ESTIMATION OF CANOPY PHOTOSYNTHETIC AND NONPHOTOSYNTHETIC COMPONENTS FROM SPECTRAL TRANSMITTANCE

Abstract

Spectral transmittance signatures (expressed as absorbances) were studied as a potential indicator of photosynthetic and nonphotosynthetic contributions to the canopy-absorbed photosynthetically active radiation (PAR). An analytical approach was made under laboratory conditions using synthetic canopies in an integrating sphere. This approach provided the basis for identifying spectral (absorbance-based) features and indices to estimate green (photosynthetic) and nongreen (structural and dead materials) contributions to canopy absorbance. A strong relationship was found between the amplitude of the first derivative of the absorbance (ARE) and green area, while the integrated absorbance in the PAR region (APAR) mainly responded to variations in total area. The ratio ARE/APAR was closely correlated to the fraction of photosynthetic area to total area (i.e., the canopy green fraction). Similarly, the ratio and normalized difference of the absorbances at 680 and 900 nm (ASR and ANDVI) closely tracked variations in the canopy green fraction. Subsequently, these indices were tested in field plots with contrasting structural characteristics. Under field conditions, ARE was a good indicator of green biomass. The indices ASR and ANDVI were also reliable indicators of green biomass but were affected by changes in sampling conditions. As in the lab study, ARE/APAR was a good indicator of canopy green fraction. Thus, ground-based measurements of canopy spectral transmittance provided a tool for determining the photosynthetic contribution to canopy-absorbed PAR by correcting for nonphotosynthetic canopy components. Moreover, ARE showed a strong correlation with conventional vegetation indices derived from spectral reflectance measurements. This technique could be a useful tool for plant ecophysiology studies and a field-validation method for remote-sensing studies.