Rising atmospheric carbon dioxide concentrations ([CO₂]) and corresponding climate change have increased the demand for a better understanding of ecosystem carbon exchange processes. Temperate forests have the potential to sequester C if the stimulation of photosynthesis from elevated [CO ₂] is greater than increases in the combined respiration of plants and soil. Autotrophic respiration (Ra) is a composite of C oxidized from above- and belowground, however, forest studies have been unable to disentangle plant specific autotrophic and heterotrophic respiration without modifying at least one component of the trees before now. This study utilized stable C isotopes (¹³C) to investigate ecosystem C exchange in three forest types; A tropical rainforest plantation, a P. deltoides plantation (Biosphere 2 Laboratory) exposed to three atmospheric [CO₂] (400, 800 and 1200 μl l^-1) and a L. styraciflua plantation (Oak Ridge National Laboratory FACE site) exposed to elevated [CO ₂] (565 μl l^-1).

Using ¹³C pulse labeling events we were able to label and track one day's photosynthate (C*) through each forest plantation to determine; 1) the amount of C*released from the ecosystem, 2) the rate at which new photosynthate is returned to the atmosphere via respiration and 3) the amount of C* released from upper canopy leaves, lower canopy leaves, stems and the root-rhizosphere. The amount of C* released varied between plantation from 54-74% of C*. Our analyses indicated that C* had a mean residence time of 2.5-4.3 days. Elevated [CO₂] did not greatly increase the rate of C* release from plantations, however, soil respiration was increased by 12-140% at elevated [CO₂]. C* accounted for 33.5 ± 8% of total soil respiration and was generally increased by elevated [CO₂] at B2L. Forests exposed to elevated [CO₂] increased losses of C from the oldest C pool we examined suggesting a substrate stimulation of heterotrophs in response to increased labile C inputs belowground. If the trends we identified (increased soil respiration, increased rates of C cycling and increased loss of old C from soils) are widespread, elevated [CO₂] may result in forested ecosystems that positively feedback on atmospheric [CO₂].