Jeffrey pine and other species. The Jeffrey pine resource on the Plateau has not been affected by Jeffrey pine beetle as it has been in the Southern Cascades and Sierra Nevada sections. The effect of insects and pathogens was minimal on western juniper in the past few years. All five-needle pines are susceptible to white pine blister rust; even though they are a minor forest component in this section, losses without replacement would lead to a decline in forest biodiversity.
Ponderosa pine. Ponderosa pines are commonly attacked by either mountain pine beetle or western pine beetle, but both bark beetles can be found in some trees. Mortality on the Modoc Plateau increased in the past few years, but other sections to the south had ponderosa pine mortality increase during the late 1980's and early 1990's. During 1994, western pine beetle attacked groups of trees across several thousand acres in the Big Valley Mountains. Ponderosa pine mortality during 1995, associated with both western and mountain pine beetle, increased across the Big Valley and Double Head Districts, Modoc National Forest.
Black stain root disease centers in ponderosa and Jeffrey pine continue to be discovered on the Modoc and Lassen National Forests. These centers become more evident when bark beetles and black stain combine to cause mortality. Black stain is present primarily on wetter sites in overstocked stands that have been previously disturbed (fig. 24).
Figure 24 A black stain root disease center on the
Modoc National Forest.
White fir. Fir engraver, high stand densities, and low annual precipitation resulted in white fir mortality in several areas of Modoc County. During 1994, about 42,000 acres of the Warner Mountain District, Modoc National Forest, had a reddish hue from white fir mortality. In 1995, the number of acres affected in the Warner Mountains increased 15% above 1994 estimates. During 1994 and 1995, several areas of white fir mortality were apparent on the Big Valley District. White fir continues to die on Fredonyer Peak in the southern part of this section. Mortality in excess of 80% of stand volume can be found in some of these areas. Some stands have been reduced to only a few nonhost species and a few remaining white fir.
Incense-cedar. Cedar bark beetles, juniper scale, and wood borers were associated with the decline and death of incense-cedars on the Big Valley District in 1995. Mortality and top-kill were evident in the Niles Flat and Roney Flat areas. Since the onset of the drought, this year is the first that incense-cedar mortality has increased.
Catches of male Douglas-fir tussock moths in early-warning, monitoring
traps indicated an increase in the number of trap locations with an average
of more than 25 moths per trap from each plot from 1994 to 1995.
Elevated trap catches were noted in plots on the Warner Mountain District,
Modoc National Forest, and around the Goose Lake area, Modoc County. Intensive
egg-mass surveys and larval sampling were conducted in these areas in the
spring and summer of 1996.
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