Elevated CO ₂ 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 CO 2 concentration ([CO 2 ]) affects within‐species drought tolerances. We tested whether elevated [CO 2 ] alters drought resistance and recovery using cuttings of two ecotypes of Populus balsamifera (Peace River, PR; Hay River, HR) grown under factorial combinations of [CO 2 ] (400 vs. 1000 μmol mol −1 ) and water regime (well‐watered vs. drought followed by re‐watering). Under drought, elevated [CO 2 ] compensated for the water deficit in net photosynthesis ( P n ) only in PR, narrowing the ecotypic difference in P n ; this pattern was accompanied by a PR‐specific increase in photosynthetic capacity ( V c max ), whereas HR showed little CO 2 ‐driven enhancement. Stomatal conductance ( g s ) declined with drought across [CO 2 ] levels, and intrinsic water‐use efficiency ( WUE ᵢ) increased with both drought and elevated [CO 2 ]. Following re‐watering, drought plants showed strong recovery of P n and photosynthetic capacity ( V c max and J max ) with g s and the ratio of intercellular [CO 2 ] to ambient [CO 2 ] 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 [CO 2 ] and drought highlight the need to consider intraspecific variation in process‐based models to improve projections of population responses under future climates.