Reply on RC2

Abstract

Climate change can profoundly alter VOC emissions from vegetation and thus influence climate evolution. Yet, the short and long-term effects of elevated CO2 concentrations on emissions in interaction with temperature are not enough understood, especially for VOCs other than isoprene. We measured CO2-response curves of leaf monoterpene emissions and photosynthetic parameters at two assay temperatures (30 and 35 °C) on four populations of holm oak saplings grown under normal and double CO2 concentrations combined with two temperature growth regimes differing by 5 °C. A stepwise reduction in CO2 resulted in a decrease in emissions, occasionally preceded by an increase, with the overall decrease in emissions being greater at 35 °C than at 30 °C assay temperature. During ramping to high CO2, emissions remained mostly unchanged at 35 °C, whereas at 30 °C they often dropped, especially at the highest CO2 levels (≥ 1200 ppm). In addition to the actual leaf temperature, the high CO2-responsiveness of emissions was modulated by the plant’s growth temperature with warm-grown plants being more sensitive than cool-crown plants. In contrast, growth CO2 had no significant effect on emission CO2 sensitivity, although it promoted plant growth and the leaf’s emission factor. Correlation analyses suggest that the emission response to low CO2 depended on the leaf’s initial carbon balance and its actual energy status, whereas the response to high CO2 depended only on the leaf’s actual energy status, which was affected by the occurrence of photooxidative stress and feedback limitation of photosynthesis. Overall, our results confirm an isoprene-analogous behavior of monoterpene emissions from holm oak. Emissions exhibit a nonlinear response curve similar to that currently used for isoprene emission in the MEGAN model, with no difference between major individual monoterpene species and plant chemotype. Simulations estimating annual VOC releases from holm oak leaves at double atmospheric CO2 indicate that the observed high-CO2 inhibition is unlikely to offset the increase in emissions due to expected warming.