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ECOSYSTEM DESCRIPTION
The PHACE experiment is at the USDA-ARS High Plains Grasslands Research Station, 10 miles west of Cheyenne, WY USA (41o 11' N, 104o 54' W; elevation 1930 m). The ecosystem is a northern mixed-grass prairie, which includes species both short and tall in stature. The plant community is made up of 55% cool season grasses, 25% warm season grasses, and 20% sedges, forbs and small shrubs. Annual precipitation is 38.4 cm, mean air temperatures are 17.5 C in summer and -2.5 C in winter, and the average wind speed is 6 m s-1 with gusts to 35 m s-1.
EXPERIMENTAL DESIGN
Free Air CO2 Enrichment (FACE)1 technology and temperature controlled infra-red heaters2 are used to expose native northern mixed-grass prairie to a factorial combination of two CO2 and two temperature regimes: (1) present ambient [390 ppm] and elevated [600 ppm] CO2, (2) present ambient and elevated temperature (1.5/3.0 C warmer day/night). The elevated CO2 and temperature levels were chosen to simulate climate conditions predicted for later this century in this region. Each treatment is replicated five times across 20 plots. See LeCain et al. 2015, Morgan et al. 2011, and associated supplementary material for details on treatment performance.
Above: Infrastructure of an elevated CO2 x elevated temperature plot. An infrared radiometer (1) measures plot air temperature and reports it to a computer-controlled system that automatically adjusts energy output of ceramic heaters suspended above the plot (2) to maintain a temperature increase of 1.5 C by day and 3.0 C by night. Perforated PVC rings (3) surrounding the plot emit CO2 gas, which mixes with the air above. CO2 concentration measured at the plot center by an infrared gas analyzer (4) and wind speed data are sent to the computer control system, which adjusts the CO2 flow rate to achieve the set-point (600 ppm).
Two water treatments are imposed on 10 additional plots: (1) a "shallow irrigation" treatment, employing frequent, small water additions to match soil water content in elevated CO2 plots, and (2) a "deep irrigation" treatment, with one large irrigation in early spring and a second in late fall. The goal of the shallow irrigation treatment is to separate direct effects of CO2 from effects of the CO2-induced increase in soil water. The deep irrigation treatment is used in comparison with the shallow irrigation treatment to evaluate how the seasonal distribution of soil water pulses influences mixed-grass prairie.
Each of the rings is split into two sections by a 25 cm steel flange. One side features undisturbed northern mixed-grass prairie, while the other is devoted to understanding how disturbance and plant invasion interact with elevated CO2 and warming. Each ring is instrumented with a minirhizotron tube for studying root responses, probes to measure soil water content at several depths and locations, continuous air & soil temperature sensors, and frames for measuring ecosystem-level gas exchange. Specific subplots are used for studying plant mediation of soil responses, particular functional groups (native and invasive forbs), and individual invasive species such as Dalmatian toadflax (Linaria dalmatica) and cheatgrass (Bromus tectorum).
Above: Native mixed-grass prairie and native-invasive plant subplot layout: (1) Biomass harvest area,(2) invasion x disturbance subplots (seeded with native & invasive forb species pairs), (3) Dalmatian toadflax and cheatgrass invasion subplots, (4) plant-free subplot, (5) soil moisture probe, (6) minirhizotron tube, (7) ecosystem gas exchange frame, (8) air/soil temperature sensors, (9) trace gas flux collars.
Above: Minirhizotron tube for measuring root growth, and Sentek soil moisture probe.
The PHACE experiment began in 2006, and will run through the end of the 2013 growing season.
1 Designed by Franco Migliette, Institute for BioMeteorology in Florence, Italy
2 Kimball, B.A. et al. 2008. Infra-red heater arrays for warming ecosystem field plots. Global Change Biology 14:309-320.
