The interacting effects of elevated atmospheric CO2 concentration, drought and leaf-to-air vapour pressure deficit on ecosystem isoprene fluxes

Isoprene is the most abundant biogenic hydrocarbon released from vegetation and it plays a major role in tropospheric chemistry. Because of its link to climate change, there is interest in understanding the relationship between CO2, water availability and isoprene emission. We explored the effect of atmospheric elevated CO2 concentration and its interaction with vapour pressure deficit (VPD) and water stress, on gross isoprene production (GIP) and net ecosystem exchange of CO2 (NEE) in two Populus deltoides plantations grown at ambient and elevated atmospheric CO2 concentration in the Biosphere 2 Laboratory facility. Although GIP and NEE showed a similar response to light and temperature, their responses to CO2 and VPD were opposite; NEE was stimulated by elevated CO2 and depressed by high VPD, while GIP was inhibited by elevated CO2 and stimulated by high VPD. The difference in response between isoprene production and photosynthesis was also evident during water stress. GIP was stimulated in the short term and declined only when the stress was severe, whereas NEE started to decrease from the beginning of the experiment. This contrasting response led the carbon lost as isoprene in both the ambient and the elevated CO2 treatments to increase as water stress progressed. Our results suggest that water limitation can override the inhibitory effect of elevated CO2 leading to increased global isoprene emissions in a climate change scenario with warmer and drier climate.