LIBERTY—Modeling the Effects of Leaf Biochemical Concentration on Reflectance Spectra

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Abstract

The conifer leaf model LIBERTY (Leaf Incorporating Biochemistry Exhibiting Reflectance and Transmittance Yields) is an adaptation of radiative transfer theory for determining the optical properties of powders. LIBERTY provides a simulation, at a fine spectral resolution, of quasiinfinite leaf reflectance (as represented by stacked leaves) and single leaf reflectance. Single leaf reflectance and transmittance are important input variables to vegetation canopy reflectance models. A prototype parameterization of LIBERTY was based upon measurements of pine needles and known absorption coefficients of pure component leaf biochemicals. The estimated infinite-reflectance output was compared with the spectra of both dried and fresh pine needles with root mean square errors (RMSE) of 2.87% and 1.73%, respectively. The comparisons between measured and estimated reflectance and transmittance values for single needles were also very accurate with RSME of 1.84% and 1.12%, respectively. Initial inversion studies have demonstrated that significant improvements can be made to LIBERTY by utilizing in vivo absorption coefficients which have been determined by the inversion process. These results demonstrate the capability of LIBERTY to model accurately the spectral response of pine needles.

Introduction

Understanding the interaction of radiation with vegetation canopies requires a knowledge of the spectral properties of individual leaves (Aber, 1994). Leaves are an important component of forest canopies, and it is the concentration of their biochemical constituents, namely, pigments, water, nitrogen, cellulose, and lignin, together with canopy structure that shapes the absorption features of remotely sensed reflectance spectra. Absorption features in the near infrared region of the spectrum (1000–2500 nm) are a function of the bending and stretching vibrations of biochemical bonds (between, for example, hydrogen–carbon and nitrogen–oxygen atoms) together with their harmonics and overtones (Kemp, 1987). In the visible region, chlorophyll and carotenoid pigments have strong absorption due to electron energy transitions (Lichtenhaler, 1987; Mackinney, 1941). Fine spectral resolution remotely sensed data can be statistically analyzed to estimate the concentration of biochemicals in forest canopies. Such information has been used to drive ecosystem simulation models and estimate photosynthetic efficiency, the rate of nutrient cycling, and the degree of vegetation stress (Curran, 1994). Strong correlations between leaf biochemical concentrations and specific wavebands of measured spectra have now been reported by Wessman et al. (1988), Martin and Aber (1990), Curran and Kupiec (1995), and Zagolski et al. (1996). However, wavebands selected by multiple linear regression using biochemical assay data are often not consistent with the absorption features of the biochemicals within the leaves. Even when known causal wavebands are selected in the regression equation, spectral correlation with biochemical concentrations are not always strong (Dawson et al., 1995a). To investigate this further, a physical modeling approach is required to describe and quantify the reflectance, scattering, and absorption of canopy leaves as a function of their biochemical and physical properties.

Some of the earliest research on leaf reflectance modeling was based upon the Kubelka–Munk (K–M) radiative transfer theory (Allen and Richardson, 1968), and this theory has since been used by others Yamada and Fujimura 1988, Conel et al. 1993. The K–M approach is a two-stream approximation to the radiative-transfer equation. Later Allen et al. (1969) proposed a single layer “plate” model. By extending the plate model to several layers with air spaces between plates Allen et al. (1970) was able to simulate multiple scattering which occurs as a result of the air space/cell wall discontinuities within the mesophyll structure of a leaf (Gausmann, 1974). The PROSPECT model (Jacquemoud and Baret, 1990) develops Allen’s multiple layer plate model and adopts a solid angle of incident radiation instead of an isotropic direction. This modification accommodates shadowing caused by the undulating shape of the leaf surface at microscopic scales (Grant, 1987). Designed to illustrate the effects of chlorophyll and water content of leaves on the spectra of those leaves, PROSPECT has now been upgraded (Jacquemoud et al., 1996) to incorporate the biochemicals of cellulose, lignin, and protein. The K–M and plate theories assume the absorption and scattering of the radiation is distributed homogeneously throughout the leaf when in fact it is characterized by the absorbing biochemicals being localized into hydrated cells which are separated by intercellular air spaces causing multiple refractive index discontinuities.

Other leaf models have utilized ray tracing (Allen et al., 1973) and stochastic methods (Tucker and Garratt, 1977), but these models are difficult to invert and require computer-intensive processing. In addition, existing leaf models have usually been based upon broadleaf vegetation where the leaf structure is considered to be an infinitely extending plane having distinct layers. Pine needles present their own particular problem. They do not contain a well-defined pallisade layer, and so a transverse section of a pine needle will display roughly spherical cells. There are also problems of how to characterize and model the pine foliage elements. Because of the complex spatial distribution of pine canopies, it may be more appropriate to treat shoots (clumped needles) as the independently located foliage element instead of individual needles (Stenberg et al., 1994). Finally, their shape and size make it difficult to accurately measure the spectral properties of individual needles even in the laboratory (Daughtry et al., 1989). LIBERTY (Leaf Incorporating Biochemistry Exhibiting Reflectance and Transmittance Yields) was developed specifically to address these problems.

The objective was to generically characterize and model conifer (particularly pine) needles at the cellular level by adapting Melamed’s theory of light interaction with suspended powders (Melamed, 1963) and infer leaf optical parameters from stacked needles in the laboratory. An enhancement of LIBERTY to accommodate a thickness parameter, thereby providing a transmittance output, facilitates the modeling of individual conifer needle elements. Therefore, based upon known parameters derived in the laboratory, LIBERTY can be coupled with a suitable canopy model for investigations of forest canopy reflectance. Inversion of LIBERTY, providing in vivo absorption coefficients, indicate significant improvements in model accuracy and hint at the potential to estimate biochemical concentrations and biophysical variables directly from high spectral resolution remote sensing (Dawson et al., 1997).

Section snippets

Infinite Reflectance Function

The total reflectance R of a medium, such as a leaf or stacked leaves, will be the sum of the scattered radiation fluxes emerging in an “upwards” direction out of the leaf, assuming a horizontal leaf surface. R is derived by first obtaining an approximate expression for the total reflectance of an individual leaf cell as a function of the optical absorption coefficient k of the cell medium and taking into account both internal and external scattering. The parameters and variables of R can then

Leaf optical properties used to parameterize liberty

To parameterize LIBERTY, we used data from the laboratory analysis of jack pine (Pinus banksiana) needles. The laboratory reflectance measurements made on stacked freeze-dried and ground needles and pure and in vitro leaf component biochemicals were made using a Perkin Elmer Lambda 19 double-beam spectrophotometer equipped with an integrating sphere. The instrument was continually purged with gaseous nitrogen to remove atmospheric moisture. The calibration of the instrument was performed using

Validation data

To compare LIBERTY output with the reflectance spectra of dried needles, spectrometer measurements were made on 160 needle samples collected from sample sites in Nipawin National Reserve, Saskatchewan, Canada as part of the Boreal Ecosystem Atmosphere Study (BOREAS) in 1994. This site, comprising jack pine (Pinus banksiana), provided 10 sample plots with 16 samples collected from each plot; eight of these samples were first year needles and eight were second year needles. The reflectance of

Results and discussion

Based upon the model variables and biochemical data outlined in Table 1, the laboratory spectrometer data and the LIBERTY predictions were highly correlated r=0.99 across all wavelengths (Fig. 4). For both the dried and fresh needle samples, the RMSE between estimated and measured spectra for the quasiinfinite setup were 2.87% for the predicted freeze-dried needles (Fig. 5a) and 1.73% for fresh needles (Fig. 6a).

A sensitivity analysis of LIBERTY indicated that it was necessary to reduce the

Conclusion

A new model, LIBERTY, has been presented which predicts, quite accurately, the reflectance spectra of both dried and fresh conifer needles. Overall root mean square differences between predicted and measured quasiinfinite reflectance outputs were 2.87% for the freeze-dried needles and 1.73% for fresh needles. For single needle reflectance and transmittance estimates, the RMS errors were 1.84% and 1.12%, respectively. For the freeze-dried needle predictions, LIBERTY mirrors the absorption peaks

Acknowledgements

The research was funded by the Natural Environment Research Council (Research grant GR3/7647 to P. J. C. and studentship GT4/94/407/L to T. P. D.) and the University of New Hampshire (Accelerated Canopy Chemistry Program grant to P. J. C.). The authors are indebted to many individuals who made this work possible, notably John Kupiec (Scottish Natural Heritage), Geoff Smith (Institute of Terrestrial Ecology, Monks Wood), John Marshall (University of Southampton), and John Aber and Mary Martin

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