I've mentioned this before on the forum but it may be worth posting again.

I have a friend who keeps ground squirrels out of his vegetable garden with a low chicken wire fence that has a hot wire attached to the top. I just phoned him to make sure I'm correct on the details, and he said it has been working great for about three years. He says the fence is low enough to step over easily, with about 6-8 inches buried in the ground. The posts extend up a little higher than the mesh, and are insulated with short pieces of plastic tubing that sleeve over the posts. Then he stretches the hot wire about an inch above the top of the chicken wire, so the squirrels have to grab it when they try to climb over the fence. It's powered by an old cattle-type electric fence charger, but should work fine with any of the units you can get at big box stores. Attach the "ground" lead to the chicken wire, and the  "hot" lead to the insulated wire.

He says they usually get zapped and go flying off the fence to run away, but a few times they've been killed. Lately, he says, they mostly don't even mess with it any more.

At any rate, if your trees are in an area that can be surrounded by a low fence and are well away from other trees or things a squirrels could jump from, his method might be worth a try--although tree squirrels are definitely more agile and devious than ground squirrels.