Photosynthesis and carbon allocation are both important predictors of genotype productivity responses to elevated CO₂ in Eucalyptus camaldulensis

Intraspecific variation in biomass production responses to elevated atmospheric carbon dioxide (eCO₂) could influence tree species’ ecological and evolutionary responses to climate change. However, the physiological mechanisms underlying genotypic variation in responsiveness to eCO₂ remain poorly understood. In this study, we grew 17 Eucalyptus camaldulensis Dehnh. subsp. camaldulensis genotypes (representing provenances from four different climates) under ambient atmospheric CO₂ and eCO₂. We tested whether genotype leaf-scale photosynthetic and whole-tree carbon (C) allocation responses to eCO₂ were predictive of genotype biomass production responses to eCO₂. Averaged across genotypes, growth at eCO₂ increased in situ leaf net photosynthesis (A_net) (29%) and leaf starch concentrations (37%). Growth at eCO₂ reduced the maximum carboxylation capacity of Rubisco (−4%) and leaf nitrogen per unit area (N_area, −6%), but N_area calculated on a total non-structural carbohydrate-free basis was similar between treatments. Growth at eCO₂ also increased biomass production and altered C allocation by reducing leaf area ratio (−11%) and stem mass fraction (SMF, −9%), and increasing leaf mass area (18%) and leaf mass fraction (5%). Overall, we found few significant CO₂ × provenance or CO₂ × genotype (within provenance) interactions. However, genotypes that showed the largest increases in total dry mass at eCO₂ had larger increases in root mass fraction (with larger decreases in SMF) and photosynthetic nitrogen-use efficiency (PNUE) with CO₂ enrichment.