Bowling Lab - Ecosystem Ecology
 

Utah Grassland Carbon and Water Fluxes

This ongoing project has been focused on the processes of respiration, photosynthesis, decomposition, evaporation, and transpiration in a Utah grassland. This is a collaboarative project with Dr. Jayne Belnap's group from the USGS Canyonlands Research Station in Moab.

The site is called Corral Pocket and is near the eastern (Needles) entrance to Canyonlands National Park in southeast Utah. This study was initially conducted during the height of the western drought (2001-2003), and after a 2-year break was restarted in January 2006.  We are examining the net ecosystem exchange of CO2 (photosynthesis respiration) and evapotranspiration (plant transpiration and soil evaporation) from the plant/soil community as a whole, using a tower-based atmospheric measurement technique called eddy covariance.

In general we have found that water vapor fluxes are greatest in the spring and fall, with the former especially dominated by transpiration rather than evaporation.  Sensible heat fluxes are greatest at midsummer before arrival of rains associated with the North American monsoon.  Carbon uptake is greatest in April and May, and summer and winter periods were marked by respiratory carbon release. Summer monsoon rains at this site usually contribute to loss of CO2 through respiration (enhanced decomposition) rather than uptake of CO2 by plants.
This figure shows the typical soil moisture pattern at 10 cm depth at the Corral Pocket. There is relatively high soil moisture in the fall and winter, a consistent decrease through the spring, with a low point in the summer before monsoon rains arrive (the monsoon period is indicated with the vertical lines)..
This figure shows the substantial variability from one year to the next in soil moisture at 10 cm depth - different colors are different years from 1999 to 2004. The timing and magnitude of monsoon rains are different every year, as is the fall/winter recharge.
This figure shows 4 days of eddy covariance measurements at Corral Pocket. The days/nights are clear from the top panel. The CO2 flux at night is positive, indicating loss of CO2 from the grassland by respiration. During the day, the CO2 flux is negative, indicating that photosynthetic uptake of CO2 is greater than respiratory release, and the grassland is gaining carbon. The periods of clouds that occurred on the last 2 days influenced the CO2 flux and also the water vapor flux, which is due primarly to transpiration from the grasses.
This figure shows a summary of the carbon fluxes during three important periods. The top panel shows soil moisture at several depths below the surface. Maximum C uptake occurs in the spring (green diamonds), when temperatures are warm but not hot and there is plenty of moisture in the soil (bottom right panel). The arrival of the monsoon rains in the summer (red squares) is associated with high soil and air temperatures (not shown) and the moisture does not penetrate very far into the soil. Carbon fluxes are respiratory all day long during the monsoon rains (at least in the year shown. Fall rains begin to recharge the soil moisture, and are associated with some carbon uptake if they are plentiful.