I'd suggest wind spells such as Gust of Wind, perhaps even Control Weather so that it blows a gale and becomes overcast, splattering rain, and dark. Trees are never quite as spooky as when there's wind howling and whistling through their branches, and an undead treant should be able to use this not only as a scare tactic, but as a weapon (throwing enemies about with wind, perhaps even summoning lightning).
Actually, this sounds like the province of a druid. A druid ghost treant would be an unusual but very strongly thematic foe indeed. I think the one in Dungeon might be a druid, from memory.
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