The tree takes up less "floor area", but it takes up considerably more volume because it is significantly taller. That's why, in most places, you probably don't want to replace large panels with a single tree, but with a forest of mini-trees.

Trees also have a "concentration of weight" problem, which can be an issue for roof-top installations. The entire weight of the tree is on its trunk, which has a lot smaller cross-section than a comparable panel. Roofs that can handle weight that is spread out often have problems with concentrated weight. Again, the solution is a forest of mini-trees.

Note that the optimal spacing for solar-panel mini-trees is different from tree spacing in nature because nature's trees aren't just trying to optimize energy collection, they're also trying to crowd out their neighbors.