This non-lethal method may or may not work on your tree squirrels, but it worked for a friend who was trying to protect his Tucson vegetable garden from round-tailed ground squirrels (which don't jump very far) and pack rats. I haven't seen his set-up, but he described it and says it works great:
Make a chicken wire fence around your garden (or tree), about 3 feet high, with the bottom edge buried. At or near the top, run a hot wire, separated an inch or so from the fence mesh with insulated supports. When the squirrel climbs the fence and grabs the hot wire, it gets zapped, jumps off the fence and runs away. You can buy various shock units at home improvement centers, feed stores, or online. Here's a link to one of them:
http://www.fishock.com.
Most climbing critters don't like to jump clear over a fence if they can avoid it, unless it's very low. If your squirrels jump over a 3 foot fence you might need to raise it. It may also help to add an overhang, with the hotwire out at the edge, or add a second hotwire a couple of inches below the top on the squirrel side. It's a lot of work, but not as much as enclosing the whole tree. Good luck!
I read an article about keeping squirrels out of bird feeders that gave a general guideline of "5-7-9" (no, it's not a new tax code proposal); it said squirrels won't jump vertically 5 feet, horizontally 7 feet, or drop 9 feet from a tree onto a feeder.