015 study found that snow water input along the edges of a canopy can be 30 to 40 percent greater than directly beneath the trees. This shows just how strongly forests can influence where snow ultimately accumulates.

Dense forests, therefore, can keep substantial quantities of snowfall from ever reaching the ground, where that water could otherwise contribute to the mountain water supply. The powdery snow clouds and hazy trails of sublimation that I casually watch are actually altering the water cycle around me.

Climate Change and Future Snowfall

That water cycle will change further as the planet’s climate continues to warm. Exactly how those changes will appear, however, remains uncertain. The outcome could be beneficial in some ways or harmful in others.

For example, wetter snow may not produce the same dramatic powdery bursts from tree branches. Instead, it could simply slide or fall away, altering how snow accumulations between branches settle and how they later melt toward tree roots and the water table. As temperatures increase, wetter snowfall is expected to become more common.

There is even a specialized area of physics devoted to understanding how snow crystals develop under different temperature conditions and how the shapes of those crystals influence their ability to accumulate on surfaces and eventually melt.

Will More Snow Reach the Ground?

If wetter snow falls from trees more quickly, forests might intercept a smaller proportion of total snowfall. That could mean more water reaches the ground and eventually soaks into the soil.

But there is another possibility: perhaps less snow will fall altogether.

Every year, I watch the snowpack charts for the Colorado River Basin with increasing concern. If only a small amount of snow falls and trees capture a large share of it, even less water may ultimately make it to the ground.

A Simple Snow Experiment

The next time snow arrives, I plan to go outside and take some measurements. I want to conduct an experiment of my own. The enormous ponderosa in my front yard will probably catch a substantial amount of the snowfall, but I would like to determine exactly how much.

I am not currently concerned about my own water supply, but I am curious enough to want to understand what is happening around me. Regardless of what the measurements reveal, I know that I will look at those bright, hazy mornings and the little clouds of powder falling from the trees with a different perspective from now on.