Since beaver activity is very unpredictable, it is highly recommended to protect valuable trees on resident properties.
Here are the basics:
Use at least 1.5 or preferably 2 inch rigid mesh. It is ideal as it is not very visible from afar.
It should be as high as 4 feet as beavers are also active in the winter.
Leave room for the tree to grow. At least a few inches.
Chicken wire mesh (poultry netting) is another solution. It is less expensive but it has to be replaced or rearranged when the tree grows. It is also less rigid.
This material is available at your local hardware stores.