Construction of metal base ring at off-site location.

Dr. Arnone’s field experiments specifically focus on quantifying the effects of environmental change on carbon and water balances in desert ecosystems. To accomplish this task, Arnone and his team will measure the uptake and release of CO₂ and the release of water vapor. A large geodesic dome tent (2 m high with diameter of 4 m) will be placed over soil and plants growing in each of nine experimental plots. Changes in the concentrations of CO₂ and water vapor that occur over a 1-2 min period will be measured. For each measurement, the dome will be lowered onto a metal-base rim embedded in the soil to make an air-tight seal.

Three base rims have been installed in each of the nine large (25 m in diameter) experimental plots. Three of these experimental plots are being exposed to current ambient levels of CO₂ (360 ppm CO₂); three plots are being exposed to CO₂ levels expected to occur in the year 2050 (550 ppm CO₂; FACE); and three plots are not being treated. Ecosystem CO₂ and water vapor fluxes will be measured monthly over three years (with different amounts of annual precipitation) to obtain an accurate representation of responses to elevated atmospheric CO₂. Because foot traffic is prohibited within experimental plots, the installation of base rims and collection of dome measurements require the use of a boom truck to lift rims into the plots and to raise and lower the domes.

Installation and placement of dome rims at the Nevada Desert FACE Facility

Three key points form the basis for the Mojave Desert FACE study:

• Since deserts are the largest terrestrial biome and are expanding at alarming rates, understanding deserts responses to environmental change is critical.
  
• Deserts are limited by water and secondarily by nitrogen. Studies have shown that elevated CO₂ can significantly enhance plant efficiency in utilizing resources. Therefore, deserts are expected to be the most responsive biome to changing concentrations of atmospheric CO₂. Availability of nitrogen in the ecosystem and water balance are keys in predicting this response.
  
• Long-term, integrated experiments involving intact ecosystems coupled with process-based models are needed to address critical feedback processes.

Container of liquid CO2 at the Nevada Desert FACE Facility

Previous work at the Nevada Desert FACE Facility has provided significant insight into the complex responses of a desert ecosystem to elevated atmospheric CO₂ with the following major results:

• Elevated CO₂ increased dominance of an exotic annual grass. This grass could alter the ecology of the Mojave Desert by causing an increase in wildfires having greater intensity.
  
• Increases in primary production in response to elevated CO₂ were greater than observed in any other ecosystem. The magnitude of the increase was strongly and positively related to input of rainfall to the ecosystem, however.
  
• Greater water storage was predicted from reduced transpiration rates under elevated levels of atmospheric CO₂. Reduced stomatal conductance and transpiration were observed, but increased atmospheric CO₂ did not produce a significant effect on soil moisture content.
  
• Elevated levels of CO₂ decreased plant-available nitrogen and increased loss of gaseous nitrogen. Isotopic evidence points to altered patterns of microbial activity and acquisition of plant nitrogen.
  
• Elevated levels of atmospheric CO₂ have doubled the loss of CO₂ from the soil.

Trays situated under creosote bushes are used to collect plant litter (leaves, stems, etc.) that falls off the plant. Litter bags (mesh bags in right foreground shown in left photo) contain known amounts of plant litter and are placed on the ground and allowed to decompose as a measure of the decomposition rate. An individual suspended above the FACE plot (right photo) is downloading temperature and humidity data from a "HoBo" data logger positioned in the soil under a creosote bush. Foot traffic is not allowed inside the FACE plots.

Based on these results and coupled with the long-term nature of FACE experiments, three sets of overarching questions are being addressed in this Mojave Desert FACE study:

• Will elevated CO₂ alter community composition and vegetation physiognomy? Specifically, will elevated CO₂ cause a disproportionate increase in an exotic grass in subsequent wet-dry cycles, causing species and/or structural changes in the system?
  
• Will elevated CO₂ alter primary production, nutrient dynamics, and water balance and will these changes be sustained over time? Will changes in ecosystem processes be further driven by potential shifts in species composition?
  
• Can the future behavior of a Mojave Desert ecosystem under elevated CO₂ be successfully simulated by applying proven models of desert ecosystem function?

Several key elements punctuate the Mojave Desert FACE study. First, research focuses on seminal interactions among species composition, primary productivity, nitrogen dynamics, and water balance. Additionally, studies are focusing on the way these interactions may be influenced by increased photosynthesis and plant efficiency in using resources under elevated CO₂. Second, this research approach explicitly addresses processes that occur across scales. Studies to date have integrated the biochemical regulation of photosynthetic responses to elevated CO₂ with growth and production across species. At the system level, production has been integrated with water balance and nutrient dynamics. These processes have been further integrated by adapting an established, validated desert ecosystem response model to system-level responses to elevated CO₂. Third, the FACE studies approach is collaborative and integrative. Current research integrates with other proposals and ongoing studies to optimize measurements of important ecological interactions. Deserts are highly unpredictable systems, especially regarding precipitation. It is crucial that the system be examined over long periods and include a strong modeling component to extend field- and laboratory-based knowledge to fully observe the stochastic behavior of deserts in a globally changing context.