Treebeard is an old tree-man. Groot is a shapeshifting, super strong (relative to size) wooden man with regeneration abilities.
In fisticuffs, Groot wins. In a battle of wits, Treebeard wins. In anything else, it's Batman with prep time.
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Treebeard may be old, but he took on hundreds of orcs at the orc's base while orcs were trying to set him on fire.
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Groot took on a fair amount of people with laser rifles and an other such thingies, which are arguably more dangerous than orcs with torches and swords.
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That and surviving a crash from high altitude.
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Well, it's debatable if he did survival. I'm pretty sure that his butt was quite broken, at least. Shattered, really.