Generally, Northern Idaho forests have evolved in a wet, cool climate. Southern Idaho forests tend to be drier and warmer. Altitude, soil type, snow, rainfall and average temperatures all influence the evolution of a forest. If forest health is a statement about trees at risk of mortality from insects, diseases, and wildfire, then much of Idaho's forest land is either unhealthy or on the verge of poor health, especially in the national forests that represent three-quarters of the state's timberlands. Firs are the more prevalent trees in Idaho's forests, which were predominantly pines before European settlers arrived in Idaho.
Firs are less resistant than pines to many insects and diseases as well as wildfire.
Prolonged drought in southern Idaho has weakened forests, making them even more susceptible to insect epidemics and wildfires. In northern Idaho, root diseases are affecting the growth potential of mature stands. In forests throughout the state, environmental, ecological, economic, and social values are at risk. The situation can be changed by using forest management practices favoring pines instead of firs and reducing competition between trees by thinning, while protecting other forest values. Two obstacles to this course of action are public policy and public trust.