Variability in the carbon isotopic composition of foliage carbon pools (soluble carbohydrates, waxes) and respiration fluxes in southeastern U.S. pine forests

Abstract

We measured the δ 13 C of assimilated carbon (foliage organic matter (δC OM), soluble carbohydrates (δC SC), and waxes (δC W)) and respiratory carbon (foliage (δC FR), soil (δC SR) and ecosystem 13 CO 2 (δC ER)) for two years at adjacent ecosystems in the southeastern U.S.: a regenerated 32m tall mature Pinus palustris forest, and a mid-rotation 13m tall Pinus elliottii stand. Carbon pools and foliage respiration in P. palustris were isotopically enriched by 2‰ relative to P. elliottii. Despite this enrichment, mean δC ER values of the two sites were nearly identical. No temporal trends were apparent in δC SC, δC FR, δC SR and δC ER. In contrast, δC OM and δC W at both sites declined by approximately 2‰ over the study. This appears to reflect the adjustment in the δ 13 C of carbon storage reserves used for biosynthesis as the trees recovered from a severe drought prior to our study. Unexpectedly, the rate of δ 13 C decrease in the secondary C 32-36 n-alkanoic acid wax molecular cluster was twice that observed for δC OM and the predominant C 22-26 compound cluster, and provides new evidence for parallel but separate wax chain elongation systems utilizing different carbon precursor pools in these species. δC FR and δC ER were consistently enriched relative to assimilated carbon but, in contrast to previous studies, showed limited variations in response to changes in vapor pressure deficit (D). This limited variability in respiratory fluxes and δC SC may be due to the shallow water table as well as the deep taproots of pines, which limit fluctuations in photosynthetic discrimination arising from changes in D. © 2012 by the American Geophysical Union.