A strong positive correlation between vegetation canopy bidirectional reflectance factor (BRF) in the near infrared (NIR) spectral region and foliar mass-based nitrogen concentration (%N) has been reported in some temperate and boreal forests. This relationship, if true, would indicate an additional role for nitrogen in the climate system via its influence on surface albedo and may offer a simple approach for monitoring foliar nitrogen using satellite data. We report, however, that the previously reported correlation is an artifact—it is a consequence of variations in canopy structure, rather than of %N. The data underlying this relationship were collected at sites with varying proportions of foliar nitrogen-poor needleleaf and nitrogen-rich broadleaf species, whose canopy structure differs considerably. When the BRF data are corrected for canopy-structure effects, the residual reflectance variations are negatively related to %N at all wavelengths in the interval 423–855 nm. This suggests that the observed positive correlation between BRF and %N conveys no information about %N. We find that to infer leaf biochemical constituents, e.g., N content, from remotely sensed data, BRF spectra in the interval 710–790 nm provide critical information for correction of structural influences. Our analysis also suggests that surface characteristics of leaves impact remote sensing of its internal constituents. This further decreases the ability to remotely sense canopy foliar nitrogen. Finally, the analysis presented here is generic to the problem of remote sensing of leaf-tissue constituents and is therefore not a specific critique of articles espousing remote sensing of foliar %N.

The Radiation Transfer Model Intercomparison (RAMI) initiative benchmarks canopy reflectance models under well‐controlled experimental conditions. Launched for the first time in 1999, this triennial community exercise encourages the systematic evaluation of canopy reflectance models on a voluntary basis. The first phase of RAMI focused on documenting the spread among radiative transfer (RT) simulations over a small set of primarily 1‐D canopies. The second phase expanded the scope to include structurally complex 3‐D plant architectures with and without background topography. Here sometimes significant discrepancies were noted which effectively prevented the definition of a reliable “surrogate truth,” over heterogeneous vegetation canopies, against which other RT models could then be compared. The present paper documents the outcome of the third phase of RAMI, highlighting both the significant progress that has been made in terms of model agreement since RAMI‐2 and the capability of/need for RT models to accurately reproduce local estimates of radiative quantities under conditions that are reminiscent of in situ measurements. Our assessment of the self‐consistency and the relative and absolute performance of 3‐D Monte Carlo models in RAMI‐3 supports their usage in the generation of a “surrogate truth” for all RAMI test cases. This development then leads (1) to the presentation of the “RAMI Online Model Checker” (ROMC), an open‐access web‐based interface to evaluate RT models automatically, and (2) to a reassessment of the role, scope, and opportunities of the RAMI project in the future.

Reliable information on the optical properties of leaves and needles is needed for parameterization of radiative transfer models and interpretation of remotely sensed data. The optical properties also convey information about the structure and biochemical constituents of the leaf or needle tissues, and can be linked to the photosynthetic processes of plants. Currently, very little is known about the optical properties of tree species in the European boreal zone. To bridge this gap, we measured directional-hemispherical reflectance and transmittance factors of the three most common tree species in this zone: Scots pine, Norway spruce and Silver birch. The measurements covered a wide spectral range from 350 to 2500 nm with a high spectral resolution of 3–10 nm. To explore the driving factors of the observed leaf-level optical properties, supplementary measurements of structural and biochemical traits of leaves and needles were made. The results showed that the transmittance of pine and spruce needles is clearly lower than reflectance, whereas for birch reflectance and transmittance were similar. In conifers, exposed needles had higher albedo than the shaded needles. Also, the spectra of needles were more dependent on canopy position than the spectra of birch leaves. The relationships between narrowband reflectances and chlorophyll and nitrogen percentage concentrations and specific leaf area were similar for all species, but the strongest correlations were observed for birch.