Limited mitigating effects of elevated CO2 in young aspen trees to face drought stress

Abstract

Elevated atmospheric CO2 concentration (eCO2) is expected to mitigate the adverse effects of moderate drought on leaf and whole-tree functioning. However, tree responses to eCO2 under severe drought and throughout the growing season remain largely unknown. One-year-old Populus tremula L. trees were grown in two controlled treatment chambers under ambient and elevated CO2 conditions, while progressive drought was imposed early (spring/summer 2019) and late (summer/autumn 2018) during the growing season. Leaf level responses to eCO2 (i.e., stomatal conductance, leaf carbon assimilation and leaf respiration) were monitored in concert with whole-tree level responses (i.e., canopy conductance, radial stem growth, stem CO2 efflux, xylem water potential and non-structural carbohydrates (NSC)). At the leaf level, eCO2 lowered the drought susceptibility of stomatal closure and delayed drought-induced reduction in leaf carbon assimilation during late season drought, but these responses were not observed during the early season drought. Drought effects on whole-tree functioning and NSC depletion remained unaltered by eCO2. Under moderate drought, stem volumetric growth ceased earlier than photosynthesis, while leaf and stem respiratory metabolism were maintained at 30 % of well-watered levels even under severe drought, independent of the CO2 treatment and timing of drought. Therefore, the ability of eCO2 to mitigate drought was mainly limited to leaf processes during the late season and under moderate drought (> − 2 MPa), while drought offset any beneficial effect of eCO2 at the whole-tree level. These results urge us to revisit predictions of forests' potential to sequester carbon under climate change scenarios.