Differential Responses of Tree Species to Elevated Ozone and Increasing Air Temperature: Implications for Foliar Functional Traits, Carbon Sequestration, and Their Relationship Under Mixed Planting

Ozone pollution and global warming are affecting plant growth and ecosystem functions considerably. However, the information is limited on the effects of these factors on foliar traits and carbon sequestration (CS). This study evaluated the effects of elevated ozone (EO, ambient air +80 ppb) and increased air temperature (IT, ambient air +2 °C) alone and the combination of these on foliar traits and CS in Quercus mongolica and Pinus tabuliformis under single (SP) and mixed planting (MP) conditions. The results showed that CS increased by 24.3% in Q. mongolica and decreased by 5.3% in P. tabuliformis under MP. EO decreased CS, while IT increased it (p < 0.05). Under MP, IT mitigated ozone’s negative impact on CS of P. tabuliformis, but exacerbated it on Q. mongolica. Structural equation modeling revealed that ozone reduced CS by reducing the photosynthesis rate (Pn) under SP and by reducing leaf length under MP in Q. mongolica. IT enhanced CS by increasing Pn, leaf thickness (LT) under SP, and LT under MP only in P. tabuliformis. Pn had the highest total effect. Overall, MP can modulate environmental stress effects on CS, but this varies by species. Future research should focus on long-term, cross-species studies to provide practical strategies for ecosystem management.