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

We measured the δ 13 C of assimilated carbon (foliage organic matter ( δ C OM ), soluble carbohydrates ( δ C SC ), and waxes ( δ C W )) and respiratory carbon (foliage ( δ C FR ), soil ( δ C SR ) and ecosystem 13 CO 2 ( δ C ER )) for two years at adjacent ecosystems in the southeastern U.S.: a regenerated 32 m tall mature Pinus palustris forest, and a mid‐rotation 13 m 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 δ C ER values of the two sites were nearly identical. No temporal trends were apparent in δ C SC , δ C FR , δ C SR and δ C ER . In contrast, δ C OM and δ C 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 δ C 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. δ C FR and δ C 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 δ C 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.