Historically, ponderosa pine forests predominated on warm-to-hot, dry sites at the lower elevations along the east slope of the mountains and in major river valleys in the Northern Rockies, Middle Rockies and Palouse grassy, semi-arid plains (steppe) Ecoregions. Mature ponderosa pine forests were commonly quite open, a condition that was maintained by intermittent low intensity fires averaging every 5 to 25 years. These surface fires consumed the needle duff and killed most understory trees. Bark beetles killed individual or small groups of aging or stressed trees, which were eventually replaced by regeneration that had survived the fires.
Ponderosa pine is now less common, having been replaced by denser forests of Douglas-fir or grand fir. Acreage decreased by 44 percent for Idaho as a whole during the period 1952-87. The change is a result of fire suppression and timber harvesting. Without fire, the more shade-tolerant Douglas-fir and grand fir become established and outcompete the ponderosa pine. Early harvesting of ponderosa pine accelerated the shift in composition toward Douglas-fir and grand fir. The net result has been a change from predominantly semi-open, mature ponderosa pine forests to dense, younger forests, many of which are multi-storied, shade tolerant species more susceptible to fire and disease.
The changes in forest composition and structure have favored a number of native insects and diseases. Douglas-fir dwarf n-Listletoe builds up to high levels in dense, slow-growing stands and when infected overstories provide an infection source for understory trees.
Bark beetles kill ponderosa pine at increased rates in the dense stands, especially during periods of drought. Defoliating insect outbreaks periodically occur, with most significant effects occurring in multi-storied Douglas-fir and grand fir stands.
Altered forest structure and composition have also increased risks from wildfire. Fire suppression has permitted greatly increased ground fuels, with the multi-storied condition creating a “fuel ladder.” Fires often bum hotter and more extensively than they did in the past, creating conditions where many fires can no longer be contained.
More than half a million acres burned between 1989 and 1994 on the Boise National Forest. In the past, fires in this forest type were primarily low to moderate intensity, and most of the large ponderosa pine survived. A relatively small amount of the forest burned severe enough to kill all the trees.
Unlike the low-moderate intensity fires of the past, some wildfires now are lethal across large areas with the potential for damaging the productivity of soils and increasing erodibilit) through the consumption of organic matter and high temperatures especially when coarse textured soils are involved.
Common throughout the western United States and the southern portion of British Columbia, the ponderosa pine is a curious tree. Individual specimens from different regions across its growing range exhibit considerable variation in needle length and thickness, cone size, bark color, and wood texture. Needles can range from 5 to 10 inches in length, and occur in bunches of two or three. Ponderosa pine cones are oval in shape, are generally from 3 to 6 inches long, and feature a stiff prickle on the end of each cone scale. The bark of a mature ponderosa pine features broad, irregular, scaly plates that give the impression of belonging to a jigsaw puzzle.
Ponderosa pines grow to heights of 230 feet, though trees in the 150-foot range are more common. The trunk of a mature ponderosa pine has few lower branches, and can be from 5 to 8 feet in diameter.
The fine-grained, moderately strong, lightweight wood of the ponderosa pine is used in such applications as decorative moulding, shelving, interior paneling, and crating material.