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 two-thirds of the state's timberlands. Firs are the most prevalent trees in Idaho's forests, which are predominantly pines before European settlers arrived in Idaho. Firs are less resistant than pines to many insect 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 effecting 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.
The suppression of wildfire has changed the composition of trees in Idaho. The high mortality rates in Idaho's national forests are a result of this shift.
Change in Idaho Forest Types
Idaho has 5 of the 22 major forest types in the United States recommended by the Environmental Protection Agency for monitoring to assess forest health. They are Douglas-fir (27.9% of the Idaho total), fir/spruce (27.6%), lodgepole pine (13.3%), ponderosa pine (9.6%), and western white pine (4.3%).
These five forest types represent 82.7% of all Idaho growing stock volume. Figure 3 illustrates changes in the composition of tree species in Idaho forests since 1952. The back row in Figure 3(a) shows that Douglas-fir has increased slightly, holding its position as the largest component of Idaho forests. Figure 3(b) shows Douglas-fir increased by roughly 1.2 billion cubic feet (top scale) or 15% (bottom scale).
The second largest component is the aggregation for Engelmann spruce, western larch, and other softwoods, primarily western redcedar and western hemlock. Taken together, spruce, larch, cedar, and hemlock increased by more than 30% from 1952 to 1987. The next component depicted in the illustration is true firs, consisting mainly of grand fir but including subalpine fir and a small amount of white fir. The true fir component has increased by 60%.
Lodgepole pine increased almost 40% during the 35-year period of analysis.
Historically, the most important timber species in Idaho were ponderosa pine and western white pine. Both have declined since 1952, ponderosa pine by 40% and western white pine by 60%. (1994) estimated that the extent of western white pine may now be only 10% of what it was in 1900.
Based on these species changes, it is obvious that something significant has happened in Idaho's forests. Ponderosa pine has been reduced because it is a desirable timber species. Through the combined effects of fire exclusion and timber harvesting, Douglas-fir has invaded sites once occupied by ponderosa pine. Western white pine, also a desirable timber species, has been reduced primarily by the introduction of the exotic white pine blister rust fungus in the region in the early 1900s.
Figure 3(b) depicts the same data in a different way, and shows that growing stock volumes of both western white pine and ponderosa pine have declined by almost 2 billion cubic feet from 1952 to 1987. During that period, the true firs increased about 2.7 billion cubic feet. Spruce, larch, and other softwoods increased approximately 1.7 billion cubic feet. Lodgepole pine and Douglas-fir have both increased more than 1 billion cubic feet.
Western white pine and ponderosa pine together have declined by almost 4 billion cubic feet, while true firs and Douglas-fir have increased by a like amount. The increases in lodgepole pine and other softwoods, therefore, represent about 3 billion cubic feet of net volume increase (or 12%) in Idaho's forests since 1952.
