I did think of that. Tolkien also had storm giants of stone. The Ring of Doom was mutagenic, strong enough to affect even adults; the orcs were mutated elves; and the soil from Lothlorien was mutagenic in a more positive way.

If I were going to do something with mutated and/or super-powered trees, I would probably look at things that trees actually do and just ... soup it up. Like alder groves are already near-immortal; with true immortality and/or regeneration they could withstand fires. Redwoods are terraformers; not far to weather control. Bristlecone pines are fireproof, and another species that's extremely long-lived; fire powers are plausible. Willow roots can break through damn near anything; that's super-strength. Sugar maples can warn each other about insect attacks; telepathy.