Elevated CO2 Modifies Within-Species Differences in Relative Drought Resistance With Implications for Recovery in Populus balsamifera.

Forecasting forest responses to climate change requires mechanistic insight into how rising atmospheric CO2 concentration ([CO2]) affects within-species drought tolerances. We tested whether elevated [CO2] alters drought resistance and recovery using cuttings of two ecotypes of Populus balsamifera (Peace River, PR; Hay River, HR) grown under factorial combinations of [CO2] (400 vs. 1000 μmol mol-1) and water regime (well-watered vs. drought followed by re-watering). Under drought, elevated [CO2] compensated for the water deficit in net photosynthesis (Pn) only in PR, narrowing the ecotypic difference in Pn; this pattern was accompanied by a PR-specific increase in photosynthetic capacity (Vcmax), whereas HR showed little CO2-driven enhancement. Stomatal conductance (gs) declined with drought across [CO2] levels, and intrinsic water-use efficiency (WUEᵢ) increased with both drought and elevated [CO2]. Following re-watering, drought plants showed strong recovery of Pn and photosynthetic capacity (Vcmax and Jmax) with gs and the ratio of intercellular [CO2] to ambient [CO2] becoming comparable to those in well-watered trees, indicating recovery of both diffusional and biochemical limitations. Moreover, whole-plant responses remained ecotype dependent: drought reduced total biomass in both ecotypes, with a greater reduction in PR, while HR expressed lower specific leaf area and leaf area ratio under drought. Ecotype-dependent responses to the interaction between elevated [CO2] and drought highlight the need to consider intraspecific variation in process-based models to improve projections of population responses under future climates.