Tree wells and SIS kill skiers and snowboarders every year.  They are truly the devils work and account for 20% of all ski deaths!  Most Aussies aren’t even aware what a tree well is little lone worry about being killed by one.   When the snow falls around trees it creates a well around the base of the trunk where the lower branches shade the trunk which gets progressively deeper as the snowpack increases.
Skiing in Australia we are not used to, or really even aware of tree wells.  A mountain snow gum would hardly create a well big enough to worrry an endangered pigmy possum.
However in North America where you can get a masses of fresh powder snow falling in one day around a 70 foot Hemlock tree creating a snow well meters in depth.
The problem is that if you are skiing or snowboarding through the trees and you accidentally end up in a tree well, usually head first, you can suffer SIS – Snow Immersion Suffocation.  Basically you drown in snow.  Once you are in head first, the only way out is to be rescued.
90% of people involved in tree well SIS hazard research experiments could not rescue themselves so chances of surviving alone are minimal.

Tips on lessening your risk of SIS

Stay out of the woods little red riding hood!
Stick to the runs, or if you must, Ski or Ride with a mate when going off piste or through the trees.
Attach a whistle to your ski jacket to attract attention. (it may look a little dorky but it could save your life)
Carry a transceiver if you have one handy
Don’t use your pole wrist straps when skiing in the trees.

Here are some tips to survival if you do find yourself in a tree well.

Remain Calm… easier said than done, but if you panic you will only disrupt the snow further causing more powder to fall in burying you deeper.  Put your energy into being cool and thinking clearly.
Don’t unclip your skis / board unless your are certain you won’t slip further.
Be a Tree Hugger
Grab onto the tree trunk or any part you can reach. The idea is to cling on so you don’t fall further down into the well.
Give yourself some breathing room
Try to make a pocket of air amid the snow surrounding you to prevent the falling snow suffocating you.
Escaping the void
If you are alone then at some point you will need to try to make your own way out.  If you have the energy you can try to escape on your own. It will take time and patience.
Use slow rocking movements with your body.  This will create space around you and make the snow a little more compact making the climb out easier, but this will take time. If you have enough room try to get upright (Most tree well victims are stuck upside down).
If you are close enough, use the tree to climb up and out of the well.
Here are some resources on SIS and how to survive.  Let’s hope you will never need it, but it’s good to know.

Stay safe out there peoples!