Seasonal Chemical Variation in Lodgepole Pine Physiology: A Predictor of Mountain Pine Beetle Colonization Success?

Throughout western North America, mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins, outbreaks have caused extensive damage to lodgepole pine, Pinus contorta Dougl, trees.  Concern exists that MPB will spread to other hosts in the boreal forest of Canada. It has been previously thought that beetle colonization success is related to tree physiology, stand density, and drought. Host volatile organic compounds (terpenes) are utilized by insects to locate and recognize suitable host trees. From June through September of 2008 and again in 2009, un-attacked trees from four plots of decreasing density were studied within the Colorado State Forest Service State Forest, Jackson County, CO. High beetle pressure was present during 2008 and 2009 seasons, and many studied trees were colonized and killed during the study. The research objective was to identify attributes of tree susceptibility, stressors, and resistance to beetle invasion by simultaneously quantifying water status (via water potentials, Ψ), foliage volatiles (using headspace SPME-GC and GC-MS), and drought history (^δC₁₃ isotopes) over a density gradient in four plots. Preliminary results from season long water potentials and needle ^δC₁₃ isotopes showed no severe water stress, yet SPME-GC analysis showed varying terpene profiles that varied with plot density. Tree needle profiles changed over the course of each summer and were markedly different in density extremes with some volatile compounds completely absent or present. The present findings indicate no severe water stress amid differing volatile emissions.  Eighteen different compounds were identified in needle volatile emissions as separated by GC retention time. The trees were dynamic and changed according to their environment. Limonene, known for its toxicity to beetles, was seven times greater in the thinnest plot than the un-thinned control plot. Limonene decreased as density increased and with tree age. Limonene was also more prevalent in trees not successfully colonized.  Volatiles fluctuated in their levels across the season within their own plots. This shows the dynamic fluctuation of tree physiology over the season.