M262B
Southern California
Mountains and Valleys

Ecology

Forest Health Issues

Fire suppression in the San Bernardino Mountains this century has resulted in a change from open stands of old growth to stands of increased density, decreased age, and with compositional changes from ponderosa and Jeffrey pines to white fir and incense-cedar (Minnich et al. 1995).

Sugar pine. Sugar pine is a component of mixed conifer stands in the Transverse Range and, to a lesser extent, the Peninsular Ranges. White pine blister rust has not yet been found in this section, and sugar pine mortality is associated with native insects and diseases, including armillaria root disease, dwarf mistletoe, and bark beetles.

Pinyons. Two species of singleleaf pinyons occur in southern California. One, Pinus californiarum, grows at lower elevations (3,600 to 4,800 ft) in the Little San Bernardino Mountains and in portions of the Peninsular Ranges, particularly in Anza-Borrego Desert State Park. Little is known of the pests associated with this species, and minimal mortality has been noted. This species has only recently been separated from singleleaf pinyon (Bailey 1987).

Singleleaf pinyon grows at 4,200 to 7,800 feet and has a much greater geographical range than does the newly identified species, with extensive stands in California, Nevada, and Utah. Wildfire is the most significant source of mortality for this species in southern California. Before this century, stand-replacing fires in pinyon were common but small (12 to 200 acres), probably because of the sparseness of the undergrowth (Minnich 1988). Recent fires have been much larger; in 1995, 12,000 acres of singleleaf pinyon burned in the Devil Fire (San Bernardino National Forest). Trees on the periphery of the fire were not killed directly, but were subsequently killed by pinyon engravers (Merrill 1994b). Pinyons are difficult to regenerate, and regeneration has not been possible with previous stand-replacing fires on the San Bernardino National Forest. Black stain root disease, often in conjunction with pinyon engravers, consistently causes moderate mortality near Baldwin Lake on the east side of the Forest. Trees do regenerate in the disease centers, but whether the percentage of tree cover has changed in this heavily infested stand is not known.

Parry pinyon grows in the Santa Rosa and Laguna Mountains, and south into Baja California. Little is known of its pest associates, and recent widespread mortality has not been observed.

Ponderosa and Jeffrey pines. Ponderosa and Jeffrey pine are the most sensitive of the native pines to ozone pollution, and they continue to suffer foliage loss and mortality where ozone concentrations are highest, particularly in the west end of the San Bernardino Mountains. Damage has been observed throughout much of the San Bernardinos, in portions of the San Gabriels, and on Alamo Mountain, Los Padres National Forest. Annosus root disease is common because of previous tree felling without treatment of stump surfaces with borate, and dwarf mistletoe has increased in some areas because of fire suppression. These two agents combine to stress trees and render them susceptible to mortality by bark beetles and flatheaded wood borers, even in nondrought years.

Coulter pine. Coulter pine has suffered severe mortality associated with drought and the western pine beetle. The most recent period of widespread mortality ended in 1992. Rainfall has been at or above average since then, but Coulter pines continue to die in low numbers in native stands and in much higher numbers where the species has been planted offsite. Trees tend to be large and long lived where Coulter pine grows in mixed-conifer stands. Coulter pine commonly invades chaparral, however, where it is much more susceptible to mortality from drought and insect attack or from fire. Coulter pine has adapted to chaparral ecosystems with short intervals between fires through early reproduction (often at 10 to 15 years of age) and serotiny (Borchert 1985). Serotinous cones are those that remain on the tree, unopened, for one or more years after they mature, usually considered an adaptation to fire. High mortality in young stands growing in chaparral or on the forest-chaparral interface should be expected in the next drought.

Other pine species. Gray, limber, knobcone, and lodgepole pines are minor components of forests in southern California. Forest health problems have not been reported in native stands, but knobcone and knobcone x Monterey crosses were extensively planted offsite in the past. These stands are now highly susceptible to mortality from bark beetles, particularly the California fivespined ips. Many clearwing moth larvae have been found in a plantation of knobcone hybrids in the San Jacinto Mountains. These insects typically have high populations only where trees are injured or stressed.

White fir. Mortality of white fir is most commonly associated with pest complexes, usually combinations of annosus root disease, mistletoe, wood borers, and fir engraver. Because of past fire suppression, white fir has invaded stands formerly dominated by pine, and trees in these situations are particularly susceptible to attack by pests. White fir has not invaded pine stands in this section to the extent that it has in northeastern California, and mortality is more scattered.

Bigcone Douglas-fir. Bigcone Douglas-fir grows on north-facing slopes and in steep canyons at low elevations; it provides important habitat for spotted owls and other wildlife. Wildfire currently presents the most significant threat to this tree, particularly because natural fires have been suppressed, allowing for increased undergrowth and more intense fires.

Hardwood species. Some hardwoods, particularly California black oak, are important components of mixed-conifer stands; most are in low-elevation woodlands or in riparian habitat. The fruittree leafroller is the most noticeable of the pests of California black oak. This species periodically goes into outbreak, most recently in the San Bernardino Mountains in 1993 and on Mount Palomar in 1995.

Alder flea beetles have caused substantial defoliation and some mortality in riparian areas. Outbreaks have been observed in the San Jacinto Mountains and in the San Bernardino Mountains. Insects have also caused defoliation on other hardwoods, such as California sycamore and cottonwood.

Air pollution. A decline in vigor of ponderosa pine reported in 1957 was confined to the Arrowhead-Crestline area in the San Bernardino Mountains. Further surveys
and long-term monitoring plots in these mountains showed that ponderosa and Jeffrey pines were the most sensitive to ozone, the principal plant-damaging gas in photochemical air pollution. California black oak and white fir were less sensitive. Incense-cedar and sugar pine rarely show injury symptoms. Between 1974 and 1983, mortality in the young-mature age-class (50-99 years) of ponderosa pine was 33% at plots in the zone of highest ozone exposure, compared to 7% in the same age-class at plots with the lowest ozone exposure. The chronic injury to crowns of ponderosa and Jeffrey pines at these plots was less severe in 1988 than in 1983.

The index describing the amount of ozone injury ranges from 0 (no injury) to 100 (very severe injury). This index increased from 38 to 47 between 1991 and 1995 at three locations annually evaluated in the Barton Flats area of the San Bernardino Mountains. The index is 65 at Camp Paivika on the western edge of the mountains. Ozone injury to ponderosa and Jeffrey pines has also been identified at monitoring plots in the Angeles and Los Padres National Forests.

Exotic pests. Exotic pests can present a threat, although none of significance to native forests are established in this section. Pine pitch canker is established to the north and west in parts of the Central and Southern California Coast sections and in agricultural settings in valleys adjacent to the Transverse and Peninsular Ranges of southern California. This disease may move into native stands. The proximity of the southern California mountains to major ports increases the likelihood of infestation by exotic pests.

The first Africanized honey bees in California were found in October 1994. Since then, the bee has expanded its range to include much of southeastern California (fig. 26). In the near future, this insect is expected to colonize much of southern California, including the forested areas. Because of its extremely defensive stinging behavior, this bee will be hazardous to forest workers and recreationists. A new management challenge will arise in recreation areas and in wildfire suppression (Merrill 1994a, Merrill and Visscher 1995).

Figure 26 — Area of California infested by Africanized honey bees in December 1995.
 

Urbanization. In some areas, urbanization presents the greatest threat to native woodlands. Low-elevation species have already suffered severe habitat loss, including certain native oak species, native walnuts, and others (Davis et al. 1995). Still other native species, such as the Cuyamaca and Tecate cypresses, have extremely limited distributions and are at risk from wildfire or other catastrophic events.

Current Forest Health Activities

In 1995, pinyons in a campground were sprayed to prevent attack by bark beetles after the Devil Fire. Because of previous pruning for dwarf mistletoe, the fire went to the ground at the campground, sparing those trees, but the stand surrounding the campground was completely destroyed. Survival of the trees in the campground is important for recreation and to serve as a seed source for pinyon regeneration in the surrounding area.

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