mainly in enclosure systems, but since the 1990s
with the advent of Free Air Carbondioxide Enrichment (FACE) technology,
plants can be studied under e[CO2] without the constraints and potential artefacts of chambers or glasshouses (Allen, 1992; Hendrey and Miglietta, 2006).
Natural CO2 springs
have also been used as an alternative option to study
plants under e[CO2] in the field (Raschi et al., 1997). Whilst they provide unique opportunities to investigate long-lived trees and whole ecosystems under life-time e[CO2] exposure, a number of other trace gases (often H2S) in such springs complicate interpretations of results (e.g. Badiani et al., 1993).