Leaf waxes in riparian trees: hydrogen isotopes, concentrations, and chain-length patterns

The stable hydrogen isotope ratios of epicuticular leaf wax n-alkanes record aspects of a plant's ecophysiological conditions. However, it remains unclear as to whether n-alkane hydrogen isotope values (δ²H) directly reflect environmental water (source water or tissue water) or environmental water in combination with a biochemical fractionation. Furthermore, it is uncertain if leaf n-alkane δ²H values reflect a single time interval during leaf expansion or if n-alkane δ²H values record the combination of inputs throughout the entire lifespan of a leaf. These different possibilities will influence how leaf wax biomarkers are interpreted in both ecological and environmental reconstruction contexts. To address these issues, we sampled leaves/buds, stems, and water sources of five common western U.S. riparian species under natural field conditions throughout the growing season. Riparian species were selected because the input water source is most likely to be nearly constant through the growing season. We found that species in this study demonstrated marked and systematic variations in n-alkane concentration, average chain length, and δ²H values. Intraspecific patterns were consistent: average chain lengths and δ²H values increased from bud opening through full leaf expansion with little variation during the remainder of the sampling interval, while leaf-wax concentration as a fraction of total biomass increased throughout the growing season. These data imply that leaf-wax δ²H values reflect multiple periods of wax growth and that the leaf wax is continually produced throughout a leaf's lifespan.