If you look carefully, you'll notice that the tree branch is not removed from the pentagram during the summoning. You need the talking tree branch from the Dark Forest, not the fake one from Mpenzak.
The alchemist in the Dark Forest also has a quest for you at the tree, which you may want to get before approaching it (the quest item is not there until you have the quest). You can talk to the tree at most twice without it turning hostile, depending on what you say to it.
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BTW, there are separate forums for Divine Divinity and Beyond Divinity.
