A free‐air enrichment system for exposing tall forest vegetation to elevated atmospheric CO₂

Abstract

Abstract A free‐air CO 2 enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO 2 concentrations ([CO 2 ] 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 [CO 2 ] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole‐stand CO 2 regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO 2 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 [CO 2 ] of 550 μmol mol –1 , ≈ 200 μmol mol –1 above ambient [CO 2 ]. The system provided spatial and temporal control of [CO 2 ] similar to that reported for open‐top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO 2 ] 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 [CO 2 ] to within ± 110 μmol mol –1 of the target [CO 2 ] for 92% of the operating time. Deviations of [CO 2 ] 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 [CO 2 ] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO 2 ] over the exposure season. CO 2 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. CO 2 consumption by the FACE system was strongly related to windspeed, averaging 50 g CO 2 m –3 h –1 for the stand for an average windspeed of 1.5 m s –1 during summer. The [CO 2 ] 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 [CO 2 ] a that were previously impossible or intractable to study in true forest ecosystems.