Interesting note. I do not know about west coast carving practices, but Iroquois carvers carve the mask ON THE TREE, then cut the tree down, continue to carve the edges and remove the mask.
What do you know... Wiki-Fu: http://en.wikipedia.org/wiki/False_Face_Society
But I was going on what my prof, a Mohawk, told me years ago...
They also make masks out of other stuff, like rolled dried corn husks: