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

Free-air carbon dioxide enrichment (FACE) provides a realistic, cost-effect ive method for evaluating the effects of supra-ambient CO2 concentrations o n growth, development, yield, and water use of agricultural crops and natur al ecosystems with very few of the problems normally associated with glassh ouse or chamber type research. There are no walls interfering with incident radiation and no artificial constraints on rooting depth. With current FAC E technology, CO2 enriched air is injected around the perimeter of circular plots and natural wind disperses the CO2 across the experimental area. Und er stable, night-time wind conditions found in FACE wheat experiments at Ma ricopa, Arizona, the blowers used to inject CO2 exerted subtle effects on t he microclimate in a manner analogous to wind machines used for orchard fro st protection. Plots equipped with blowers had nighttime foliage and air te mperatures that averaged 0.6-1.0 degrees C warmer than controls without blo wers. A secondary effect of these elevated temperatures was that plots equi pped with blowers displayed differences in dew duration (time that leaves w ere wet was reduced 30%), plant development (anthesis occurred 4 days earli er), and senescence [as measured with the normalized difference vegetation index (NDVI)]. Natural wind and turbulence appear to overcome the blower ef fect during daytime treatments and on some nights. Aerial thermal imagery ( 8-12 mu m) acquired during the 1998 FACE experiment with grain sorghum prov ided additional evidence of the blower effect on canopy temperatures. Since increased plant tissue temperatures also occur when elevated CO2 induces p artial stomatal closure and reduces transpiration, not all instances of can opy temperature elevation in CO2 enriched plots can be ascribed solely to t he presence of blowers. It is concluded that proper controls for FACE facil ities should have similar air hows to those used in the FACE plots. Advanta ges and disadvantages to nighttime CO2 enrichment are discussed. (C) 2000 E lsevier Science B.V. All rights reserved.