Submitted to: Water Resources Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/12/2007
Publication Date: 2/20/2008
Citation: Emmerich, W.E., Verdugo, C.L. 2008. Precipitation thresholds for CO2 uptake in grass and shrub plant communities on Walnut Gulch Experimental Watershed. Water Resources Research. 44, W05S16, doi:10.1029/2006WR005690. Interpretive Summary: Precipitation is the driving force for plant growth and carbon uptake from the atmosphere in arid environments. The timing, frequency, and amount of precipitation influence how grass and shrub communities grow. Precipitation thresholds for a carbon dioxide uptake response for a shrub and grass plant community were determined. Precipitation timing and frequency were found to be more important than amount for thresholds and for total carbon uptake. This study will allow scientists to better understand how shrub and grass plant communities interact and estimate what plant communities will prosper and which will not with climate change.
Technical Abstract: In semiarid ecosystems, precipitation is the major driving force for carbon uptake and subsequent plant growth. The hypothesis for this study was that the timing, frequency, and precipitation amount would produce different carbon dioxide and moisture fluxes for grass and shrub plant communities. Eight years of precipitation inputs to a grass and shrub ecosystem were evaluated for the influence on carbon dioxide fluxes and uptake totals for spring and summer seasons. Bowen ratio energy balance systems were used to measure carbon dioxide and moisture fluxes. Precipitation threshold amounts needed for an uptake response were determined for spring and summer events. In the spring at the shrub site, close to or above long-term average spring precipitation was required to produce an uptake response, whereas the grass site needed much less to produce an uptake response. A spring response reduced the threshold values for a summer response at both sites. Threshold values for multiple summer responses were smaller for each subsequent response at the sites. Summer precipitation threshold values were higher than spring values relating to the high evapotranspiration demand. Precipitation timing and frequency had a major influence on threshold values and could be more important than total precipitation for carbon uptake. The influence of precipitation timing and frequency along with the variability in precipitation timing and frequency resulted in no definitive conclusions as to differences between the grass and shrub threshold values, except that the grass site had slightly lower thresholds.