Estimation of canopy photosynthetic and nonphotosynthetic components from spectral transmittance

Spectral transmittance signatures (expressed as absorbances) were studied a s a potential indicator of photosynthetic and nonphotosynthetic contributio ns to the canopy-absorbed photosynthetically active radiation (PAR). An ana lytical approach was made under laboratory conditions using synthetic canop ies in an integrating sphere. This approach provided the basis for identify ing spectral (absorbance-based) features and indices to estimate green (pho tosynthetic) 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 (A(RE)) and green area, while the i ntegrated absorbance in the PAR region (A(PAR)) mainly responded to variati ons in total area. The ratio A(RE)/A(PAR) was closely correlated to the fra ction 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 (A(SR) and A(NDVI)) closely tracked variations in the canopy gr een fraction. Subsequently, these indices were tested in field plots with c ontrasting structural characteristics. Under field conditions, A(RE) was a good indicator of green biomass. The indices A(SR) and A(NDVI) were also re liable indicators of green biomass but were affected by changes in sampling conditions. As in the lab study, A(RE)/A(PAR) was a good indicator of cano py green fraction. Thus, ground-based measurements of canopy spectral trans mittance provided a tool for determining the photosynthetic contribution to canopy-absorbed PAR by correcting for nonphotosynthetic canopy components. Moreover, A(RE) showed a strong correlation with conventional vegetation i ndices derived from spectral reflectance measurements. This technique could be a useful tool for plant ecophysiology studies and a field-validation me thod for remote-sensing studies.