To Sanctaphrax's point you could go full Swamp Thing with him, if you'll pardon the nerd reference. Current Swamp Thing is the chosen Avatar/champion of The Green, which is the manifest embodiment of all plant life on Earth, and along with the Champions of The Red (aka Animal Life) they Battle The Black/The Rot and its more death-themed champions.
So maybe your Father Tree is the embodiment of the Decay of Life, nominally as part of the Cyclic Renewal of life, but ultimately a necromantic force. It's a Death-themed thing, but very much Not Winter. You could make it a Summer thing and give it a Winter counterbalance (maybe in this instance Summer:decay::Winter:cryo-stasis?)
Another route is to have your Father Tree but make him not actually have plant/nature themes of his own Powers. The Kingkiller Chronicles, as an example, features an Evil Tree that is prescient enough that supposedly simply speaking to it is going to set you down a Butterfly-effect path that will lead ievitably to tragedy, simply because he can see all the possible branching timelines. But its not really an Evil Tree, rather it's some creature that is Possessing (or maybe imprisoned by) the tree. Some legends have Loki once getting imprisoned in a Tree. Hell, you could take him a level higher and make him some sort of walking manifestation of the World Tree itself, opening his theme up to all kinds of Cosmic, Fate, and Reality-level stuff. At that point he wouldnt need to be evil so much as overly-powerful and ignorant of modern mortal life, with some overriding Purpose that ends up looking like the corrupting effect you describe.