Seasonal and canopy height variation in n-alkanes and their carbon isotopes in a temperate forest

Abstract

The stable carbon isotopic composition (δ13C) of terrestrial leaf wax components, such as n-alkanes (δ13Calk), is used extensively to address questions about past changes in vegetation, climate and the carbon cycle. To interpret sedimentary δ13Calk values, characterization of the environmental and biological controls on carbon isotopic fractionation during plant metabolism is required, especially for fractionation that occurs during photosynthesis (ɛleaf-CO2 or Δleaf) and n-alkane biosynthesis (εalk-leaf). Although much is understood about controls on ɛleaf-CO2, little is known about seasonal or within canopy variation in n-alkane composition and εalk-leaf. To address the gap, we sampled 5 common tree species (Acer rubrum, Acer saccharum, Juniperus virginiana, Sassafras albidum and Ulmus americana) from buds to senescing leaves from within a single temperate forest in southwestern Ohio, USA. We measured n-alkane concentration, δ13Calk, leaf biomass δ13C (δ13Cleaf), δ13C of atmospheric CO2 within the canopy, luminous flux and specific leaf area (SLA). In angiosperms, δ13Calk values were low in the buds, increased substantially (4–6‰) to the highest values in young leaves and then decreased (1–3‰) in mature leaves. Seasonal variation in δ13Calk values generally tracked changes in δ13Cleaf, but the fractionation between leaf and n-alkanes (εalk-leaf) was variable, with lower values in buds (−6.7 ± 1.1‰) than in young leaves (−4.9 ± 2.3‰). Later in the growing season, εalk-leaf values stabilized and ranged between −2.5 and −6.5‰, with a mean of −4.3 ± 1.4‰ in angiosperms. Stabilization of δ13Calk values coincided with settling of SLA and increase in average chain length (ACL). For the conifer, temporal variation in δ13Calk values was smaller (ca. 2‰) compared with the angiosperms, and εalk-leaf values slightly increased when n-alkane production increased. Average εalk-leaf value in the conifer at the end of the growing season was −5.0 ± 0.7‰. We speculate that εalk-leaf variation in both angiosperms and the conifer reflects changes in the source of carbon for plant metabolism, as well as the timing of wax synthesis. We also tested canopy effects on n-alkane concentration and carbon isotopic fractionation at different heights and extent of canopy closure. n-Alkane concentrations were higher at the top of the canopy by 2–8 times than in the lower canopy. The εleaf-CO2 values were more negative at the lower two thirds of the canopy than the upper one third of the canopy by 2–4‰ in two species with closed canopies. The εalk-leaf values did not vary with height and extent of canopy closure. To further explore the link between canopy leaves and leaf litter, we compared δ13Calk values of mature canopy leaves with those in leaf litter collected during senescence, and the offsets were within ca. 1‰. The results confirm indications that leaf litter δ13Calk values reflect those of the upper canopy leaves due to not only high leaf biomass in the upper canopy but also to high n-alkane production.