my guess is that linux starts off with one main file and braches off to all the other stuff like module loading, disk access, to plug and pray stuff.
however, if you look on the web for 3d representations of the kernel ( check slashdot for past linux stuff ) you will see it looks more like a nebula in space, but then if you look on think geek for the linux kernel source poster you will see they have shown it in circles and little line drawings for each file
but it is still called a linux tree no matter what