Did you talk to the alchemist to the west of there? He also wants something from that area, and you only have two shots at getting close to the tree without being attacked.
Did you talk to the elves in the area about the tree? I ran into them just north of the tree, but the cut scene probably has a larger trigger area. They mention the tree is afraid of wood cutters....
spoiler: [color:"#3A495A"]IIRC you have to ask for a branch first, then tell the tree there are some wood cutters nearby, but you would not tell them about it, since you were getting along so well. After this you can get close again once by saying you were just leaving.[/color] (highlight the text to read)