Remember the cutscene conversation where the elves near the talking tree stated that the tree was dangerous and could not be cut down by normal means, but had heard the other trees talking about dwarven woodcutters, and was afraid of them? If not, you didn't go through all the speech options. I believe the alchemist also gives you this information about the tree.
Anyway, IIRC to approach you have to say something like 'I just wanted to see if you were still here', followed by a something about dwarven woodcutters being in the area. I'm not sure about the opening, but mentioning the woodcutters is the key. Quick save as you get near the tree, and you will be able to check different speech options.
You can only approach the tree twice (the second time you need to say something like 'sorry, I was just leaving'), after which there are no options for a peaceful encounter. There are about 10 or 12 zombies around the tree. If you can kill them in a couple hits and keep moving, it should be easy to avoid the tree's fireballs, which stop once the zombies are all dead. I killed the tree shortly before the blessing ceremony, but that could be difficult the first time you get to the tree.
