How can I reach the massive tree in the south, near Bald Mountain

This is a location you can only visit during the main quest. It's a pretty late quest, closer to the end of the game.

As far as I know you can't get there before the quest. I did read about a few people managing to get there, but running into glitches as this wasn't intended. I would just ignore that location, it'll come up at some point during the main quest.