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A mass balance approach for calculating stem respiration in trees.
Teskey, Robert*,1, McGuire, Mary Anne 1, 1 University of Georgia, Athens, GA
ABSTRACT- The rate of respiration of a tree stem has commonly been estimated from measurements of CO2 efflux to the atmosphere. These estimates are based on the assumption that all efflux of CO2 originates from respiration of local tissues and that all CO2 produced by local tissues escapes to the atmosphere through the bark. However, dissolved CO2 can be transported in the xylem and the CO2 concentration in the stem can be up to three orders of magnitude greater than that of the atmosphere, suggesting that measurements of CO2 efflux to the atmosphere do not account for all CO2 produced by respiration. We developed a new mass balance approach for estimating the respiration rate of tree stems that accounts for both external and internal fluxes of CO2. We demonstrate this approach in three trees of different species using measurements of CO2 efflux, sap flux, and internal CO2 concentration to calculate the total rate of CO2 production of a segment of stem tissue in situ. When we applied the mass balance approach, we found that CO2 produced by respiration of stem tissues moves in internal and external flux pathways in different proportions at different times of day and in different environmental conditions. Our calculations show that a substantial portion of respired CO2 is not accounted for in measurements of efflux to the atmosphere alone. In some cases, more than 75% of the respired CO2 moved in the internal pathway, with less than 25% released to the atmosphere. During daylight hours when sap is flowing, a large proportion of respired CO2 is carried away in the xylem stream, while at night more respiratory CO2 escapes to the atmosphere through bark. Measurements made using the mass balance approach can lead to an improved understanding of woody tissue respiration rates.
Key words: internal carbon dioxide transport, plant respiration