Grizzly vs Black Bear

So today I had the most amazing wildlife encounter. I was driving a bus full of people on the David Thompson highway in Alberta, Canada and got stopped for an hour due to a motorcycle accident that happened when a biker stopped to see a bear on the side of the road and was hit by another biker. (a lot of road rash and maybe some broken bones but they survived). Turns out the little black bear was in the tree because a huge grizzly bear was at the bottom of the tree trying to get him (or her?). We watched the poor little black bear hang on for dear life and make a number of attempts to get away. I had to leave before the end of this encounter and I can only hope the black bear made out okay although his chances did not look good.

Update: August 29, 2012.
I've had some feedback from Parks Canada staff and others (see comments below)
It seems most likely that the black bear was never in any serious danger at all. The grizzly bear probably had a carcass (deer or elk) in the bushes and the black bear kept trying to get a piece for itself. Of course, the grizzly bear was having none of it as you can see in the video. So, it turns out the grizzly was just keeping the black bear away from its food.and not stalking the black bear at all.