The effect of elevated CO2, soil and atmospheric water deficit and seasonal phenology on leaf and ecosystem isoprene emission.

Two cottonwood plantations were grown at different CO2 concentrations at the Biosphere 2 Laboratory in Arizona to investigate the response of isoprene emission to elevated [CO2] and its interaction with water deficits. We focused on responses due to seasonal variation and variation in the mean climate from one year to the next. In fall and in spring, isoprene emission rate showed a similar inhibition by elevated [CO2], despite an 8-10°C seasonal difference in mean air temperature. The overall response of isoprene emission to drought was also similar for observations conducted during the spring or fall, and during the fall of two different years with an approximate 5°C difference in mean air temperature. In general, leaf-level isoprene emission rates, measured at constant temperature and photon-flux density, decreased slightly, or remained constant during drought, whereas ecosystem-level isoprene emission rates increased. The uncoupling of ecosystem- and leaf-scale responses is not due to differential dependence on leaf area index (LAI) as LAI increased only slightly, or decreased, during the drought treatments at the same time that ecosystem isoprene emission rate increased greatly. Nor does the difference in isoprene emission rate between leaves and ecosystems appear to be due solely to increases in canopy surface temperature during the drought, though some increase in temperature was observed. It is possible that still further factors, such as increased penetration of PPFD into the canopy as a result of changes in leaf angle, reduced sink strength of the soil for atmospheric isoprene, and decreases in the mean Ci of leaves, combined with the small increases in canopy surface temperature, increased the ecosystem isoprene emission rate. Whatever the causes of the differences in the leaf and ecosystem responses, we conclude that the overall shape of the leaf and ecosystem responses to drought was constant irrespective of season or year.

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.

Effect of elevated CO2 concentration and vapour pressure deficit on isoprene emission from leaves of Populus deltoides during drought.

To further our understanding of the influence of global climate change on isoprene production we studied the effect of elevated [CO2] and vapour pressure deficit (VPD) on isoprene emission rates from leaves of Populus deltoides Bartr. during drought stress. Trees, grown inside three large bays with atmospheres containing 430, 800, or 1200 µmol mol-1 CO2 at the Biosphere 2 facility, were subjected to a period of drought during which VPD was manipulated, switching between low VPD (approximately 1 kPa) and high VPD (approximately 3 kPa) for several days. When trees were not water-stressed, elevated [CO2] inhibited isoprene emission and stimulated photosynthesis. Isoprene emission was less responsive to drought than photosynthesis. As water-stress increased, the inhibition of isoprene emission disappeared, probably as a result of stomatal closure and the resulting decreases in intercellular [CO2] (Ci). This assumption was supported by increased isoprene emission under high VPD. Drought and high VPD dramatically increased the proportion of assimilated carbon lost as isoprene. When measured at the same [CO2], leaves from trees grown at ambient [CO2] always had higher isoprene emission rates than the leaves of trees grown at elevated [CO2], demonstrating that CO2 inhibition is a long-term effect.