There is a need for methodology to warm open-field plots in order to study the likely effects of global warming on ecosystems in the future. Herein, we describe the development of arrays of more powerful and efficient infrared heaters with ceramic heating elements. By tilting the heaters at 451 from horizontal and combining six of them in a hexagonal array, good uniformity of warming was achieved across 3-m-diameter plots. Moreover, there do not appear to be obstacles (other than financial) to scaling to larger plots. The efficiency [g h (%); thermal radiation out per electrical energy in] of these heaters was higher than that of the heaters used in most previous infrared heater experiments and can be described by: g h 5 10 1 25exp(À0.17 u), where u is wind speed at 2 m height (m s À1 ). Graphs are presented to estimate operating costs from degrees of warming, two types of plant canopy, and site windiness. Four such arrays were deployed over plots of grass at Haibei, Qinghai, China and another at Cheyenne, Wyoming, USA, along with corresponding reference plots with dummy heaters. Proportional integral derivative systems with infrared thermometers to sense canopy temperatures of the heated and reference plots were used to control the heater outputs. Over month-long periods at both sites, about 75% of canopy temperature observations were within 0.5 1C of the set-point temperature differences between heated and reference plots. Electrical power consumption per 3-m-diameter plot averaged 58 and 80 kW h day À1 for Haibei and Cheyenne, respectively. However, the desired temperature differences were set lower at Haibei (1.2 1C daytime, 1.7 1C night) than Cheyenne (1.5 1C daytime, 3.0 1C night), and Cheyenne is a windier site. Thus, we conclude that these hexagonal arrays of ceramic infrared heaters can be a successful temperature free-air-controlled enhancement (T-FACE) system for warming ecosystem field plots.
Summary• Sorghum ( Sorghum bicolor ) was grown for two consecutive seasons at Maricopa, AZ, USA, using the free-air CO 2 enrichment (FACE) approach to investigate evapotranspiration of this C4 plant at ample and limited water supplies.• Crop evapotranspiration (ET) was measured using two CO 2 concentrations (control, c. 370 µmol mol -1 ; FACE, ambient + 200 µmol mol -1 ) and two irrigation treatments (well watered and water-limited). Volumetric soil water content was measured before and after each irrigation using neutron scattering techniques.• Averaged over both years, elevated CO 2 reduced cumulative ET by 10% when plants were given ample water and by 4% under severe drought stress. Water-use efficiency based on grain yield (WUE-G) increased, due to CO 2 enrichment, by 9% and 19% in wet and dry plots, respectively; based on total biomass, water-use efficiency (WUE-B) increased by 16% and 17% in wet and dry plots, respectively.• These data suggest that in the future high-CO 2 environment, water requirements for irrigated sorghum will be lower than at present, while dry-land productivity will increase, provided global warming is minimal.
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