Yep those are bald face hornets. They are actually wasps in spite of their name. However they are the biggest and meanest around. I've been stung in the face by them before as well and for no good reason. I was just standing around and then bam, one lands on my face and stings me. Now I have a hatred for them, although they do eat tent caterpillars which eat my trees.
If I were you I'd get a pressure washer and blast that nest and then run like hell. Another trick is WD-40, I've discovered it kills wasps and hornets nearly instantly. WD-40 would probably hurt your tree though. Another technique I've used when it's cold and they are inactive is simply to spray expand-o-foam in the entrance of their nest. Not only does it block the opening of the nest nearly instantly it expands to fill the rest of the nest engulfing and suffocating all of the occupants.