Growth and defense characteristics of whitebark pine (Pinus albicaulis) and lodgepole pine (Pinus contorta var latifolia) in a high elevation, disturbance prone mixed-conifer forest in northwestern Montana, USA

Nickolas Kichas and Dave McWethy, Earth Sciences, Montana State University, Bozeman, MT, Amy Trowbridge, Entomology, University of Wisconsin-Madison, Madison, WI, Kenneth F. Raffa, Entomology, University of Wisconsin, Madison, WI, Shealyn Malone, Forest and Wildlife Ecology, University of Wisconsin, Madison, WI, Sharon Hood, Fire, Fuel, and Smoke Science Program, USDA Forest Service, Missoula, MT, Richard G. Everett, Department of Natural Resources, Salish Kootenai College, Pablo, MT, Gregory T. Pederson, Northern Rocky Mountain Science Center, U.S. Geological Survey, Bozeman, MT

Background/Question/Methods Recent, widespread tree mortality in the western U.S. resulting from changes in climate, pathogens, insect activity, and forest management practices has led to concerns for many ecologically and culturally important species. Within conifers, resin-based defenses have long been recognized as a primary defense mechanism against a variety of insects and pathogens. Oleoresin produced by trees contain complex mixtures of terpenoids that have numerous insecticidal and fungicidal properties. Research has also identified links between resin duct characteristics and increased probability of survival during bark beetle outbreaks. Whitebark pine (Pinus albicaulis) is a culturally significant high elevation species that provides numerous ecological services within subalpine and alpine ecosystems. Whitebark pine has co-evolved with a suite of biotic and abiotic disturbances. Individual trees allocate resources towards growth and resin-based defenses, making it a good candidate species to evaluate growth and defense relationships and tradeoffs. In this study we compared constitutive resin chemistry, tree growth and resin duct anatomy between similarly aged whitebark and lodgepole pine (P. contorta var latifolia) growing in proximity within a disturbance-prone, mixed-conifer forest in northwestern Montana. These two host species have varying degrees of historical exposure to mountain pine beetle. Results/Conclusions Our research yields four important findings. First, we did not find evidence of a tradeoff between tree growth and tree defenses (resin duct morphology and resin chemistry). This suggests that trees growing under favorable field conditions can experience high growth rates and still allocate ample resources towards defense. Second, we found that resin ducts and constitutive mono- and sesqui- terpenes were not correlated in lodgepole pine while duct size, production, and area were positively related to constitutive terpenes in whitebark pine. The lack of distinct, consistent relationships between these defensive features suggests that trees present beetles with numerous, complex combinations of resin-based defenses. Third, based on constitutive terpene profiles, bark beetles are more likely to enter lodgepole pine but more likely to successfully elicit mass attacks in whitebark pine, which agrees with beetle attack and success patterns observed in the field. Fourth, overstory competition, particularly by Engelmann spruce (Picea engelmannii), can influence tree defenses, specifically constitutive terpene concentrations. Competitive tree interactions could lead to altered bark beetle-conifer interactions as host and nonhost species migrate in response to changing climate. Our results suggest that strategies designed to support whitebark pine populations can benefit from better understanding interactions among growth, competition and physical and chemical defenses in responding to multiple disturbance.