Wen it comes to mountain safety, the most obvious hazard that comes to mind is the threat of an avalanche. Often times the severity of tree well hazards is overlooked, but they can be just as deadly, or potentially more-so than avalanches, due to complacency and lack of knowledge. Even the potential to encounter tree wells on in-bounds gladed terrain should be taken seriously. All skiers and boarders planning on tree skiing or riding should familiarize themselves with proper safety measures and precautions before proceeding. 

Tree wells are dangerous due to snow-immersion suffocation (SIS).  A tree well forms when snow accumulates around the base of a tree, but not under the lower branches or around the trunk. This results in a hole forming around the base of the tree, which gets progressively deeper as the snowpack height increases. Often times, a skier will fall and wind up head first down a tree well. If there is snow on the tree branches, the skier's fall and contact with the tree will often cause the snow to fall down on top of the skier and bury them in the hole. This snow, much like an avalanche, can trap the skier and leave them without fresh air to breathe, thus leading to suffocation.

Most tree well/SIS accidents have happened during or just after big snow storms or storm cycles. In general terms, the more fresh snow, the higher the risk. 

• Assume all trees have tree wells under them, even seemingly small ones. Tree wells are not easy to identify by sight as you are skiing due to the lower hanging branches obscuring the hidden holes beneath.
• Always ski with a buddy. 90% of people involved in tree well/SIS hazard research experiments could NOT rescue themselves.
• Maintain visual and verbal contact with your partner at all times on gladed runs. If you can't see or hear them, stop, wait and re-establish contact. If you cannot immediately find them, assume they are in a tree well.
• In larger groups, have established check-in points (ex, a cat track, natural benches in terrain or the slope), to maintain close proximity on the run. If you are too far down the run or at the chair, you will not be any help to your friend who is stuck a few hundred feet uphill in a tree well. With SIS, time is crucial, and deep snow is difficult to make your way uphill in. 
• Attaching a whistle to your jacket zipper may also prove useful in getting your partner’s attention should you end up in a tree well.

• Try to grab at the branches, the trunk, or anything to prevent you from sliding further into the hole.
• Do not struggle after you have fallen in, it will only lead to more snow falling on top of you and you sinking further in the hole.
• Try to make a space for breathing or an air pocket around your face and mouth. A pocket of air can buy you precious time to wait for your partner to make their way to you.
• Yell or whistle to get your partner’s attention, and try to stay as calm as possible to preserve air.

• If you cannot immediately locate your missing partner, call ski patrol and identify your partners last seen location. Then begin searching for them while you wait for patrol to arrive.
• If you and your partner are wearing beacons, switch to search mode and locate them using the same techniques you would as if in a slide.
• Try listening for whistling or shouting. If you can visually see your partner’s tracks, follow them.
• Do not try to pull your partner out the way they fell in. Instead, determine where their head is and tunnel in from the side.
• When tunneling directly for the airway, be careful not to knock more snow into the hole. Continue expanding the tunnel to the airway until you can pull your partner out. Efficient “strategic shoveling techniques” with multiple rescuers is very useful, such as conveyor type methods as taught in AST 1 courses (Avalanche Skills 1).

For more information about SIS and tree well safety, visit www.deepsnowsafety.org/index.php/ 

Source: Whistler Blackcomb