I wanted to put in some raised beds that are about 1' deep above ground, and that wrap circularly around a Northern Arizona cedar tree. Do I need to plastic or a rock/cage around the tree trunk to protect it from the new wet environment? The trees are about 1-3' in diameter.
What is the diameter of the planting bed in relation to the drip line of the tree? If you raise the soil grade one foot around the tree you will kill the roots in that area that the tree uses to exchange air. It is not advised to cover much more than forty percent of the surface of the tree from the drip line to the trunk. I have seen many trees near new house construction slowly killed by this kind of work.
Younger trees are better able to adapt than older trees to this kind of change but you increase your chances of success by building a tree well as wide as you can tolerate in your design around the base the tree.
Another solution is to raise the grade slowly. By adding two or three inches a year you can achieve what you want in three years.
Another solution is more intensive and involves using 4-inch perforated plastic pipe arranged in 5 to 6 horizontal lines radiating from the tree well like spokes in a wheel to a point beyond the branch spread. This allows air exchange and can be covered with soil.
Yet another solution is to: