Yes, it's a dead tree so shouldn't have needles or enough twigs to carry enough snow.
I'm thinking an energy-draining attack, maybe one that does additional cold damage and leaves frozen solid corpses?
We've got the usual SRD cold spells to think of - cone of cold, fire shield (chill shield only), freezing sphere, ice storm, wall of ice or sleet storm.
Any of those would be handy.
Either unholy blight or blight would seem to be thematically appropriate. How about a special attack that combines both effects, doing extra damage to Good creatures, Outsiders and Plants?
Speaking of which, can they create spawn? Maybe they can only turn other plant monsters or just other treants into more of their kind?