DominatorJacquesPhones and whistles won't do you any good cus you won't be able to get to them.
Tree wells scare the piss outta me but I disagree partially with this statement. The more resources the better.
Attach the whistle to your backpack strap. Totally accessible unless your hands are somehow immoble. Also many packs have built in whistles on the chest strap.
I agree on phones. Gonna be difficult to get to and if you did get to it, there's no telling if it will be dead or if you've got service. Although, rocking headphones (assuming they don't get ripped out), "OK Google, call 911" could work. There was a story about a snowboarder a few years ago who managed to call ski patrol and they came and dug him out. Totally lucky though since air doesn't last long.
Anyone have thoughts about avalung? They kinda seem perfect for tree wells to give you a little more time to get dug out before suffocating.
OP definitely don't ride alone. There's probably always someone on the lift or nearby who you can tag along with in the trees if you so desire. It's nearly impossible to free yourself if you fall headfirst into a tree well which is most likely the way you would fall.
Here ya go OP: http://www.deepsnowsafety.org/