Rabbits and hares injure or kill many seedlings, and pocket gophers are especially destructive. In areas where pocket gopher populations are high all seedlings and many saplings may be destroyed. Squirrels and porcupines attack sapling and pole-size trees and, although rarely killing them, deform the stems on which they feed. Repeated browsing by deer has stunted seedlings for 50 years or more (13,55). In the absence of regulation, sheep and cattle have damaged reproduction by trampling, bedding, and occasional browsing (13).
At least 108 species of insects attack P. ponderosa var. ponderosa, and 59 species attack P. ponderosa var. scopulorum (13). The most damaging of the tree-killing insects are several species of Dendroctonus. Trees die from the combined effects of a blue stain fungus transmitted by the beetle and extensive larval consumption of the phloem. The western pine beetle (D. brevicomis) is a common cause of mortality in overmature, decadent trees within the range of ponderosa pine from Baja California, north into Oregon, Washington, western Canada, Idaho, and western Montana. During epidemics, however, apparently healthy, vigorous trees are also killed. During the drought years of the 1930's, losses from western pine beetle in the Pacific Northwest were so heavy that many foresters feared for the pine stands' continued existence. The mountain pine beetle (D. ponderosae) is the most destructive and aggressive enemy in the central and southern Rocky Mountains. During the 1894-1908 outbreak in the Black Hills of South Dakota, this insect killed between 5.7 and 11 million m³ (1 and 2 billion/fbm) of ponderosa pine (13). Tree killing by D. ponderosae has increased with the conversion of old-growth to young-growth stands in the Pacific Northwest. High stand density is believed to reduce vigor of some of the larger trees in a stand and, therefore, is an underlying factor in the occurrence of bark beetle outbreaks. D. adjunctus, D. approximatus, and D. valens are other species of the genus that often kill ponderosa pines.
Among bark beetles, Ips species are second in destructiveness only to Dendroctonus (21). Ips are present naturally in all stands, where they usually breed in slash. In abundant slash from forestry activities, Ips can kill vigorous ponderosa pine up to 66 cm (26 in) in d.b.h. when populations reach explosive levels. Eleven species of Ips have been found attacking ponderosa pine. Of these, I. latidens, I. emarginatus, I. pini, I. lecontei, and I. paraconfusus have the most impact.
Several insects mine buds and shoots, primarily of young trees. Although seldom killed, trees are retarded in growth when infestations are severe. Pine tip moths (Rhyacionia spp.) and the gouty pitch midge (Cecidomyia piniinopis) kill the buds and shoots they mine. A more insidious pest, until recently overlooked and overrated, is the western pineshoot borer (Eucosma sonomana) (21). Larvae of this species bore within the pith of the terminal shoot, stunting but seldom killing them. Shoots that are potentially more robust are more likely to be infested than are weaker shoots. Accordingly, direct comparisons of infested vs. uninfested shoot lengths will underestimate actual growth loss. Each terminal shoot infested by a larva that developed to maturity was reduced in length that year by more than 25 percent in one study (59).
The pine reproduction weevil (Cylindrocopturus eatoni), a native of California and, presumably, Oregon, can be a threat to slow-growing plantations. Its impact has declined, however, with the improvement in planting stock and control of competing vegetation.
Defoliating insects, such as the pine butterfly (Neophasia menapia) and the pandora moth (Coloradia pandora), periodically cause damage over extensive areas. The pine needle sheathminer (Zelleria haimbachi) can be locally severe in young stands.
Dwarf mistletoe (Arceuthobium vaginatum ssp. vaginatum in the Southwest, and A. campylopodium in California and the Northwest) is ponderosa pine's most widespread disease, absent only in the Black Hills (25). It seems to be particularly devastating in the Southwest, where it infects trees on about one-third of the commercial acreage. At Fort Valley Experimental Forest in northern Arizona, dwarf mistletoe has caused up to 36 percent of the mortality (55). On trees not killed, the parasite is responsible for a significant loss in growth, primarily in height, and is reported to reduce seed viability as much as 20 percent. In the Northwest, A. campylopodium has little effect on vigorous, young trees because height growth will usually exceed its upward spread, relegating the parasite to the lower crown (5).
Several diseases attack ponderosa pine roots. Black stain root disease [Leptographium (syn. Verticicladiella) wageneri] causes a diffuse dark staining of the root wood and kills roots (6). Heterobasidion annosum causes an insidious lethal root disease that is spread by airborne spores to the surfaces of freshly-cut stumps. It and L. wageneri kill trees of all ages and usually result in group mortality that is sometimes mistaken for the work of bark beetles, which are frequently secondary invaders. Armillaria sp., previously considered weak root and butt decayers, are causing increased mortality in young plantations and thinned stands where the disease can build up in dead root systems (3). Active infection centers of L. wageneri and H. annosum spread about 1 m (3 ft) per year. The rate is usually less for Armillaria sp.
The most damaging heart rot in the southern Rocky Mountains and the Black Hills is western red rot caused by Dichomitus squalens (25). It is a major cause of loss of sound wood in commercial stands. Because ponderosa pines older than 100 years have substantially greater defect, shorter rotation ages should eliminate much of the heart rot. Phellinus pini is the major heart rot in the Pacific Coast States.
A needle cast, Elytroderma deformans, found throughout ponderosa pine's wide range, is the most serious foliage disease (6). It is unique among the needle casts in being perennial and in its capacity to infect the host twigs, which enables it to maintain its populations even under adverse environmental conditions. Although less destructive than the alarming appearance of affected trees suggests, it can slow growth and kill trees of sawtimber size. Bark beetles are prompt to attack infected trees. Severe damage from E. deformans was reported on the Ochoco National Forest in Oregon, where 555,900 m³ (98,148,000 fbm) of dying and dead trees were removed from 1946 to 1950 (13).
Several rusts of the Cronartium coleosporioides complex are damaging to ponderosa pine. Locally, especially in the Southwest, limb rust (Peridermium filamentosum) attacks middle or upper crowns of mature trees, killing branches in both directions as it spreads (46). The western gall rust (Endocronartium harknessii) attacks ponderosa pine from the Black Hills to the Pacific Northwest (25). It infects all ages, resulting in round and pear-shaped galls, distortions, and trunk lesions. Young trees may be killed. Comandra blister rust (Cronartium comandrae) is found in all states west of the Rocky Mountains but is most common in California, Idaho, Montana, Utah, and Wyoming. It causes scattered mortality in well-stocked sapling and small pole stands. In thinned stands, however, the disease may cause substantial damage (3).
Air pollution is an increasing and vexing source of foliar damage to ponderosa pine. Ozone is the major plant-damaging constituent of photochemical oxidant air pollution. Ozone becomes concentrated enough to cause damage near the border of air basins and in the predominant summer downwind direction from heavily populated areas. Because ponderosa pine, especially var. ponderosa, is susceptible, and because it grows near areas heavily polluted, ozone damage can be great. Typical injury is a chlorotic mottling accompanied by premature abscission of old needles (6). Moderately or severely injured trees are attacked more frequently by bark beetles and Heterobasidion annosum root disease (28).
Basal fire scars are common on the thick-barked stems in old-growth ponderosa pine forests. Uncontrolled fire was common before European colonization. These surface fires consumed branches, fallen trees, understory vegetation, and some living trees. The fires burned from 1 to 47 years apart, with most at 5- to 20-year intervals (3). Low-intensity fires kept many pine forests open and parklike. They also helped to maintain ponderosa pine in areas where more tolerant climax species would have attained dominance, because saplings or larger-sized ponderosa pine are more fire resistant than many of the true firs and Douglas-fir.
Survival and growth of ponderosa pine usually are affected little if 50 percent or less of the crown is scorched in a fire. Six years after a fire in Arizona, however, no poles and only 5 percent of the sawtimber-size trees were living if more than 60 percent of the crown had been destroyed (13). Low tree vigor and cambium damage increase the likelihood of mortality. Vigorous young trees have survived, on occasion, when 100 percent of their crowns were scorched. Because buds are protected by thin long scales, late season fires cause less mortality. Continued accumulation of food reserves after diameter growth ceases in late summer also increases the ability of the tree to withstand fire injury. When crowns are scorched, young, fast-growing trees on good sites have the best chance of survival and old, slow-growing trees on poor sites the poorest chance.
Snow often injures saplings and larger trees. Stem bending and breaking from unusually wet snowfalls that overload tree crowns can seriously damage dense pole-size stands (49). Stem deformation by snow pressure and movement on mountain slopes is a threat to sapling stands (38), especially where ponderosa pine is planted above its optimum elevational limit.