24 FACE Value: Perspectives on the Future of Free-Air CO2 Enrichment Studies

Free-air CO2 enrichment (FACE) studies are the ultimate test bed for hypothe-ses that seek to explain how plants respond to rising [CO2]; and they provide the most realistic conditions for simulating the impact of future elevated (e)[CO2] levels (see Chapter 2). FACE studies have many benefits over con-trolled environment and open-top chamber (OTC) experiments. FACE allows the investigation of an undisturbed ecosystem and does not modify the vege-tation’s interaction with light, temperature, wind, precipitation, pathogens and insects (Long et al. 2004). This, in combination with the large size of FACE plots, allows the integrated measurement of many plant and ecosystem processes simultaneously in the same plot, avoids many of the problems asso-ciated with edge effects prevalent in OTCs (Long et al. 2004), enables signifi-cantly more plant material to be harvested without compromising the exper-iment, and allows plants to be studied throughout their life cycle, including trees that have enough space to develop to canopy closure. FACE experiments are not without their problems (see Chapter 2). In par-ticular, there has been criticism that the application of a step increase in [CO2] to ecosystems imposes a strong perturbation (Luo and Reynolds 1999; New-ton et al. 2001). Luo and Reynolds (1999) modelled the effects of a step increase in [CO2] and found that the demand for N caused by the additional carbon influx from the step increase is much greater than that caused by a gradual increase in [CO2], resulting in an initial overestimation of carbon sequestration in experiments using a step increase in [CO2]. This should be a minor problem in annual agro-ecosystems where plants have spent their whole life-cycle under FACE. However, in natural ecosystems, or perennial agro-ecosystems, a step increase in [CO2] is more likely to have a significant effect. Continued support is crucial for many existing FACE studies if short