Risk and hazard of spruce beetle attack in unmanaged stands on the Kenai Peninsula, Alaska, under epidemic conditions

Abstract

Forest community type was the most important variable determining risk of a spruce beetle ( Dendroctonus rufipennis Kby) outbreak. Black spruce ( Picea mariana B.S.P.) communities exhibited low to moderate risk overall, but stands in these communities with deep accumulations of fibrous organic matter on the soil surface (more than 7.5 cm) were at significantly greater risk than stands with shallower accumulation of fibrous organic matter. Differences in risk between black spruce communities with shallow and deep fibrous organic layers were not paralleled by differences in hazard as measured by either affected stems or basal area. Low-elevation mixed spruce-paper bitch ( Picea-Betula papyrifera Marsh.) communities, in which the principal spruce species were white ( Picea glauca (Moench) Voss) and/or Lutz spruce ( Picea X lutzii Little), exhibited the greatest overall risk of spruce beetle outbreak, and the greatest hazard was associated with these commuinities. Within this group, stands with open canopies had a significantly greater incidence of outbreaks than stands with closed canopies, but open-and closed-canopy stands did not differ with respect to hazard. The group of forest communities with the lowest overall risk of spruce beetle outbreak consisted of a diverse mixture of communities. Communities in which white and/or Lutz spruce were the principal overstory species also occurred in this group. Within the group, increasing elevation was associated with increasing risk and hazard. In low-elevation stands (less than 150 m), a further division between low- and high-risk stands could be made on the basis of forest community. In stands of intermediate elevation (150–300 m), depth of the fibrous organic layer exceeding 5 cm was associated with increased risk.