If you could populate the trees on a virtual ground that is initially the same as the actual ground, and animate the virtual ground in a way that would tilt away from the avalanche, and if you have rotation to ground set up properly, theoretically you could have them fall over to one side. I'm thinking aloud now, mind you.
Of course the tilting will have to be done on a small scale in lines perpendicular to the avalanche's path, or the front trees will dive underground, so you might need a sinus and a twist and shear shader to make some sort of ripples/waves away from the avalanche.
The virtual ground itself shouldn't be visible, by the way, so you need another planet with the same ground to sit your trees on.
If you get my point.....
