The impact of pine beetle infestation on monoterpene emissions and secondary organic aerosol formation in western North America

Abstract

Over the last decade, an extensive beetle outbreak has impacted western North America resulting in the mortality of over 100,000 km2 of forest throughout British Columbia and the western United States. Climate change has aided the expansion and continuation of this beetle infestation for more than a decade as beetles survive milder winters and expand northward and to higher elevation areas. Studies have been conducted to investigate the impact of this disturbance on forest carbon stocks, beetle-fire interactions, and meteorological variables, as well as to affirm the importance of including beetle infestation in models. In recent years there has been increased interest in the impact of beetle mortality and attack on atmospheric composition. Numerous studies have demonstrated that insect attack can prompt elevated emissions of volatile organic compounds (VOCs) in a variety of plant and tree species, including mountain pine beetle attacking lodgepole pine, the main beetle-host combination in the current outbreak. These enhanced VOC emissions are likely a defense mechanism of the tree, consisting of increasing emissions of compounds that are toxic to the beetles and attract predators of the beetles as well as increasing sap flow to help remove beetles from the trunk. This impact has not yet been modeled; however, beetle attack may have a significant impact on atmospheric composition and air quality in western North America. In this study, we use 14 years of beetle mortality data for 13 beetle species and beetle-induced monoterpene concentration data in the NCAR Community Earth System Model (CESM) to investigate the impact of beetle mortality and attack on monoterpene emissions and secondary organic aerosol (SOA) formation in western North America. Needleleaf vegetation is decreased each year based on the annual mortality data while emissions of certain compounds in needleleaf trees under attack are scaled-up based on recent beetle-induced VOC data for lodgepole pine (pine scenario) and Engelmann spruce (spruce scenario). As the mountain pine beetle has had the most extensive impact on mortality, we compare changes in emissions of VOCs and subsequent SOA formation caused by the mountain pine beetle to changes caused by the other 12 beetles combined. Beetle infestation impacts monoterpene emissions through both decreased emissions as trees are killed off (mortality effect) and increased emissions in trees under attack (attack effect). Regionally, beetle infestation may have a significant impact on monoterpene emissions and SOA concentrations with up to a 4-fold increase in monoterpene emissions and up to a 40% increase in SOA concentrations in some years. Responses to beetle attack can vary greatly over space and time as the areas affected as well as the magnitude of the impact depend on the extent of previous mortality and the number of trees under attack in a year. The model captures highly localized impacts on smaller-scales, while on larger-scales, the cumulative mortality effect often mutes the ongoing attack effect. The mountain pine beetle alone has an impact similar to that of the other 12 beetles combined, and the spruce scenario has an impact 3-4 times greater than the pine scenario due to differences in the magnitude of the observed enhancement in monoterpene emissions. In North America, the pine scenario would likely dominate since lodgepole pine is the main species impacted; however, smaller regions of spruce may see higher localized impacts on monoterpene emissions and SOA concentrations. Placed in the context of OM and PM2.5 IMPROVE network measurements, the changes in SOA concentrations due to beetle attack are in most cases small compared to the large annual and interannual variability in the measurements of total organic aerosol, indicating that most beetle-induced SOA changes are not likely detectable in current observation networks. However, in areas with especially large emissions enhancements (e.g. areas of spruce under attack) and lower variability in measurements of OM, beetle-induced changes in SOA may be observable. Due to the large potential impacts that beetle infestation may have on monoterpene emissions, SOA formation, and degradation of air quality, it is important that beetle infestation be included in future models.