The most life - threatening type deals with the second law of energy flow. The law states that every system must have a continuous supply of energy to remain in an orderly state - healthy. As energy input decreases, the likelihood of operating near the limits increases. Call it primary stress. Because trees are living systems, they must maintain a continuous flow of energy.
Trees burn glucose to release energy to power the forces of life.
This is the same for humans and other life forms. When energy begins to become limiting, the system begins to operate near its genetically designed limits. There is no known way to feed a tree in the sense of adding an energy source. Food is a substance made up of elements essential for life and an energy source. Animals can be fed.