To that end, perhaps there's a Flamebreak-type sweeper that hits players, too? You want an opponent who is trying to beat the Tree in wall-mode to be punished for overextending.
I think Slagstorm is the closest, even though it's modal.
Volcanic Fallout is probably the next best, since it can't be countered.
"The Ancients teach us that if we can but last, we shall prevail."
—Kaysa, Elder Druid of the Juniper Order