Remote sensing of canopy nitrogen as a window to the functioning of ecosystems

How plants interact with sunlight is central to the existence of life and provides a window to the functioning of ecosystems.  Although the basic properties of leaf spectra have been known for decades, interpreting spectral patterns of whole ecosystems is more challenging because leaf-level effects are complicated by a host of stem- and canopy-level traits. Progress has been made through empirical analyses and models, although both methods have been hampered by a series of persistent challenges.

In this study, we examined spectral properties of forests across North America in relation to concentrations of nitrogen in foliage and a suite of related ecosystem properties.  We also explored the effects of spectral resolution, spatial resolution and radiometric resolution on estimates of canopy %N using remote sensing data from AVIRIS (the Airborne Visible/InfraRed Imaging Spectrometer), Landsat and MODIS (Moderate Resolution Imaging Spectroradiometer).  Interestingly, we observed virtually no loss of %N estimation accuracy with coarser bandwidths, but instead saw accuracy decline as radiometric resolution and spatial resolution declined. We also found that regression models were driven primarily by reflectance in the near infrared (NIR) region, with little contribution from the visible or mid infrared regions. These results suggest that much of the variability in canopy N concentration is related to reflectance properties in the NIR region, and suggests a synergy among the scattering effects of leaf-, stem- and canopy-level traits that becomes accentuated in the near-infrared (NIR) region. This poses a challenge for remote detection of specific plant properties, but suggests an emergent property of ecosystems that results from optimization of plant form and function across multiple scales.  It also presents an interesting paradox that has yet to be fully explored: that we can often gain more insight about the functioning of plants by examining wavelengths that are not used in photosynthesis than by examining those that are.