Effects of shoot-level structure on narrowband vegetation indices for Scots pine

We measured simultaneously the spectral albedo of ten Scots pine shoots, and the needles constituting the shoots, in Zurich, Switzerland. Next, we used the spectral information to calculate the photochemical reflectance index (PRI) which allows to retrieve photosynthetic productivity from imaging spectroscopy data. Using p-theory, we showed that PRI values can be scaled between needle and shoot levels using photon recollision probability and the harmonic mean of needle spectral albedo at the wavelengths used in the index. As a comparison, we used narrowband NDVI and demonstrated that despite mathematical similarity, the two indices scale differently. Contrary to NDVI, the needle-level information contained in PRI scaled well to the level of the shoot.