Simulating tree responses to elevated CO₂ and climate change in agroforestry system

This study extends the Hi-sAFe agroforestry model by incorporating the effects of elevated atmospheric CO₂ on tree growth. Hi-sAFe is a process-based biophysical model that represents tree-crop interactions, but until now lacked a mechanism to simulate CO₂-driven physiological responses of trees. We introduced CO₂ sensitivity into the Light Use Efficiency (LUE) module and simulated the growth of Juglans nigra (black walnut) under current and future climate conditions (550 ppm CO₂, + 3 °C, - 10% precipitation). Elevated CO₂ increased tree height, diameter and root development in both forestry and agroforestry systems, but responses were stronger and more persistent in forestry. In agroforestry, CO₂ effects were more variable over time and were strongly modulated by competition with crops, while belowground responses indicated greater root system plasticity. Climate change reduced tree growth in both systems, but CO₂ partially offset these effects, particularly in forestry. These results demonstrated the importance of explicitly representing CO₂ fertilisation in agroforestry models to realistically capture vegetation-climate interactions and assess the resilience of tree-crop systems under global change.