Trees cannot actively fight or move away from harm, but that does not leave them defenseless.  Trees have a number of features that serve as protection: thick bark, thorns, leaf hairs, thick cuticles, and any others.  In addition, certain cellular materials may resist decay or may be indigestible by insects.  Another defense mechanism is the production of chemicals that resist insect feeding, pathogen infection, or decay.

A developmental process unique to trees is the ability to compartmentalize, or “wall off,” decay.  Compartmentalization is the process by which trees limit the spread of discoloration and decay.  After a tree has been wounded, reactions are triggered that cause the tree to form boundaries around the wounded area.  (From the International Society of Arboriculture)