Biochemical and reflectance variation throughout a Sitka spruce canopy

The reflectance properties of tree leaves and canopies are influenced by their biochemical concentration. Modelling and empirical studies have been used to better understand this relationship for the remote sensing of foliar biochemical concentration. The success of these studies have been predicated on two implicit assumptions; first, that variability in biochemical concentration and reflectance within any one canopy is small, and second, that foliar samples from a point (usually the top) of the canopy could be used to represent the canopy as a whole. To evaluate these two assumptions the three-dimensional variation of various biochemical and reflectance characteristics of a Sitka spruce canopy were examined. Biochemical concentrations varied with canopy depth and needle age: chlorophyll and cellulose concentrations increased slightly with canopy depth, water and lignin concentrations were greatest in the lower canopy, and nitrogen concentrations were similar throughout the canopy. Reflectance in visible wavelengths decreased with canopy depth, as did reflectance at the ‘red edge’. Biochemical concentration also varied with needle age: chlorophyll concentration increased with needle age; lignin and cellulose concentrations were similar for all ages of needles though variations in water and nitrogen concentration were more complex. Overall, however, these variations in biochemical concentrations were slight and it was concluded that foliage samples taken from near the top of the canopy and incorporating needles of a variety of ages would be representative of the canopy as a whole for the purposes of the remote sensing of foliar biochemical concentration. This paper also explores the strength of the relationships between biochemical concentration and reflectance and found them all to be weak, with the exception of those between chlorophyll concentration and reflectance.