Isoprene emissions from plants are mediated by atmospheric CO 2 concentrations

The tropical African tree species Acacia nigrescens Oliv. was grown in environmentally controlled growth chambers at three CO 2 concentrations representative of the Last Glacial Maximum (∼180 ppmv), the present day (∼380 ppmv), and likely mid‐21st century (∼600 ppmv) CO 2 concentrations. Isoprene (C 5 H 8 ) emissions, per unit leaf area, were greater at lower‐than‐current CO 2 levels and lower at higher‐than‐current CO 2 levels relative to controls grown at 380 ppmv CO 2 . Changes in substrate availability and isoprene synthase (IspS) activity were identified as the mechanisms behind the observed leaf‐level emission response. In contrast, canopy‐scale emissions remained unaltered between the treatments as changes in leaf‐level emissions were offset by changes in biomass and leaf area. Substrate concentration and IspS activity‐CO 2 responses were used in a biochemical model, coupled to existing isoprene emission algorithms, to model isoprene emissions from A. nigrescens grown for over 2 years at three different CO 2 concentrations. The addition of the biochemical model allowed for the use of emission factors measured under present day CO 2 concentrations across all three CO 2 treatments. When isoprene emissions were measured from A. nigrescens in response to instantaneous changes in CO 2 concentration, the biochemical model satisfactorily represented the observed response. Therefore, the effect of changes in atmospheric CO 2 concentration on isoprene emission at any timescale can be modelled and predicted.