Historical and current roles of insects and pathogens in eastern Oregon and Washington forested landscapes.

Abstract

This paper examines by climax conifer series, historical and current roles of many important pathogens and insects of interior Northwest coniferous forests, and their unique responses to changing successional conditions resulting from management. Insects and pathogens of the subalpine fir and mountain hemlock series historically reduced inter-tree competition for site resources, and generated most of the coarse woody debris between fires. Severity of growth and mortality effects was proportional to the abundance of susceptible seral species such as Douglas-fir, grand fir, and lodgepole pine within and adjacent to subalpine fir and mountain hemlock forests. Laminated root rot, a mortality factor, influenced successional status, fire intensity, and fire behavior. Insect and disease disturbances in present day western hemlock and western redcedar climax forests are much the same as those occurring historically, but increased scale of fire disturbance resulting from fire exclusion, has increased the scale of insect and pathogen disturbances associated with changing successional conditions. Spectacular differences are apparent when comparing historical and current roles of pathogens and insects of the Douglas-fir and grand fir series. Before the advent of fire control on public lands, late successional and climax forest stands were relatively scarce in comparison with current distribution. A century of fire protection has produced a steady shift away from parklike ponderosa pine and western larch forests toward denser late-successional fir forests. Harvesting of high-value seral overstories accelerated conversion to insect- and pathogen-susceptible late-successional forests. Douglas-fir and grand (white) fir are highly susceptible to root pathogens, bark beetles, defoliators, and dwarf mistletoe. Excluding fire from grand fir and Douglas-fir forests has perhaps been the single greatest detriment to diversity of eastside forests, and a primary factor in current susceptibility to major pathogens and insects. Low intensity fires, once common to historical ponderosa pine climax forests, maintained low fuel loads, minimized fuel ladders, and spaced trees struggling to survive under severe moisture-limited growing conditions. The western pine beetle and mountain beetle thinned densely stocked areas missed by fire, and killed trees injured by wind and weather, or weakened by root disease, dwarf mistletoe, Pandora moth, or advanced age. With fire control, overstocked conditions became widespread and bark beetles assumed the role of underburning to the elimination of trees in excess of site potential. Regeneration of historical lodgepole pine forests was predicated on mountain pine beetle outbreaks and subsequent stand replacing fire events. Today, with fire control, mountain pine beetle outbreaks affect larger areas, for longer periods, often with greater intensity than historical outbreaks. Specific solution to elevated insect and disease disturbance in current forests is complicated by great variety in environmental and vegetal conditions where rehabilitation might be needed, and change in biological and physical potentials as a direct result of management. Still, much can be done. Stocking can be reduced where long-term carrying capacity is exceeded. The shift toward late-successional, fire intolerant, pathogenand insect susceptible forests can be reversed by developing a seral-dominated forest matrix. Management activities can promote landscape structure, composition, and pattern, consistent with historical disturbance regimes and land potentials. Future research on forest pathogens and insects should address three primary subject areas: insect and pathogen population dynamics in managed and unmanaged forests; ecological roles and effects of native and introduced pathogens and insects; and, effects of natural disturbances and management practices on native insects, pathogens, and their