i have one that i'll slap onto my BC pack if its really deep.
you are a beginner, and riding with a backpack (unless you are in the backcountry where it is required for safe travel) beginner or not is going to really mess with your balance and slow your progression.
it may be the backpack that puts your ass in that treewell....
keep in mind that you have to be wearing the mouthpiece IN your mouth for it to work. you'll put an avalung in your mouth before you drop in on a wide open face you just hiked up, but whenever you dip into the trees? i wouldn't.
if you're nervous about riding in the trees its probably your self-preservation instinct because you just might not be there yet. riding in the trees requires you to keep your speed up and to think and visualize like 3 turns ahead of you. you not only have to have the skills, but you have to have the confidence that you KNOW you have the skill... an avalung isn't going to be what makes you safer and a backpack isn't worth wearing unless you have to.