Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2003
Publication Date: 3/20/2004
Citation: Scott, R.L., Edwards, E.A., Shuttleworth, W.J., Huxman, T.E., Watts, C., Goodrich, D.C. 2003. Interannual and seasonal variation in fluxes of water and carbon dioxide from a riparian woodland ecosystem. Agricultural and Forest Meteorology. 122:65-84. Interpretive Summary: Riparian corridors provide valuable habitat in semiarid regions such as the southwestern United States. Rural and urban development often impact the vitality of riparian areas by changing land use and by diverting water and lowering the water table. New data are needed to improve our understanding of the ecological functioning and water use of major riparian ecosystems so that improved management strategies can be developed that will ensure the long-term health of these vital ecosystems. This scientific paper reports on environmental measurements that were made in 2001 and 2002 at a riparian woodland site along the San Pedro River in southeastern Arizona to better understand the carbon dioxide (photosynthesis and respiration) and water exchanges. It is shown that the mesquites do rely mainly on groundwater as a water source, though high aridity and a relatively short growing season limit their water use. Also, the paper shows a disconnect between the carbon accumulation and carbon loss by the ecosystem. The ecosystem acquires carbon dioxide from the atmosphere throughout the growing season, but it loses significant amounts of carbon only during a limited time after rainfall when the soil microbes can function. This paper provides improved estimates of groundwater use by riparian vegetation in the San Pedro Valley, which in turn will improve the groundwater models used by scientists and management agencies to better manage the regions water resources. It also provides provocative data to improve scientific understanding of carbon cycling of terrestrial ecosystems.
Technical Abstract: Fluxes of water, energy and carbon dioxide were measured using the eddy covariance technique over a mesquite (Prosopis velutina) woodland along the San Pedro River in southeastern Arizona for the entire growing seasons of 2001 and 2002, between the last freeze event of spring and the first of fall. Although the general pattern of ecosystem response to climate forcing was similar in both years, latent heat and CO2 fluxes show significant variations between and within the growing seasons. The main differences between the two years were a consequence of an extended drought that lasted from October 2001 to July 2002. Most of the within season variability is attributable to the timing and magnitude of mid-summer precipitation associated with the North American Monsoon. Following new tree leaf production and prior to the monsoon onset, there is little precipitation, daytime air temperatures are high and relative humidity low. Evapotranspiration and water level data suggest the mesquite trees always have ready access to groundwater. Nonetheless, decreases in afternoon transpiration and carbon dioxide uptake suggest stomatal regulation of leaf gas exchange in response to the high vapor pressure deficit. Because near-surface soil moisture is limited prior to the summer rains, ecosystem respiration is low and there is little evapotranspiration from understory plants and soil. With the arrival of the monsoon rains, understory vegetation activity and, consequently, total ecosystem evapotranspiration increases. Total ecosystem photosynthesis also increases, but the net uptake of carbon decreases, due to enhanced respiration from the abundant carbon sources, stimulated by the precipitation and warm temperatures. The nighttime measurements of carbon dioxide fluxes are questionable, but, if interpreted correctly, imply the ecosystem is a net sink of carbon dioxide for most of the two growing seasons