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.
Trees get their energy by a process that traps the energy of the sun in a molecule called glucose.
Glucose is made as chlorophyll is stimulated to form ATPs that later power the formation of glucose. When the process does produce glucose, some is used for metabolism, some for structural parts and some for storage. The stored energy is used to start new growth when the next growth cycle starts and for defense. When stored energy reserves are low, defense is low. You cannot feed it.