|Huxman, T. - UNIVERSITY OF ARIZONA|
Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2006
Publication Date: September 25, 2006
Citation: Scott, R.L., Huxman, T.E., Cable, W.L., Emmerich, W.E. 2006. Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a chihuahuan desert shrubland. Hydrological Processes. Special Issue on Emerging Issues in Rangeland Ecohydrology, eds. Wilcox , B. and Thurlow, T. 20:3227-3243. Interpretive Summary: Encroachment by woody plants into former grasslands has been a widely reported phenomenon across many semiarid landscapes around the world. We do not yet understand how this pervasive on-going change in vegetation will affect water and nutrient cycling, and so we cannot predict the outcomes of this change on society. We investigated the how water and carbon dioxide are cycled in a Chihuahuan Desert shrubland in southeastern Arizona to begin to collect data that will help scientists to decipher this issue. We used innovative measurements to better understand specifically how the shrubs affected water and carbon dioxide exchange. Results suggest that the ecosystem lost the most carbon at the start of the summer rainy season when the shrubs were still dormant, but once the shrubs became active they were able to efficiently acquire both water and carbon dioxide throughout the growing season.
Technical Abstract: The partitioning of evapotranspiration into biological and non-biological sources is one of the keys to understanding the consequences of woody plant encroachment on water and carbon cycling in semiarid ecosystems. To better understand how precipitation is partitioned into evaporation and transpiration and how precipitation affects carbon dioxide (CO2) respiration and plant uptake, we measured whole plant transpiration, evapotranspiration, and CO2 fluxes over the course of a growing season at a semiarid Chihuahuan desert shrubland site in southeastern Arizona. Whole plant transpiration was measured using the heat balance sap flow technique, while evapotranspiration and net ecosystem exchange of CO2 (NEE) were quantified using both the Bowen ratio and eddy covariance technique. Before the summer rainy season began, all water and CO2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO2 fluxes were dominated by respiration as it took approximately ten days for the shrubs to respond to the monsoon start. During the growing season, periods immediately following rain events (< 2 days) were dominated by evaporation and respiration while transpiration and CO2 uptake peaked during the interstorm periods. The surface of the coarse, well-drained soils dried quickly which rapidly reduced evaporation. The peak respiration responses following rain events generally lagged a couple days after the evaporation peak and were better correlated with peak transpiration rates. Peak transpiration rates and CO2 uptake also decayed rather quickly during interstorm periods indicating that optimal plant soil moisture conditions were rarely encountered. Net ecosystem exchange of CO2 was increasingly more negative as the growing season progressed indicating a greater net uptake of CO2 and greater water use efficiency due mainly to decreases in respiratory efflux. Overall, the ratio of total transpiration to evapotranspiration was 57% but it was around 70% during the months when the plants were active.