# Presentation: 065303

## Seasonal Changes in Water and CO2 Fluxes in 

**Seasonal Changes in Water and CO****2**** Fluxes in **
**Great Basin Ecosystems**** **
**Lawrence E. Hipps****1****, Ron Ryel****2**** and Sasha Ivans****1**** **
**1.Dept. of Plants, Soils, and Biometeorology, Utah State University, Logan, UT 84322 (E-mail: biomet@cc.usu.edu)**
**2. Dept. of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322**

_**Introduction**_

_**Objectives**_

**Research activities at the Great Basin Ameriflux sites focus on the seasonal changes in water and CO****2**** fluxes in three plant communities, as well as the biophysical processes that govern these balances. In particular, the responses of the different life forms to temporal changes in precipitation is being investigated. The plant communities include: perennial grass, *****Agropyron desertorum***** (crested wheatgrass - CRW); shrub, *****Artemisia tridentate***** (big sagebrush); and tree *****Juniperus osteosperma***** (Utah juniper). **

**Determine seasonal changes in NEE of ****CO****2**** and water vapor for each plant community. Integrate these results with measurements of soil ****CO****2**** fluxes and soil microbial studies.**
**Determine how the three communities each respond to temporal variations in precipitation. Examine the biophysical processes that govern the responses of each life form.**
**Integrate a process-based simulation model of ****CO****2**** with the above measurements **** to identify mechanisms underlying the CO****2**** and water vapor exchanges in these ecosystems.**** **

_**Current Results**_

**Despite some missing data due to operational problems, results are sufficient to show seasonal variations in water vapor and CO****2**** fluxes in each plant community for 2001. The summer of 2001 was exceptionally hot and dry, and spring precipitation was below normal. **
**Data were divided into 3 seasonal periods. These corresponded to spring, where soil water was available, summer, which was hot and dry, and fall, which was cool and dry. The total fluxes for water vapor and CO****2**** are listed by season for each site in Table1. (Note that the length of periods is not the same for each season shown.) **

**Daily values of CO****2**** fluxes changed sign rather abruptly in the spring. This happened earlier in CRW than juniper. An example is depicted in Figure 1, which shows the diurnal patterns of the flux during the late spring for CRW. The negative fluxes diminished and afternoon fluxes rapidly become positive during the transition. **

**Table 1.  Total water vapor and CO****2**** fluxes for each season in 2001 **

**Spring was characterized by CO****2**** assimilation during the daytime at all three sites. Total flux values for the period were negative for all sites, except the sagebrush. Larger nighttime positive fluxes in the sagebrush resulted in a positive total CO****2**** flux. The largest assimilation totals were observed for the CRW and juniper. **

**Figure 1.   Transition from negative to positive daily CO****2**** flux in CRW **

**Turbulence measurements were made with a CSAT 3-D sonic anemometer and a LiCor 7500 open-path water vapor and CO****2**** sensor. Instruments were mounted 2.0 m above surface at the crested wheat grass and sagebrush sites, and 7.0 m over the surface at the juniper site. Upwind fetch is on the order of 1 km for the juniper site and a minimum of 500 m for the crested wheatgrass and sagebrush sites. **

_**Methods**_

**The study sites are near Rush Valley, UT (112******** 28’ W, 40******** 17’ N, elev. 1600 m) situated in the Northern Great Basin of the western United States. Climate at the site is characterized by cold winters (mean Dec-Feb, -2 C) and warm summers (mean high Jun-Aug, 30 C). Annual precipitation averages about 270 mm with soil recharge coming fall to spring, and summers subjected to infrequent thundershowers. Vegetation height and cover for the communities are: crested wheatgrass h ~ 0.5 m with 40-50% cover; sagebrush h ~ 0.75 – 1 m with 30% cover; and juniper h ~ 4 m in height with 15-35% cover. During the April-October growing season, these plants remain physiologically active except for crested wheatgrass which typically senesces in early summer. **

**Sagebrush site in Rush Valley**

**Juniper site in Rush Valley**

**Crested wheatgrass site in Rush Valley**

## Future activities

**	****During fall, NEE values were generally positive, though negative values were observed during the day in juniper, suggesting that the plants were active.**
**Crested wheatgrass, sagebrush and juniper were net sources of CO****2**** during the growing season in the year 2001. The total evaporation fluxes were largest in juniper and smallest in CRW. This likely reflects differences in the ability of the different plants to extract water during the summer months.**

** **

_**Future activities**_

**Modeling**
**A process-based simulation model will be developed for two purposes: **
**Provide a tool for filling in missing flux data**
**Improve our understanding of mechanisms that underlie exchange of CO****2**** and water vapor between each ecosystem and atmosphere.**

**Acknowledgments**

**Figure 5.  Model of Community H****2****O Fluxes**

**Figure 3.  Response of LE to rain at the Juniper site. **

**	****Daytime CO****2**** fluxes increased rapidly after rain events. Only in the juniper (not shown here) were negative flux values observed after rain. The responses to rain generally lasted for a few days.**

**	****In summer, water vapor fluxes rapidly decreased to very small values in each community. However, the juniper and sagebrush communities had much larger responses of evaporation after rain events, suggesting they remained physiologically active. This is seen in Figure 3. The evaporation responses in the crested wheatgrass community to rain events were much smaller. **

**	****This model  links above and below ground processes to predict CO****2**** and water vapor fluxes for a vegetation community.**

**The model has two basic units:**
**Whole canopy photosynthesis model**
**Model of soil processes which affect carbon and water fluxes (Fig. 4 and 5). **

**The two units are linked through the root-shoot continuum. The physiologically-based canopy photosynthesis model is based on calculating gas-exchange at the single-leaf level and scaling up to the whole canopy.**

**The soil process model is comprised of two major components: **
**unsaturated water flow dynamics, including root uptake and hydraulic redistribution**
**soil respiration (which includes microbial respiration and root respiration).**

**Diffusion of CO****2**** within the soil column is also modeled, accounting for diffusion resistances that occur due to soil properties under different moisture and temperature conditions.**

**Figure 4.  ****Model of Ecosystem-Atmosphere CO****2**** Exchange**

**Eddy Covariance Measurements and Analyses**
**Data from 2002 will also be examined to determine how the plant communities respond to different weather conditions than the previous year. The changes in fluxes over time in each community will be integrated with the observed environmental conditions, to determine the processes that govern the water and CO****2**** balances, and how they may change from year to year.**
**A profile system for CO****2**** will be combined with very sensitive wind sensors to study the flows of CO****2**** at night, and determine why fluxes are not seen by the eddy covariance systems. A model for soil CO****2**** emission will be developed from the soil-based measurements. This will be used to fill in the  missing night periods**
**A new system is being implemented to collect and save all time series data from each site. This is important, since it allows more sophisticated analyses to be done to evaluate the fluxes.**

**	****In this case, soil emission of CO****2**** was larger than assimilation by the vegetation. These plant communities responded very rapidly to water availability when a critical stage is reached. A punctuated response to drying conditions is evident.**
**	****It is evident that there are problems with the flux values at night. Little to no upward flux is observed by the sensors, suggesting that there is horizontal transport of CO****2**** near the surface.**
**	****Total LE values for the spring period were largest in the juniper and sage site. This is related to the early senescence of the crested wheatgrass.**
**	****Summer was characterized by very hot and dry conditions, which affected the flux values at all sites. Daily CO****2**** flux values were positive throughout the summer. Any assimilation by vegetation was compensated by larger respiration rates of the vegetation and soil. There were, however, short responses to occasional summer rain events in the juniper and sagebrush (Figure 2). **

**The National Science Foundation for project funding**