Problems which arise from the environment and not caused by living things such as insects or diseases are called abiotic. Soil compaction, Air pollution, Nutrient deficiencies are examples of Abiotic factors.
Fertilization provide trees with the nutrients and compounds they need to metabolize and grow. In the past a great deal of emphasis was placed on the element Nitrogen. Nitrogen makes green leaves greener and promotes growth.  It tends to move through the root zone rapidly, often making it in short supply. ; As more research is done on tree fertilization the more the realization that a lot of Nitrogen may not be the answer for old trees and those that are in trouble.
A new "breed" of fertilizers in tree care are the biostimulants. These include benifical bacteria, Humic acid that feeds soil microorganisms, the vitamin B complex which stimulates root growth and enzymes which aid in the uptake of water and nutrients. Biostimulants are oten added to traditional fdertilizer
Plant Growth Regulators
Paclobutrazol was originally developed as a fungicide. It inhibits the development of the plant hormone Gibberellin which controls shoot elongation in plants. A little more study revealed that if the plant was no longer producing Gibberellin then it had additional resources to use for things such as more chlorophyll, denser finer roots, and more defense compounds.
A tree treated with Pacloburazol has smaller thicker greener leaves, more feeder roots and more leaf hairs. In addition limiting the growth of a tree, stressed trees can be treated with Paclobutrazol to improve chances for recovery and survival. Trees damaged by construction are good candidates for paclobutrazol. A very useful tool for the Arborist. It is marketed under several labels: Cambistat, Profile and Shortstop.