Affiliation: | (1) Department of Oceanography, Florida State University, Tallahassee, FL 32306-4320, USA;(2) Nicholas School of the Environment and Earth Sciences, Duke University, LSRC, Box 90328, Durham, NC 27708-0328, USA |
Abstract: | Temporal variability in the 13C of foliage (13CF), soil (13CS) and ecosystem (13CR) 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 13CF, 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 13C of forest floor CO2 efflux dominated the 13CR, while in the younger pine forest, the 13C of foliage respired CO2 dominated 13CR. This dependence resulted in a more variable 13CR 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 13CR, 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 13CR. |