See http://www.krcrtv.com/news/26919815/detail.html . The expert rider was physically fit and dressed properly, which become irrelevant factors after falling into a tree well.
Kindly see http://www.treewelldeepsnowsafety.com/ for more information about tree wells.
My son could have easily lost his life to a tree well last year, and the experience is one we'll never forget. I hope the posting of these sad reminders will help save a life.