Altitude effect on leaf wax carbon isotopic composition in humid tropical forests

The carbon isotopic composition of plant leaf wax biomarkers is commonly used to reconstruct paleoenvironmental conditions. Adding to the limited calibration information available for modern tropical forests, we analyzed plant leaf and leaf wax carbon isotopic compositions in forest canopy trees across a highly biodiverse, 3.3 km elevation gradient on the eastern flank of the Andes Mountains. We sampled the dominant tree species and assessed their relative abundance in each tree community. In total, 405 sunlit canopy leaves were sampled across 129 species and nine forest plots along the elevation profile for bulk leaf and leaf wax n-alkane (C₂₇–C₃₃) concentration and carbon isotopic analyses (δ¹³C); a subset (76 individuals, 29 species, five forest plots) were additionally analyzed for n-alkanoic acid (C₂₂–C₃₂) concentrations and δ¹³C. δ¹³C values display trends of +0.87 ± 0.16‰ km⁻¹ (95% CI, r² = 0.96, p < 0.01) for bulk leaves and +1.45 ± 0.33‰ km⁻¹ (95% CI, r² = 0.94, p < 0.01) for C₂₉ n-alkane, the dominant chain length. These carbon isotopic gradients are defined in multi-species sample sets and corroborated in a widespread genus and several families, suggesting the biochemical response to environment is robust to taxonomic turnover. We calculate fractionations and compare to adiabatic gradients, environmental variables, leaf wax n-alkane concentrations, and sun/shade position to assess factors influencing foliar chemical response. For the 4 km forested elevation range of the Andes, 4–6‰ higher δ¹³C values are expected for upland versus lowland C₃ plant bulk leaves and their n-alkyl lipids, and we expect this pattern to be a systematic feature of very wet tropical montane environments. This elevation dependency of δ¹³C values should inform interpretations of sedimentary archives, as ¹³C-enriched values may derive from C₄ grasses, petrogenic inputs or upland C₃ plants. Finally, we outline the potential for leaf wax carbon isotopes to trace biomarker sourcing within catchments and for paleoaltimetry.