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Assuming that your concern is the tree roots themselves (and the uneven ground / heaving of the concrete that can come with these roots) you"could" do what I did and build a "sandbox" out of treated lumber (you could cut the bottom edges to fit over tree roots as required to keep the top profile level) and fill with crushed unwashed gravel.
Personally, I would build a circular offshoot of the deck around the hot tub in between the 2 trees with the tub sunk in aroung the deck. Put some pea gravel on the ground and go from there. Take out the railing lit up by the sun to access the new deck and if you really wanted to get fancy, put a railing/privacy fence around the new deck. Just don't forget the tire swing and a place to put the boob tube to watch the Packers.And of course a spot for the grill.
Cut the tree down.
Okay, how long has your tub been on this? Has it shifted at all? Did you tamp it thoroughly prior to installing the tub? I've considered the gravel pad idea, I'm hesitant because I'm afraid of the one saying:"If you can't be a good example, be a horrible warning"*lol*