Drought-accelerated Parasitism of Conifers in the Mountain Ranges of Northern California

Abstract

Drought conditions in Northern California during 1975–77 caused widespread mortality in conifer forests mainly from attacks by disease organisms—including dwarf mistletoes and Fungi—and from epidemic outbreaks of various bark beetles. A federal survey has estimated the volume losses sustained through the affected four years (1975–79) as 9.6 thousand million board feet (45.3 million cubic metres) on 6.3 million acres (2.55 million ha) of national forest. As the dominant host species in afflicted forests is Pinus ponderosa, the most destructive agents have been the parasites of this species—primarily the Western Pine Beetle (Dendroctonus brevicomis) and the Western Dwarf Mistletoe (Arceuthobium campylopodum). However, interrelationships between many stress-inducing factors may be of more significance than the direct attack by any single parasitic species on a host.Three approaches to the reduction of conifer losses have been studied. The first involves the use of silvicultural practices based on the elimination of susceptible, or high-risk, trees. This is most applicable in countering bark beetles which have the mobility and selectivity to reach potential hosts over large areas.The second approach is direct control, or the physical removal or destruction of parasites. The steady spread of dwarf mistletoe, formerly contained by natural fire, can only be slowed through pruning, logging, or burning. Bark beetles can be directly controlled if the work is done before new broods hatch and mature. Toxic pesticides such as Lindane have proven ineffective—mainly for economic reasons, but also due to serious reservations among some foresters about detrimental effects on the forest ecosystem. Sex attractants have been used to draw bark beetles into trees of which the removal is planned.The third approach, applicable to bark beetles, is biological control, or reliance on organisms—such as lizards, woodpeckers, predaceous beetles and their larvae, parasitic hymenopterans, and various Fungi, which in some cases the forester has options to encourage or discourage. At the very least these controls suppress bark beetle populations when normal precipitation patterns return.Unfortunately, the ideal of a natural equilibrium being reached solely with the use of biological controls is unrealistic. The most fundamental stress on the commercial forests of California is human use, in spite of occasional drought-accelerated epidemic losses to parasites. Salvage must continue to be aggressively pushed upon a reluctant industry, but, above all, more flexible and immediate responses to the first signs of increasing bark-beetle activity, coupled with indirect, pre-epidemic silvicultural practices based on the broader lessons of forest parasitology, must be developed.