Teskey, Robert , Aubrey, Doug .
Xylem transport of CO2 derived from root respiration indicates that tree root respiration has been substantially underestimated.
We measured the quantity of CO2released from root respiration that moved internally in xylem sap in nine Populus deltoides trees throughout a growing season. We compared that CO2 flux with theCO2 efflux from the soil surface, which is generally considered to measure soil (autotrophic plus heterotrophic) respiration. The internal transport of root-derived CO2 was calculated from measurements of sap flow and dissolved CO2 concentration ([CO2]) in the xylem at the base of the stem. Soil CO2 efflux was measured using the [CO2] gradient approach. We found that on a daily basis the amount of CO2 that fluxes upward from tree root systems into stems via the xylem stream rivals the amount of CO2 diffusing from the soil surface. Over the growing season, the internal transport of root-derived CO2 increased total belowground respiration by 33%. This indicates that autotrophic respiration was substantially higher than previously estimated, and also higher than heterotrophic soil respiration. The quantity of internally transported CO2 was influenced by both seasonal and daily environmental factors that influenced sap flow rates. We observed high concentrations of CO2 in xylem sap, ranging from 1% to 20% [CO2] among and within individual trees through time. Our results provide evidence that belowground autotrophic respiration consumes a larger amount, and stem respiration consumes a smaller amount, of carbohydrates than previously realized. Our findings have important implications for our understanding of the physiological functioning of trees and carbon cycling in forests. Our observations of an alternative pathway for root-derived CO2 flux highlights the inadequacy of using CO2 efflux to the atmosphere alone to measure root respiration. We suggest the internal transport of root-derivedCO2 should be measured concurrently with CO2 efflux to the atmosphere to more fully understand rates of respiration of trees.
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1 - University of Georgia, Forestry and Natural Resources, 180 E Green Street, Athens, GA, 30602, USA
Presentation Type: Oral Paper:Papers for Topics
Location: Forsyth Room/Chase Park Plaza
Date: Monday, July 11th, 2011
Time: 2:20 PM