Plant-derived diterpenoids are commonly used as conifer-specific biomarkers and for chemotaxonomic assignment or confirmation. Numerous studies have reported on the utility of diterpenoids as chemotaxonomic indicators, but few have quantitatively analyzed diterpenoid concentrations, especially with respect to conifer phylogeny. In addition, the carbon isotope values (δ13C) of diterpenoids provide a means to track conifer-specific δ13C values, which is useful for tracking water availability and the carbon cycle. To expand on previous work, we measured diterpenoid concentrations and carbon isotopes of 43 conifer species, and Ginkgo biloba, collected at the University of California Botanical Garden at Berkeley. In this collection, all extant conifer families and almost two-thirds of extant genera are present, including many subtropical and Southern Hemisphere groups that were once common and widespread components of fossil assemblages. Overall, we found diterpenoid concentrations were highly variable among species and families. Despite this variability, there is coarse family-level phylogenetic structuring with the lowest concentrations in Pinaceae. When diterpenoid concentrations are fitted to a model of trait evolution (Brownian motion), we however find that there is no phylogenetic signal. In contrast, when terpenoids are analyzed by the proportion of diterpenoid compound structure classes (abietanes, labdanes, pimaranes, tetracyclics, and totarols/sempervirol), there was significant phylogenic signal for the abietane and tetracyclic structures. Diterpenoid biosynthetic carbon isotope fractionation, as expressed between diterpenoids and leaf tissue (εditerpenoid), also contained a phylogenetic signal, as well as the broad phylogenetic structuring observed in total diterpenoid concentrations. Overall, these results indicate that the Pinaceae is unique among conifer clades with respect to terpenoid structure classes, concentrations and εditerpenoid. When diterpenoids are applied to taxonomic assignment of fossils, it would be useful to combine several traits (concentration, proportion, and εditerpenoid). In this context, it should be possible to broadly distinguish three major conifer groups: Cupressaceae, Podocarpaceae and Pinaceae. However, based on these results, we recommend against assuming that closely related species have similar diterpenoid compositions.
