Free-air CO2 enrichment (FACE): blower effects on wheat canopy microclimate and plant development

Free-air carbon dioxide enrichment (FACE) provides a realistic, cost-effective method for evaluating the effects of supra-ambient CO2 concentrations on growth, development, yield, and water use of agricultural crops and natural ecosystems with very few of the problems normally associated with glasshouse or chamber type research. There are no walls interfering with incident radiation and no artificial constraints on rooting depth. With current FACE technology, CO2 enriched air is injected around the perimeter of circular plots and natural wind disperses the CO2 across the experimental area. Under stable, nighttime wind conditions found in FACE wheat experiments at Maricopa, Arizona, the blowers used to inject CO2 exerted subtle effects on the microclimate in a manner analogous to wind machines used for orchard frost protection. Plots equipped with blowers had nighttime foliage and air temperatures that averaged 0.6-1.0°C warmer than controls without blowers. A secondary effect of these elevated temperatures was that plots equipped with blowers displayed differences in dew duration (time that leaves were wet was reduced 30%), plant development (anthesis occurred 4 days earlier), and senescence [as measured with the normalized difference vegetation index (NDVI)]. Natural wind and turbulence appear to overcome the blower effect during daytime treatments and on some nights. Aerial thermal imagery (8-12 micrometer) acquired during the 1998 FACE experiment with grain sorghum provided additional evidence of the blower effect on canopy temperatures. Since increased plant tissue temperatures also occur when elevated CO2 induces partial stomatal closure and reduces transpiration, not all instances of canopy temperature elevation in CO2 enriched plots can be ascribed solely to the presence of blowers. It is concluded that proper controls for FACE facilities should have similar air flows to those used in the FACE plots. Advantages and disadvantages to nighttime CO2 enrichment are discussed.