Precipitation thresholds for CO2 uptake in grass and shrub plant communities on Walnut Gulch Experimental Watershed

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 precipitation thresholds for uptake of carbon dioxide in grass and shrub plant communities. Eight years of precipitation data were used to determine the amount needed for carbon dioxide uptake thresholds in spring and summer seasons. Bowen ratio energy balance systems were used to measure carbon dioxide and moisture fluxes. In spring at the shrub site, close to or above long-term average spring precipitation of 59 mm was required to produce an uptake response. At the grass site a minimum of 23 mm was needed to produce an uptake response, which was much less than the long-term precipitation average of 68 mm. At both sites, spring or multiple summer responses reduced the threshold values for a summer response. Summer threshold ranges for the shrub site were 57-94 mm with a spring response and 123-140 mm without. Grass site summer thresholds were 51-95 mm with a spring response and 80-148 mm without. Summer precipitation threshold values were higher than spring values relating to the high summer evapotranspiration demand. The influence and variability of precipitation timing and frequency on carbon dioxide uptake threshold values resulted in no definitive conclusions as to differences between the grass and shrub plant communities, except that the grass site had slightly lower thresholds. Precipitation timing and frequency influence on total carbon uptake in some situations were more important than total precipitation. The lower grass site threshold values, along with a shift in climate toward more frequent and smaller precipitation events, may give grass ecosystems a competitive growth advantage.