Relationships between conifer constitutive and inducible defenses against bark beetles change across levels of biological and ecological scale

Plants resist herbivores using both constitutive and induced defenses. Limited resources and biosynthesis costs of anti-herbivore compounds may impose tradeoffs between these modalities, but the evidence is conflicting. We postulated that biological and ecological scale may influence potential tradeoffs and analyzed constitutive-induced relationships from genotypic to interspecific levels. We focused on one system, conifer phloem monoterpenes that resist bark beetle attack, to minimize variation from feeding guild or phylogeny. We examined 45 half-siblings in common gardens (n = 763), seven populations of five species with tree-level data (n = 436) and 29 studies of nine species with population-mean data in forests. All studies evaluated mature trees to correspond with beetle behavior in nature and employed common methods of induction and analysis. Rather than a global tradeoff between constitutive and induced defenses, relationships varied with level of biological organization. In common gardens, constitutive concentrations and inducibility (I-C) were inversely related across families and individuals. These relationships did not extend to forest populations, however, where constitutive concentrations and inducibility were positively related. Across populations and species, constitutive concentrations and inducibility were likewise positively related. There was no evidence for inducibility tradeoffs between monoterpene quantity and composition: trees showed no or positive relationships between constitutive concentrations and induced compositional changes. These results suggest environmental variability can supersede underlying physiological tradeoffs in some systems, allowing plants with greater resource pools to excel in multiple defense components. This is further supported by microsite effects on inducibility exceeding family effects in the common gardens, and by relationships between constitutive and absolute induced concentrations being positive at all levels. We present a general model for understanding how underlying physiological tradeoffs are variably manifested across scales. These results can contribute to the protection and resilience of agronomic and native ecosystems and help contextualize organismal-level allocations to ecosystem-level patterns across heterogeneous landscapes.