Approach and Methodology

There are several approaches one might use to determine the impact of global change on crop production. This chapter will discuss the use of naturally sunlit chambers known as soil—plant—atmosphere research (SPAR) units, which are capable of controlling and monitoring temperature, water, nutrients and [CO2] systematically. The SPAR facility has been described in detail elsewhere (K.R. Reddy et al., 1992b; V.R. Reddy et al., 1995b; V.R. Reddy and K.R. Reddy, 1998). It has the advantage of having near-natural levels of radiation incident on the plants, and excellent control of other physical factors. Crop data obtained in this manner are less ambiguous and allow understanding of the responses to environmental variables and nutrient status. SPAR units have the disadvantage of containing a limited number of plants, so experiments that require much destructive sampling during the growing season must be avoided. Experiments using SPAR units have shown unambiguously that cotton responds to several environmental factors of concern in a changing climate (Table 8.1). Original references should be checked to determine how individual experiments were conducted.

This chapter will also discuss some results from the experiments conducted in open-top CO2-enrichment chambers (OTCs) and free-air carbon dioxide enrichment (FACE) settings in Arizona (Table 8.2). The OTCs enable plants to be grown in normal field soils and in aerial environments that approach open-field conditions but with elevated levels of CO2 (Kimball et al., 1992b, 1997; Kimball and Mauney, 1993). However, like other chamber methods, the walls shade the plants somewhat, and the wind-flow patterns and energy exchange processes are different from those outside. Generally, air temperatures are somewhat warmer and humidities are somewhat higher inside than outside (Kimball et al., 1997).

The FACE approach allows experimental plants to be grown under conditions as representative of open fields in the future high-[CO2] world as is possible to create them today (Hendrey, 1993; Dugas and Pinter, 1994). Although precise techniques have detected a slight disturbance of the microclimate if the crop is enriched at night (Pinter et al., 2000), the FACE plants generally experience full sunlight and normal wind, air temperature and humidity conditions. The relatively large plot size enables frequent destructive sampling and also enables researchers from many different disciplines to make measurements on nearly identically grown plant material (Kimball et al., 1997). A major disadvantage of the FACE technique is the high cost of the prodigious amounts of CO2 required. However, because the relatively large plot size provides an economy of scale, the FACE approach is least expensive per unit of high-[CO2]-grown plant material (Kimball, 1993).

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