Evaluating growth-defense trade-offs in ponderosa pine (Pinus ponderosa) in response to stimulated bark beetle attack

Abstract

The mountain pine beetle (MPB; Dentroctonus ponderosae, Hopkins, Coleoptera: Curculionidae: Scolytinae) and its associated blue stain fungi are considered to be among the greatest natural threat to conifer ecosystems worldwide. In response to a rapidly changing climate, namely more frequent and hotter droughts, bark beetles benefit through a combination of higher reproductive capacity and greater availability of weakened and stressed host trees. Conifers have potent constitutive and induced chemical defenses to resist the bark beetle-fungi complex, but investment in these carbon-based defenses may be constrained by trade-offs associated with a tree's inherent growth rate. Although there are trade-offs that exist among all processes, it is unclear whether those particular growth-defense trade-offs are measurable and can be compared between individuals within various populations. Here we assess the concentration and composition of constitutive and induced terpene chemical defenses as a function of intraspecific variation in growth rates (slow-versus fast-growing mature ponderosa pines). We examine these relationships in the context of classical plant defense theories as well as the current state of the field. By determining the variation in chemical responses to stimulated bark beetle attack we can further our understanding of potential resistance trade-offs that might exist in stands that are being selectively bred for fast growth.