Fuel Complex Alterations Associated with Spruce Beetle-Induced Tree Mortality in Intermountain Spruce/Fir Forests

Abstract

Abstract Tree mortality caused during extensive outbreaks of spruce beetle (Dendroctonus rufipennis Kirby [Coleoptera: Curculionidae]) has been assumed to increase hazardous fuel loads and consequently influence fire behavior, occurrence, and effects. However, little research has been done to quantify or qualify how spruce beetle-induced tree mortality may alter fuel complexes during the course of an outbreak. The objective of our research was to determine how fuel complexes differ between stands with endemic populations of spruce beetle, stands experiencing current outbreaks, and stands with postoutbreak spruce beetle-induced tree mortality. We measured ground, surface, and aerial fuels in spruce–fir stands assigned into one of three spruce beetle condition classes: endemic, epidemic, or postepidemic. These stands were located on the Manti-LaSal and Fishlake National Forests in southcentral and southern Utah. We used analysis of variance (ANOVA) to compare mean differences in stand attributes and fuel variables between the three spruce beetle condition classes. The mean amount of down woody surface fuel (tonnes/ha) in the 100-hour size class in epidemic stands was significantly greater than that in endemic stands. Mean litter depth and amount was significantly greater in epidemic stands than in endemic and postepidemic stands. A significant increase for most estimates of live and dead herbaceous material was detected in epidemic stands and postepidemic stands compared with that in stands not affected by the spruce beetle. Likewise, significant increases in live shrub material were detected in postepidemic stands. Spruce–fir stands also had significantly less live aerial fuels after outbreaks than endemic stands.