Some day in the distant future, I'd like to make the engine foliage friendly. By this I mean have different levels of details for trees, bushes, and the like, and turn them into billboards at a distance to conserve polygons, allowing for things like woodsball fields with lots of trees. I believe Far Cry uses this method.
As for the md2 limit -- maybe 4096? 8192? Some power of two, probably. If you're hitting the limit, though, it probably won't play well in game. You should really be using transparent textures for small branches and leaves instead of modeling them. If blender has an SMD exporter, you could create SKM models (what's used for the players). I don't think those have a hard limit on vertices. Still not a good idea to be using thousands of them, though, for a single tree.
