Avalanche
In the backcountry, avalanches are often skiers’ number one concern. It makes sense that we are always looking for ways to reduce risk and consequence in the backcountry. Can trees assist in decreasing avalanche risk?
The answer: it’s complicated.
Trees typically run vertically through the entire snowpack, and in doing so, they can hold the snowpack in place and prevent a collapse from turning into a full-blown fracture.
With that being said, there are some limits as to how effective tree anchors can be. The general rule of thumb is that trees must be too tight to ski through to effectively anchor a snowpack.
On your average danger day, trees actually increase avalanche risk. Say you were skiing through a nicely spaced forest, and all of a sudden the snowpack fractured and you found yourself in an avalanche. Within seconds, you’d be moving at tremendous speeds, the avalanche could slam you into a tree with incredible force, causing you to sustain serious injuries and even death. About one-third of avalanche fatalities are from backcountry recreationists being slammed into trees.
Additionally, trees and other anchors (including rocks, shrubs, etc) add stress on the snowpack. As the snowpack strains against the anchor holding it in place, the stress on the snowpack increases, making a fracture more likely. This is why avalanche fracture lines tend to run from anchor to anchor. Essentially, triggering an avalanche when skiing through sparse trees can actually be more likely than on an open slope that does not have the same tension points.
Another consideration in snowpack anchor efficacy is slab stiffness. An anchor will be less effective at anchoring a soft slab, like a storm slab, than a hard slab, like a wind slab. This has to do with how much the anchor is “cemented” into the layers. An anchor in a soft slab is like pulling a stick through sugar; an anchor in a hard slab is like pulling a stick through hardened concrete.
In conclusion, while trees can help hold a snowpack in place and increase stability, generally, they are detrimental to avalanche risk.
Final myth/fact verdict: MYTH.
Even the conclusion disagrees with the article…
Question on some math in this. Studies show 24% of avy deaths are from trauma, and 75% are from asphyxiation. How can 33% of deaths be from people hitting trees if only 25% of the total deaths are from trauma?
Terrible article that does not address the main effects from forests on snowpack stability, i.e. the forest canopy disrupting snowfall and weak layer formation.
Thanks for your feedback. The purpose of this article was to examine how forest influences avalanche stability and risk in the moment of a weak layer collapse or potential fracture & carry. However, you’re absolutely right that forests influence the formation and persistence of weak layers. Since that was not the focus of the article, I decided not to mention these factors, but now that you’ve brought it up, I thought I might include those details in this comment for anyone interested.
First and foremost, forests are practically a surface hoar factory. Surface hoar is essentially frost that forms on the snow surface and forms large, feathery crystals. These crystals can gradually become loaded with new snow on top of them, and can take a lot of force as long as it’s not loaded suddenly. However, when sudden force is applied (such as a skier, cornice fall, or rapidly accumulating snow), these crystals can collapse, initiating a fracture and avalanche. The two primary factors that break down surface hoar before it has become buried are sun and wind, neither of which are often strong in forested areas. Hence, surface hoar tends to form and persist more readily in wooded areas than those on open, exposed slopes. This increases avalanche risk once snow has accumulated on top and provided the necessary ingredients for fracture initiation.
Secondly, in forested areas, weak layers can be much more stubborn and persistent. Typically, weak layers in a given snowpack will eventually gain strength through the reversal of the snowpack temperature (and in reality, vapor pressure) gradient. This happens when one of two things occurs: 1.) the snowpack becomes deep enough that the vapor pressure gradient becomes more gradual and encourages rounding of weak layers within the snowpack or 2.) when the average day/night temperatures become high enough to ease the vapor pressure gradient or even isothermalize the snowpack. Forested areas tend to block sun due to canopy cover, and hence receive less solar radiation on the snow surface throughout the snow season. Compared to other, sunnier areas, this reduces the temperature of the snow surface, but more importantly, reduces the available moisture for downward transport throughout the snowpack. The confluence of the cold snow surface and lack of surface moisture means that rounding of weak layers in the snowpack takes a.) much deeper snowpack and b.) warmer diurnal temperatures. Hence, persistent weak layers can be more persistent in wooded areas.
Hopefully, this gives a brief and easy-to-understand explanation of how trees can influence avalanche risk before one occurs. The objective of this article was not to explain that, but hopefully this comment did a decent job of explaining that in a bit more depth.
I gravely disagree with this article. I think it’s best to leave this type of thing to the experts