The effect of elevated CO₂ on hyperspectral leaf reflectance in mature trees

Experimentally elevated CO₂ does not significantly alter the overall leaf reflectance of mature Quercus robur L., but increases Plant Senescence Reflectance Index (PSRI) suggesting a change in the ratio of chlorophyll to carotene content. Rising atmospheric CO₂ concentrations, driven by anthropogenic emissions, are projected to reach 550 ppm by 2050. Elevated CO₂ (eCO₂) is expected to have a fertilisation effect on forests, influencing productivity, water relations, and phenology. However, the impact of eCO₂ on leaf reflectance in mature forests remains poorly understood, despite its critical role in radiative transfer processes and remote sensing of forest health. Utilising the Birmingham Institute of Forest Research (BIFoR) Free-Air CO₂ Enrichment (FACE) experiment, we investigated the hyperspectral leaf reflectance of 180-year-old Quercus robur L. trees exposed to eCO₂ for 7 years. Our results demonstrate that overall leaf reflectance under eCO₂ is similar to that of leaves exposed to ambient CO₂, but the Plant Senescence Reflectance Index (PSRI) is significantly higher under eCO₂. This index relates to the ratio of foliar chlorophyll and carotene pigments. These findings suggest that Q. robur reflectance will not significantly shift under future CO₂ conditions, but the relative content of pigments will change, altering the reflectance of specific wavelengths and providing insights into the leaf level physiological and phenological responses of mature trees to eCO₂.