The reflectance properties of tree leaves and canopies are influenced by their biochemical concentration. Modelling and empirical studies have been used to better understand this relationship for the remote sensing of foliar biochemical concentration. The success of these studies have been predicated on two implicit assumptions; first, that variability in biochemical concentration and reflectance within any one canopy is small, and second, that foliar samples from a point (usually the top) of the canopy could be used to represent the canopy as a whole. To evaluate these two assumptions the three-dimensional variation of various biochemical and reflectance characteristics of a Sitka spruce canopy were examined. Biochemical concentrations varied with canopy depth and needle age: chlorophyll and cellulose concentrations increased slightly with canopy depth, water and lignin concentrations were greatest in the lower canopy, and nitrogen concentrations were similar throughout the canopy. Reflectance in visible wavelengths decreased with canopy depth, as did reflectance at the ‘red edge’. Biochemical concentration also varied with needle age: chlorophyll concentration increased with needle age; lignin and cellulose concentrations were similar for all ages of needles though variations in water and nitrogen concentration were more complex. Overall, however, these variations in biochemical concentrations were slight and it was concluded that foliage samples taken from near the top of the canopy and incorporating needles of a variety of ages would be representative of the canopy as a whole for the purposes of the remote sensing of foliar biochemical concentration. This paper also explores the strength of the relationships between biochemical concentration and reflectance and found them all to be weak, with the exception of those between chlorophyll concentration and reflectance.

Airborne imaging spectrometers can record spatially-explicit information on the absorption features associated with foliar biochemicals in a forest canopy [1]-[8]. The spectra of a single species pine canopy were recorded by the National Aeronautics and Space Administration's (NASA) Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) [9]. Up to three wavebands were correlated to the concentration of chlorophyll, nitrogen, lignin, and cellulose (R(2) = 0.96, 0.94, 0.93, and 0.61, respectively) and the content of these four biochemicals (R(2) = 0.98, 0.91, 0.88, and 0.92, respectively). The AVIRIS data were used, for the first time, to map the content of these biochemicals within the forest canopy and the accuracy was between 3-7% of the mean.

A major goal of airborne imaging spectrometry is to estimate the biochemical composition of vegetation canopies from reflectance spectra. Remotely-sensed estimates of foliar biochemical concentrations of forests would provide valuable indicators of ecosystem function at regional and eventually global scales. Empirical research has shown a relationship exists between the amount of radiation reflected from absorption features and the concentration of given biochemicals in leaves and canopies (Matson et al., 1994, Johnson et al., 1994). A technique commonly used to determine which wavelengths have the strongest correlation with the biochemical of interest is unguided (stepwise) multiple regression. Wavelengths are entered into a multivariate regression equation, in their order of importance, each contributing to the reduction of the variance in the measured biochemical concentration. A significant problem with the use of stepwise regression for determining the correlation between biochemical concentration and spectra is that of 'overfitting' as there are significantly more wavebands than biochemical measurements. This could result in the selection of wavebands which may be more accurately attributable to noise or canopy effects. In addition, there is a real problem of collinearity in that the individual biochemical concentrations may covary. A strong correlation between the reflectance at a given wavelength and the concentration of a biochemical of interest, therefore, may be due to the effect of another biochemical which is closely related. Furthermore, it is not always possible to account for potentially suitable waveband omissions in the stepwise selection procedure. This concern about the suitability of stepwise regression has been identified and acknowledged in a number of recent studies (Wessman et al., 1988, Curran, 1989, Curran et al., 1992, Peterson and Hubbard, 1992, Martine and Aber, 1994, Kupiec, 1994). These studies have pointed to the lack of a physical link between wavelengths chosen by stepwise regression and the biochemical of interest, and this in turn has cast doubts on the use of imaging spectrometry for the estimation of foliar biochemical concentrations at sites distant from the training sites. To investigate this problem, an analysis was conducted on the variation in canopy biochemical concentrations and reflectance spectra using forced entry linear regression.

The remote sensing of foliar biochemical concentration assumes that leaf biochemical absorption features will be manifest in canopy reflectance. This is a reasonable assumption providing the effect of a given change in foliar biochemical concentration has a similar effect on both leaf and canopy reflectance. A comparison between canopy and leaf reflectance was made to determine if canopy effects (composite of leaf area index, biomass, structure, multiple scattering and shadow) could alter the leaf biochemical information in canopy reflectance spectra.Differences in leaf biochemical concentrations and leaf biomass were induced by the application of fertilisers to large plots of slash pine (Pinus elliottii var elliottii) in Florida, U.S.A. The reflectance of plot canopies was measured using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). The reflectance of samples of leaves drawn from each plot were measured using a laboratory spectrometer.The differences between airborne and laboratory reflectance ratios (fertilised/control spectral were used to isolate the effects of the canopy in AVIRIS reflectance spectra.From this study it was concluded that the canopy influenced leaf reflectance substantially at wavelengths beyond the water absorption feature at 1400 nm and leaf biochemical information was transmitted virtually unchanged from the leaf to the canopy in near-infrared wavelengths.