Early-growth results within a Eucalyptus globulus breeding population suggest limited scope for selection focused on CO₂ responsiveness

Increasing [CO₂] may influence commercial crop and timber yield. While selection of genotypes sensitive to elevated [CO₂] (e[CO₂]) appears possible in agricultural crops, there is limited evidence for genotype-by-CO₂ (G × CO₂) interactions in commercial tree species. We examined [CO ₂] responsiveness in 124 open-pollinated Eucalyptus globulus ssp. globulus (E. globulus) families with the aim of assessing whether G × CO₂ interactions are detectable in seedlings for early-age screening. Plants were grown in ambient (a[CO₂]; ~ 405 μmol mol⁻¹) and e[CO ₂] (640 μmol mol⁻¹) and harvested 25 days after germination. Total, shoot, and root dry weights were determined for each plant. Carbon isotopic discrimination against ¹³C (Δ ¹³C) was determined at the family level. We observed highly significant (p < 0.0001) increases in mean total, shoot, and root dry weights. Mixed-model equations were used to estimate the main and interaction effects of the G × CO₂ for each mass trait. The main effects from the mixed-model output ([CO₂] and individual-tree effects) were significant for all traits. However, [CO₂]-by-individual tree interactions were non-significant for all traits, indicating little G × CO₂ interaction. A secondary aim was to examine the correlation between greenhouse and mature-age growth from breeding trials that use common families conducted under ambient [CO₂]. These correlations were non-significant, suggesting early growth is not necessarily indicative of later-age responses. Our results suggest that while early growth of E. globulus is enhanced under e[CO₂], genotypes respond relatively uniformly to e[CO₂] and little opportunity exists for seedling-based selection at the population level based upon the response of plants during the first weeks of growth.