I. A Forest is Always Changing
 

Since most trees live longer than humans, many people believe that forests are constant environments, such as in a museum.  Instead, forests are continually changing!  Healthy trees rely upon the death of their counterparts to survive.  In fact, 1 in 5 trees is required to die in order for the remaining trees to grow 1 inch in diameter. 
However, as a Forest Landowner, you most likely have trees you consider to be valuable or desirable for one reason or another, and would like to see grow healthy.   
The value of these trees will most likely depreciate as the tree becomes more unhealthy or declines depending on your goals.  Forest Insect and Disease Management deals with being able to provide the right growing conditions for desirable trees, identifying tree insect and disease, and providing control measures to retain forest health. 

II. Primary Stressors

A tree usually will die within 3-5 years if it is continually under stress after showing signs or symptoms of decline.  It is usually a combination of stressors rather than one, that leads to tree mortality.  These stressors can be broken into 3 categories: Predisposing Factors, Primary Invaders, and Secondary Invaders.