The effect of elevated CO on hyperspectral leaf reflectance in mature trees.

Experimentally elevated CO does not significantly alter the overall leaf reflectance of mature , but increases Plant Senescence Reflectance Index (PSRI) suggesting a change in the ratio of chlorophyll to carotene content. Rising atmospheric CO concentrations, driven by anthropogenic emissions, are projected to reach 550 ppm by 2050. Elevated CO (eCO) is expected to have a fertilisation effect on forests, influencing productivity, water relations, and phenology. However, the impact of eCO 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 CO Enrichment (FACE) experiment, we investigated the hyperspectral leaf reflectance of 180-year-old L. trees exposed to eCO for 7 years. Our results demonstrate that overall leaf reflectance under eCO is similar to that of leaves exposed to ambient CO, but the Plant Senescence Reflectance Index (PSRI) is significantly higher under eCO. This index relates to the ratio of foliar chlorophyll and carotene pigments. These findings suggest that reflectance will not significantly shift under future CO 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 eCO.