How do drought and elevated temperatures influence CO₂ fertilization effects on tree seedling performance? A global meta‐analysis

Climate change is expected to simultaneously alter many of the abiotic factors that structure forest communities. Increases in atmospheric CO 2 , rising temperatures and intensification of droughts are some of the most significant changes. However, most experimental studies focus on a single factor, and when more factors are accounted for, we still do not fully understand whether the combined impacts are additive, synergistic (amplified) or antagonistic (lessened). We conducted a meta‐analysis to quantitatively synthesize the literature on the interactive effects of climate change variables on tree seedlings. The objective of this analysis was to determine the combined effects of CO 2 , drought and temperature on a set of plant performance metrics. We focused on recruitment, that is, germination, establishment and seedling performance, as these ontogenetic stages are most likely to be affected by environmental change and are the stages targeted by most experiments. Most species studied focused on temperate or boreal species, with very little representation of tropical trees. Overall, we found that the positive effects of elevated CO 2 weakened under drought, while CO 2 combined with elevated temperature tended to enhance plant performance. In contrast, drought and temperature together generally reduced plant performance. Most combined effects were additive, that is, total impact was consistent with the sum of individual effects. However, we also observed some antagonistic effects in germination and photosynthesis, that is, the joint impact was less severe than expected based on the individual effects. We found no evidence of synergistic effects, that is, the combined effects were never greater than expected from the sum of individual treatments. Plant responses were generally consistent across performance metrics evaluated, suggesting coordinated responses across different organs. However, the analysis revealed genus‐specific differences in performance responses, with Pinus and Quercus having the strongest responses. Synthesis . These results suggest that the fertilization effect of CO 2 at the seedling stage may be limited when additional environmental stressors, such as drought, are present. The combined effect of drought, temperature and CO 2 mainly resulted in additive effects, where each stressor independently influences plant performance rather than interacting to amplify or lessen their effects.