Resin ducts as resistance traits in conifers: Linking dendrochronology and resin-based defenses.

Conifers have evolved different chemical and anatomical defenses against a wide range of antagonists. Resin ducts produce, store and translocate oleoresin, a complex terpenoid mixture that acts as both a physical and a chemical defense. Although resin duct characteristics (e.g., number, density, area) have been positively related to biotic resistance in several conifer species, the literature reporting this association remains inconclusive. Axial resin ducts recorded in annual growth rings are an archive of annual defensive investment in trees. This whole-life record of defense investment can be analyzed using standard dendrochronogical procedures, which allow us to assess interannual variability and the effect of understudied drivers of phenotypic variation on resin-based defenses. Understanding the sources of phenotypic variation in defenses, such as genetic differentiation and environmental plasticity, is essential for assessing the adaptive potential of forest tree populations to resist pests under climate change. Here, we reviewed the evidence supporting the importance of resin ducts in conifer resistance, and summarized current knowledge about the sources of variation in resin duct production. We propose a standardized methodology to measure resin duct production by means of dendrochronogical procedures. This approach will illuminate the roles of resin ducts in tree defense across species, while helping to fill pivotal knowledge gaps in plant defense theory, and leading to a robust understanding of the patterns of variation in resin-based defenses throughout the tree's lifespan.