Phylogenetic, Adaptational, and Environmental Controls on Hydrogen Isotope Apparent Fractionation in Leaf Waxes from a Synthesis of 3,000+ Plant Specimens

Abstract:

The hydrogen isotopic composition of plant waxes (δD_wax) stored in sediment archives is widely used as a proxy for hydroclimate. δD_wax corresponds to the δD of local meteoric water after a series of offsets as plants uptake soil water, use transpiration-enriched leaf water during biosynthesis, and undergo numerous biosynthesis steps to produce leaf wax. The sum of these offsets, apparent fractionation, varies widely due to environmental conditions and plant traits. However, these relationships are poorly constrained and complicate inferences of past climate. Here, we present a synthesis of published leaf wax measurements made on living plants, including 3,144 individuals belonging to 662 species from 96 data sources. Our compilation includes the δD of multiple compound classes and chain lengths and the δD of environmental waters, leaf water, and xylem water.We also include environmental data for each location from gridded climate products, and taxonomic and plant trait information derived from global databases.

Plant growth form, photosynthetic pathway, and plant phylogeny have similar relationships with apparent fractionation regardless of source water. For simplicity, we focus on ε_wax-xylem whose grand average is -116 ± 35 ‰. We find that graminoids have a large ε_wax-xylem of -170 ± 27 ‰. In agreement with previous efforts, a phylogenetic-approach reveals this signal to be driven by strong discrimination in the Bambusoideae-Oryzoideae-Pooideae clade. C4 plants have a larger ε_wax-xylem of -146 ± 22 ‰ whereas C3 and CAM plants are close to the -116 ± 35 ‰ dataset average. These factors appear to influence biosynthetic fractionation and not plant water fractionation, as there are no phylogenetic or photosynthetic patterns of enrichment of leaf or xylem waters relative to environmental water. We do, however, find that the enrichment of leaf water relative to xylem water has strong relationships with relative humidity and vapor pressure deficit consistent with increased leaf water transpiration in drier environments. These relationships are present, albeit weakly, in ε_wax-xylem, suggesting taxonomic overprinting and the possible use of some xylem water for precursors to leaf waxes during biosynthesis. We apply forward models of varying complexity to explore the relative importance of these controls on δD_wax.