Impacts Of Douglas Fir Beetle On Overstory And Understory Conditions Of Douglas Fir Stands PDF Download

Recent bark beetle outbreaks have caused extensive tree mortality in conifer forests across western Northern America, which has altered forest fuels. These changes have raised concerns about forest health and wildfire risk. Studies focused on interactions between bark beetles, forests fuels, and changes in fire behavior have been primarily led in upper elevation forests characterized by high-severity fire regimes, principally in lodgepole pine (pinus contorta Douglas ex Loudon) and Engelmann spruce (Picea engelmannii Parry ex Engelm) forests. Few studies to date have addressed bark beetle fuel interactions in lower to middle montane forest characterized by a mixed-severity fire regime, with available research focused strictly on assessing fuel load conditions or stand structural changes. The goal of this research was to quantify and characterize surface and canopy fuel changes in middle montane interior Douglas-Fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.)) forest infested by Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins), while also measuring physical and chemical changes to foliage in terms of moisture content and terpenes, which are known to play important roles in foliage flammability. Our results revealed few changes in surface fuels following Douglas-fir beetle infestations aside from a significant increase in litter depth and loading in red stage sample plots. Substantial changes to canopy fuels were detected in the red stage of an outbreak with a significant reduction in foliar moisture content measured as tree crowns faded from a healthy green phase to red. During this period, volatile emissions and within-needle concentrations of terpenes increased, including some terpenes previously associated with increased foliage flammability in other tree species. Furthermore, aerial fuel parameters that estimate the likelihood of crown fire initiation, including canopy bulk density and canopy base height, showed a substantial reduction in gray stage sample plots. Based on our findings we judge the influence of Douglas-fir beetle activity on altering fuels is most pronounced in the aerial fuels complex. Our results suggest bark beetle affected interior Douglas-fir stands with a high percentage of trees in yellow and red crown phases could have an increased threshold for crown fire initiation based on higher levels of flammable monoterpenes and lower foliar moisture content.