Once a tree is planted or transplanted it will need help establishing.
Established is the point at which a tree has developed a root system with which it can support itself.
It can take around five years for a tree to extends it’s root system to this point. For larger trees or those in sub optimal planting locations, this could necessitate a much longer period.
During this period the tree needs primarily watering to compensate for a reduced root system and its much reduced ability to collect moisture.
There is also the complex issue of soil health, the Soil Food Web and fungi extending symbiotically the root system of the tree. Without these elements any tree will struggle. The soil should be aerated and have a supply of organic matter (leaves or mulch). These with moisture will create the right environment for a healthy soil. We can add all the elements of the Soil Food Web apart from insects to ensure it is present in the soil.
A tree should have sufficient rooting area and of satisfactory quality to support the tree. Without these elements the tree will always struggle.