Why not? The gloss has "root" right in there. Not "root, but only one you can eat". Not "root, but only one from a small plant".

Botanically speaking, out of the four examples we know from TNK ('anyan 'oQqar "onion", gharlIq 'oQqar "garlic", patat 'oQqarmey "potatoes", and qe'rot 'oQqar "carrot"), only carrots are technically roots, and only potatoes are technically tubers. Garlic and onions are bulbs. (And bulbs and tubers are apparently modified stems. This is all very complicated.) I don't think 'oQqar is a very botanically-strict term.

Based on the glosses "root" and "tuber", and the four examples from Earth vegetables, it seems like the meaning of 'oQqar is mostly "part of a plant that's underground and maybe you can eat it". There's no reason it couldn't apply to a tree root.