A free‐air CO2 enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO2 concentrations ([CO2]a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO2] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole‐stand CO2 regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO2 enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO2] of 550 μmol mol–1, ≈ 200 μmol mol–1 above ambient [CO2]. The system provided spatial and temporal control of [CO2] similar to that reported for open‐top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO2] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol–1 (mean ± SD). The FACE system maintained 1‐minute average [CO2] to within ± 110 μmol mol–1 of the target [CO2] for 92% of the operating time. Deviations of [CO2] outside of this range were short‐lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO2] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO2] over the exposure season. CO2 consumption by the FACE system was much higher than for open‐top chambers on an absolute basis, but similar to that of open‐top chambers and branch bag chambers on a per unit volume basis. CO2 consumption by the FACE system was strongly related to windspeed, averaging 50 g CO2 m–3 h–1 for the stand for an average windspeed of 1.5 m s–1 during summer. The [CO2] control results show that the free‐air approach is a tractable way to study long‐term and short‐term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO2]a that were previously impossible or intractable to study in true forest ecosystems.

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Summary Arid and semiarid climates comprise roughly 40% of the earth’s terrestrial surface. Deserts are predicted to be extremely responsive to global change because they are stressful environments where small absolute changes in water availability or use represent large proportional changes. Water and carbon dioxide fluxes are inherently coupled in plant growth.  No documented global change has been more substantial or more rapid than the increase in atmospheric CO2. Free Air CO2 Enrichment (FACE) technology permits manipulation of CO2 in intact communities without altering factors such as light intensity or quality, humidity or wind. The Nevada Desert FACE Facility (NDFF) consists of three 491 m2 plots in the Mojave Desert receiving 550 μL L–1 CO2, and six ambient plots to assess both CO2 and fan effects. The shrub community was characterized as a Larrea–Ambrosia–Lycium species complex. Data are reported through 12 months of operation.

A rising global population and demand for protein-rich diets are increasing pressure to maximize agricultural productivity. Rising atmospheric [CO2] is altering global temperature and precipitation patterns, which challenges agricultural productivity. While rising [CO2] provides a unique opportunity to increase the productivity of C3 crops, average yield stimulation observed to date is well below potential gains. Thus, there is room for improving productivity. However, only a fraction of available germplasm of crops has been tested for CO2 responsiveness.Yield is a complex phenotypic trait determined by the interactions of a genotype with the environment. Selection of promising genotypes and characterization of response mechanisms will only be effective if crop improvement and systems biology approaches are closely linked to production environments, that is, on the farm within major growing regions. Free air CO2 enrichment (FACE) experiments can provide the platform upon which to conduct genetic screening and elucidate the inheritance and mechanisms that underlie genotypic differences in productivity under elevated [CO2]. We propose a new generation of large-scale, low-cost per unit area FACE experiments to identify the most CO2-responsive genotypes and provide starting lines for future breeding programmes. This is Correspondence: E. A.

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