Leaf wax composition and carbon isotopes vary among major conifer groups

Leaf waxes (e.g. n-alkanes, n-alkanoic acids) and their carbon isotopes (delta C-13) are commonly used to track past changes in the carbon cycle, water availability, and plant ecophysiology. Previous studies indicated that conifers have lower n-alkane concentrations than angiosperms and that C-13 fractionation during n-alkane synthesis (epsilon(n-alkane)) is smaller than in angiosperms. These prior studies, however, sampled a limited phylogenetic and geographic subset of conifers, leaving out many important subtropical and Southern Hemisphere groups that were once widespread and common components of fossil assemblages. To expand on previous work, we collected 43 conifer species (and Ginkgo biloba) from the University of California Botanical Garden at Berkeley, sampling all extant conifer families and almost two-thirds of extant genera. We find that Pinaceae, including many North American species used in previous studies, have very low or no n-alkanes. However, other conifer groups have significant concentrations of n-alkanes, especially Southern Hemisphere Araucariaceae and Podocarpaceae (monkey puzzles, Norfolk Island pines, and yellowwoods), and many species of Cupressaceae (junipers and relatives). Within the Cupressaceae, we find total n-alkane concentrations are high in subfamilies Cupressoideae and Callitroideae, but significantly lower in the early diverging taxodioid lineages (including bald cypress and redwood). Individual n-alkane chain lengths have a weak phylogenetic signal, except for n-C-29 alkane, but when combined using average chain length (ACL), a strong phylogenetic signal emerges. The strong phylogenetic signal in ACL, observed in the context of a common growth environment for all plants we sampled, suggests that ACL is strongly influenced by factors other than climate. An analysis of epsilon(n-alkane) indicates a strong phylogenetic signal in which the smallest biosynthetic fractionation occurs in Pinaceae and the largest in Taxaceae (yews and relatives). The relationship between phylogeny and epsilon(n-alkane) may be related to differences in carbon metabolism among conifer clades. These results have important implications for interpreting n-alkane delta C-13 values in sedimentary archives, especially outside of North America. (C) 2015 Elsevier Ltd. All rights reserved.