Leaf reflectance and transmission properties (350-2500 nm): Implications for vegetation indices

摘要

At moderate to high leaf area index (values 3-5), many ratio-based vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), reach an asymptote where the linear relationship between leaf area index and vegetation index value breaks down. The red and near infrared channels are used to calculate most ratio vegetation indices when using sensors such as Landsat; however, these channels sense very different depths in vegetation canopies due to differences in transmittance, which may explain this breakdown of vegetation indices. In laboratory-simulated canopies composed of four deciduous species, visible wavelengths (similar to 400-700 nm) were mostly attenuated by the first or second leaf layer, while near infrared wavelengths were substantially transmitted beyond the sixth or seventh leaf layer. Absolute changes in reflectance > 1% were seen in some canopies up to four leaf layers thick in the near infrared wavelengths. Therefore, in natural canopies, near infrared wavelengths have a greater probability of penetrating to the soil/litter background than visible wavelengths, which may impact vegetation indices that use both visible and near infrared wavelengths for canopies between two and seven layers thick. While this was a preliminary study that isolated the canopy depth variable, polynomial regression analysis showed that differences in canopy thickness explained most of the observed variability in canopy reflectance. These results will facilitate the development and assessment of spectral vegetation indices that would probe canopies to consistent depths.