Wrap the tree trunk in foil or place a big piece on the ground under the tree. Put dirt or rocks on top to keep it in place, then poke holes in it to let water reach the soil.
Use a heavy type of aluminum foil in your yard so that it can withstand the elements and last longer, though eventually they'll weaken and will have to be replaced.
Birds will hear the crinkle of the foil as it twists and knocks into the branches. They don't understand what kind of creature this object could be, so they'll stay clear of it.
If a bird or squirrel steps onto the area, the foil will feel strange and sound crinkly, which will sound an alarm for your visitors, encouraging them to stay away from the garden.