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

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 (P_n) only in PR, narrowing the ecotypic difference in P_n; this pattern was accompanied by a PR-specific increase in photosynthetic capacity (Vc_max), whereas HR showed little CO₂-driven enhancement. Stomatal conductance (g_s) 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 P_n and photosynthetic capacity (Vc_max and J_max) with g_s 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.