I agree that a spinner knob is a very good investment. They are $10.00 at TSC and come in JD green for us smaller PT owners and Red, too, for you folks with the "real" machines.
Tree wrap occurs when you wrap the PT around a tree trunk so that the trunk is between the front and real wheels on the same side of the tractor. The wheels spin on the trunk and you cannot go anywhere. This can happen on flat land or a slope. On flat land, I have had great success getting away from the tree by steering all the way away from the object, lowering the implement enough to raise the front tires off the ground just a tad, then turning all the way towards the object. Then raise the implement and repeat until you crab walk away from the object enought to drive away from it. This also works if you drive into and up against a fence or building.
The real problems happen when you are on the uphill side of something and crab down the hill up against a tree or fence. For a tree, you can always try to drive around to the downhill side(if there are not other trees in the way), then crab walk away from it. But it you are up sideways against the uphill side of a fence or building, it is time to get the winch, come along, tow strap, etc....