Species, Elevation, And diameter affect whitebark pine and lodgepole pine stored resources in the sapwood and phloem: Implications for bark beetle outbreaks

Stored resources in trees reflect physiological and environmental variables and affect life history traits, including growth, reproduction, resistance to abiotic stress, and defense. However, less attention has been paid to the fact that stored resources also determine tissue nutritional quality and may have direct consequences for the success of herbivores and pathogens. Here, we investigated whether stored resources differed between two hosts of the mountain pine beetle (Dendroctonus ponderosae Hopkins, 1902): lodgepole pine (Pinus contorta Douglas ex. Loudon), a common host, and whitebark pine (Pinus albicaulis Engelmann), a more naive host that grows at higher altitudes. Phloem and sapwood were sampled in small- and large-diameter trees at two elevations, and nitrogen, phosphorus, nonstructural carbohydrates, and lipids were measured. We found that concentrations of stored resources increased with elevation and tree diameter for both species and that whitebark pine had thicker phloem than lodgepole pine. Overall, stored resources were higher in whitebark pine such that small-diameter whitebark pine trees often had resource concentrations higher than large-diameter lodgepole pines. These results suggest that whitebark pine is of higher nutritional quality than lodgepole pine, which could have implications for the current expansion of mountain pine beetles into higher altitude and latitude forests in response to climate warming.