Temporal Variability in^{13}\text{C}$of Respired CO₂ in a Pine and a Hardwood Forest Subject to Similar Climatic Conditions

Temporal variability in the^{13}\text{C}$of foliage ($\delta {}^{13}\text{C}{}_{\text{F}}$), soil ($\delta {}^{13}\text{C}{}_{\text{S}}$) and ecosystem ($\delta {}^{13}\text{C}{}_{\text{R}}$) respired CO₂ was contrasted between a 17.2-m tall evenly aged loblolly pine forest and a 35-m tall unevenly aged mature second growth mixed broadleaf deciduous forest in North Carolina, USA, over a 2-year period. The two forests are located at the Duke Forest within a kilometer of each other and are subject to identical climate and have similar soil types. The$\delta {}^{13}\text{C}{}_{\text{F}}$, collected just prior to dawn, was primarily controlled by the time-lagged vapor pressure deficit (VPD) in both stands; it was used for calculating the ratio of intercellular to ambient CO₂ (Ci/Ca). A remarkable similarity was observed in the relationship between Ci/Ca and time-lagged VPD in these two forests despite large differences in hydraulic characteristics. This similarity emerged as a result of physiological adjustments that compensated for differences in plant hydraulic characteristics, as predicted by a recently proposed equilibrium hypothesis, and has implications to ecophysiological models. We found that in the broadleaf forest, the$\delta {}^{13}\text{C}$of forest floor CO₂ efflux dominated the$\delta {}^{13}\text{C}{}_{\text{R}}$, while in the younger pine forest, the$\delta {}^{13}\text{C}$of foliage respired CO₂ dominated$\delta {}^{13}\text{C}{}_{\text{R}}$. This dependence resulted in a more variable$\delta {}^{13}\text{C}{}_{\text{R}}$in the pine forest when compared to the broadleaf forest due to the larger photosynthetic contribution. Given the sensitivity of the atmospheric inversion models to$\delta {}^{13}\text{C}{}_{\text{R}}$, the results demonstrate that these models could be improved by accounting for stand characteristics, in addition to previously recognized effects of moisture availability, when estimating$\delta {}^{13}\text{C}{}_{\text{R}}$.