The impact of elevated CO₂ concentration on photosynthesis, growth and hydraulics of evergreen and deciduous tree seedlings from a subtropical forest in Southwest China

Elevated CO₂ concentration (eCO₂) in the atmosphere is expected to impact plant water relations and growth in several ecosystems across the globe. However, we still know little about such impact on tree species in subtropical regions. The present study investigated the impact of eCO₂ on leaf gas exchange, nitrogen and phosphorus concentrations, leaf and stem hydraulic conductivity, and growth of seedlings of four evergreen and four deciduous tree species from a subtropical forest in Southwest China. We found that both evergreen and deciduous tree species at eCO₂ had higher leaf area-based photosynthetic rates and lower leaf stomatal conductance. Further, leaf mass-based photosynthetic rate was more enhanced in evergreen than in deciduous trees at eCO₂. Biomass of evergreen and deciduous species was significantly higher at eCO₂, with large species-specific variation among the evergreen species. Leaf-specific hydraulic conductivity was more enhanced in deciduous tree species than that of evergreen tree species with eCO₂, which was mainly driven by the increase of biomass at eCO₂. Interestingly, eCO₂ significantly strengthened the coordination of stem hydraulic conductivity with leaf-gas exchange, leaf phosphorus concentration, and plant biomass across evergreen and deciduous species. These results highlighted greater enhancement of photosynthesis and greater species-specific variation in biomass at eCO₂ for evergreen species compared to deciduous species, and stronger hydraulic-photosynthesis correlations at eCO₂ than at aCO₂ for tree species from subtropical forests. The present study provides important insights on the potential impacts of eCO₂ on plant eco-physiology, growth and forest succession in a subtropical forest under global climate change.