|Scott, Russell - Russ|
Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2007
Publication Date: 11/15/2007
Citation: Yepez, E.A., Scott, R.L., Cable, W.L., Williams, D.G. 2007. Intraseasonal variation in water and carbon dioxide flux components in a semiarid riparian woodland. Ecosystems. 10:1100-1115. Interpretive Summary: Natural and anthropogenic changes in the climate system anticipated during this century will have large impacts on ecosystem structure and function. Within this context, there is a compelling need to understand how precipitation influences sources and sinks of carbon dioxide in seasonally dry ecosystems because these ecosystems would likely be more sensitive to changes in precipitation than to other global changes. In this paper, we investigated how the distribution of precipitation over a growing season influenced the dynamics of carbon and water cycling in semiarid riparian mesquite woodland. We made measurements of carbon dioxide exchange between the woodland and the atmosphere as well as evaporation of water and its partitioning into its component fluxes of soil evaporation, understory transpiration, and overstory transpiration. We found that in this ecosystem where the trees access groundwater, gross ecosystem production (GEP) and ecosystem respiration (Reco) were often uncoupled because of their different sensitivities to growing season rainfall. Also, the majority of the precipitation that fell on this ecosystem is returned to the atmosphere via plant uptake and transpiration. These results suggest that future changes precipitation will likely have the greatest impact on the carbon cycling of riparian woodlands with increases in precipitation likely leading to reduced carbon storage and decreases leading to storage gains.
Technical Abstract: We investigated how the distribution of precipitation over a growing season influenced the interaction between the carbon and water cycles of a semiarid riparian woodland dominated by mesquite trees (Prosopis velutina). We found that in this ecosystem where the trees access groundwater, gross ecosystem production (GEP) and ecosystem respiration (Reco) were often uncoupled because of their different sensitivities to growing season rainfall. Soon after the first monsoon rains, Reco was high and was not proportional to slight increases in GEP. During the wettest month of the growing season (July), the system experienced a net carbon loss equivalent to 46% of the carbon accumulated over the six-month study period (114 g C m-2; May-October). This large CO2 efflux likely resulted from the rapid decomposition of a large pool of labile organic material accumulated from previous years’ litter production. Using stable isotopes to partition the water vapor flux, the relative contribution of plant transpiration (T) to total evapotranspiration (ET) (T/ET) was 0.80 for the entire growing season, with T/ET reaching a value of 1 during dry conditions and dropping to as low as 0.65 when the soil surface was wet. The evaporation fraction (E) was equivalent to 41% of the precipitation received during the study period (253 mm) while trees and understory vegetation transpired 28 and 31 %, respectively, of this water source. Results suggest that the magnitude of evaporation likely limited Reco because it contributed to a rapid depletion of shallow soil moisture that was not recharged by continued precipitation events later in the growing season.