The Propagation of Foliar Biochemical
Absorption Features in Forest Canopy
Reflectance: A Theoretical Analysis
T. P. Dawson,* P. J. Curran,
†
P. R. J. North,
‡
and S. E. Plummer
‡
R
emotely sensed estimates of the foliar biochemical and the canopy were identified at a number of wavelengths
or wavelength regions. Elsevier Science Inc., 1999content of vegetation canopies could be used to derive in-
dicators of ecosystem functioning at regional to global
scales. In the past decade, a number of studies have re-
ported strong correlations between the reflectance spectra INTRODUCTION
of vegetation canopies and their foliar biochemical con-
Estimates of the foliar biochemical content of vegetation
tent. However, these studies have commonly employed
canopies would improve our understanding of terrestrial
multiple regression techniques or spectral indices to de-
ecosystem functioning. Many of the biogeochemical pro-
termine biochemical content, which have been found to
cesses, such as photosynthesis, net primary production,
be highly sensitive to variation in canopy architecture
evapotransporation, and decomposition are related to the
[such as leaf area index (LAI) and canopy closure] and
content of chlorophyll, nitrogen, water, lignin, and cellu-
understory. To date, these effects combined with the low
lose in leaves (Goetz and Prince, 1996; Running, 1990;
signal-to-noise ratios of airborne spectrometers have in-
Running and Coughlan l988). Remote sensing, with the
hibited the development of robust and portable spectral
use of airborne or spaceborne spectrometers, offers the
techniques for the estimation of canopy biochemical con-
potential for estimating foliar biochemical content at re-
tent. This paper reports on a theoretical study in which
gional to global scales by using methods originally devel-
a leaf model, LIBERTY (leaf incorporating biochemicals
oped for laboratory spectroscopy studies. On the basis of
exhibiting reflectance and transmittance yields), charac-
the quantification of absorption features in the reflec-
terized specifically for conifer needles, was coupled with
tance spectra of laboratory samples, statistical techniques
a hybrid geometric/radiative transfer bidirectional reflec-
such as multiple linear regression have been used to esti-
tance distribution function FLIGHT (forest light) model.
mate biochemical contents of leaves (Aber et al., 1993;
By varying leaf biochemical content, LAI, canopy closure
Wessman et al., 1988). However, in the transition from
and understory, we analyzed the simulated canopy re-
the controlled conditions of the laboratory to airborne
flectance spectra to determine if the biochemical absorp-
studies, a number of perturbing effects are introduced,
tion features in leaf spectra were preserved at the canopy
including variable solar illumination intensity and angle,
scale. Absorption features or wavelength regions that were
viewing geometry, atmospheric conditions, vegetation can-
both related to a specific biochemical of interest (water,
opy architecture [leaf area index (LAI) and canopy cover]
lignin-cellulose) and persistent at the scale of both the leaf
and understory. Under these conditions, statistical meth-
ods lack robustness and portability, which led to the ad-
vancement of analytical techniques for estimating foliar
* Environmental Change Unit, University of Oxford, Oxford OX1
3UB, UK
biochemical content from canopy reflectance spectra,
† Department of Geography, University of Southampton, High-
such as radiative transfer modeling (Allen and Richard-
field, Southampton S017 1BJ, UK
son, 1968; Conel et al., 1993; Fourty et al., 1996; Jacque-
‡ Section for Earth Observation, Institute of Terrestrial Ecology,
Monks Wood, Abbots Ripton, Cambridgeshire PE17 2L5, UK
moud et al., 1996; Verdebout et al., 1995) or spectral
Address correspondence to T. P. Dawson, Environmental Ex-
matching techniques; (Gao and Goetz, 1995; Wessman
change Unit, University of Oxford, 5 South Parks Road, Oxford OX1
et al., 1993). Other studies have concentrated on the de-
3UB, UK.
Received 11 May 1998; revised 11 August 1998.
velopment of spectral indices that are insensitive to un-
REMOTE SENS. ENVIRON. 67:147–159 (1999)
Elsevier Science Inc., 1999 0034-4257/99/$–see front matter
655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(98)00081-9