Scaling PRI between coniferous canopy structures

We measured simultaneously the spectral albedo of ten Scots pine shoots and the needles constituting the shoots. Next, we used the spectral information to calculate the photochemical reflectance index (PRI) which allows to retrieve photosynthetic productivity from imaging spectroscopy data. We showed that PRI can be scaled from needle to shoot level using a strictly positive scaling factor. The scaling factor is a function of photon recollision probability and the harmonic mean of needle spectral albedos at the wavelengths used in the index. The method is applicable to other normalized difference indices, as demonstrated here using normalized difference vegetation index (NDVI). At shoot level, both PRI and NDVI depend on the angle between view and illumination directions. This anisotropy, however, is not a direct function of scattering angle