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Reply toTownsend et al.: Decoupling contributions from canopy structure and leaf optics is critical for remote sensing leaf biochemistry

DOI:10.1073/pnas.1301247110
Authors:
Yuri Knyazikhin at Boston University
Yuri Knyazikhin
Philip Lewis at University College London
Philip Lewis
M. Disney at University College London
M. Disney
Pauline Teresa Stenberg at University of Helsinki
Pauline Teresa Stenberg
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Abstract

Townsend et al. (1) agree that we explained that the apparent relationship (2) between foliar nitrogen (%N) and near-infrared (NIR) canopy reflectance was largely attributable to structure (which is in turn caused by variation in fraction of broadleaf canopy). Our conclusion that the observed correlation with %N was spurious (i.e., lacking a causal basis) is, thus, clearly justified: we demonstrated that structure explained the great majority of observed correlation, where the structural influence was derived precisely via reconciling the observed correlation with radiative-transfer theory. What this also suggests is that such correlations, although observed, do not uniquely provide information on canopy biochemical constituents. We, therefore, disagree with the assertion in ref. 1 that we “did not provide an adequate rationale for the inference that %N and other leaf properties cannot be characterized from imaging spectroscopy”; our analysis showed precisely that. Our analysis also led to the conclusion that “NIR and/or SW broadband satellite data cannot be directly linked to leaf-level processes,” and any such link must be indirect and will be a function of structure. This is true for all wavelengths in the interval 423–855 nm (figure 7B and figure S2 in ref. 3), not primarily for the 800- to 850-nm spectral band, as misstated in ref. 1. None of the leaf biochemical constituents can be accurately estimated without accounting for canopy structural effects.
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LETTER
Reply to Townsend et al.: Decoupling contributions
from canopy structure and leaf optics is
critical for remote sensing leaf biochemistry
Townsend et al. (1) agree that we explained
that the apparent relationship (2) between
foliar nitrogen (%N) and near-infrared
(NIR) canopy reflectance was largely attrib-
utable to structure (which is in turn caused
by variation in fraction of broadleaf can-
opy). Our conclusion that the observed cor-
relation with %N was spurious (i.e., lacking
a causal basis) is, thus, clearly justified: we
demonstrated that structure explained the
great majority of observed correlation, where
the structural influence was derived precisely
via reconciling the observed correlation with
radiative-transfer theory. What this also sug-
gests is that such correlations, although ob-
served, do not uniquely provide information
on canopy biochemical constituents. We,
therefore, disagree with the assertion in ref.
1thatwe“did not provide an adequate ra-
tionale for the inference that %N and other
leaf properties cannot be characterized from
imaging spectroscopy”; our analysis showed
precisely that. Our analysis also led to the
conclusion that “NIR and/or SW broadband
satellite data cannot be directly linked to leaf-
level processes,”and any such link must be
indirect and will be a function of structure.
This is true for all wavelengths in the interval
423–855 nm (figure 7B and figure S2 in
ref. 3), not primarily for the 800- to 850-nm
spectral band, as misstated in ref. 1. None of
the leaf biochemical constituents can be ac-
curately estimated without accounting for
canopy structural effects.
We identified a structural variable, the
directional area scattering factor (DASF),
which was determined entirely by canopy
geometrical properties such as shape and
size of the tree crowns, spatial distribution
of trees on the ground, within-crown foliage
arrangement, and properties of the leaf sur-
faces. In dense vegetation, this parameter
can be directly retrieved from the reflectance
spectrum without the use of canopy-reflec-
tance models, prior knowledge, or ancillary
information regarding leaf optical properties
(3). Equations S4.1–S5.3 in ref. 3 explain the
background physics, but Townsend et al.
(1), nonetheless, misinterpret this as “the
authors used a single leaf spectrum derived
from one PROSPECT simulation.”We
clearly demonstrated that DASF provides
information critical to accounting for struc-
tural contributions to measurements of leaf
biochemistry from remote sensing.
Lastly, we do not claim that “links be-
tween leaf biochemistry (e.g., %N) and
‘hyperspectral’reflectance data are ob-
scured by variation in leaf-surface albedo,”
as overstated in ref. 1. We emphasized that
some radiation is scattered at the surface
of leaves and, therefore, contains no infor-
mation on leaf biochemistry; this presents
an additional confounding factor, unless it
can be accounted for.
Statistical relationships between leaf bio-
chemistry and canopy reflectance spectra
have indeed been repeatedly demonstrated.
However, analyses of underlying physical
mechanisms that generate the remotely
measured signal, which are required to
distinguish causality from correlation (4),
such as ours, have been lacking thus far.
This is absolutely necessary to obtain ac-
curate information on leaf biochemistry
from space (5). We agree that analyses in-
cluding both biologically and physically
based approaches will help reveal the sub-
tleties of the empirical relationships.
Yuri Knyazikhin
a,1
, Philip Lewis
b
,
Mathias I. Disney
b
, Pauline Stenberg
c
,
Matti Mõttus
d
, Miina Rautiainen
c
,
Robert K. Kaufmann
a
, Alexander
Marshak
e
, Mitchell A. Schull
f
, Pedro
Latorre Carmona
g
, Vern Vanderbilt
h
,
Anthony B. Davis
i
, Frédéric Baret
j
,
Stéphane Jacquemoud
k
, Alexei Lyapustin
e
,
Yan Yang
a
, and Ranga B. Myneni
a
a
Department of Earth and Environment,
Boston University, Boston, MA 02215;
b
Department of Geography and National
Centre for Earth Observation, University
College London, London WC1E 6BT, United
Kingdom; Departments of
c
Forest Sciences and
d
Geosciences and Geography, University of
Helsinki, FI-00014, Helsinki, Finland;
e
Climate
and Radiation Laboratory, Code 613, National
Aeronautics and Space Administration
Goddard Space Flight Center, Greenbelt, MD
20771;
f
Hydrology and Remote Sensing
Laboratory, US Department of Agriculture–
Agricultural Research Service, Beltsville, MD
20705;
g
Departamento de Lenguajes y Sistemas
Informáticos, Universidad Jaume I, 12071
Castellón, Spain;
h
Biospheric Science Branch,
Earth Science Division, National Aeronautics
and Space Administration Ames Research
Center, Moffet Field, CA 94035;
i
Jet Propulsion
Laboratory, California Institute of Technology,
Pasadena, CA 91109;
j
Unité Mixte de
Recherche 1114 Environnement Méditerranéen
et Modélisation des Agro-Hydrosystèmes,
Institut National de la Recherche Agronomique
Site Agroparc, 84914 Avignon, France; and
k
Institut de Physique du Globe de Paris–
Sorbonne Paris Cité, Université Paris Diderot,
Unité Mixte de Recherche Centre National de la
Recherche Scientifique 7154, 75013 Paris,
France
1Townsend PA, Serbin SP, Kruger EL, Gamon JA (2013)
Disentangling the contribution of biological and physical properties
of leaves and canopies in imaging spectroscopy data. Proc Natl Acad
Sci USA, 10.1073/pnas.1300952110.
2Ollinger SV, et al. (2008) Canopy nitrogen, carbon assimilation, and
albedo in temperate and boreal forests: Functional relations and potential
climate feedbacks. Proc Natl Acad Sci USA 105 (49):19336–19341.
3Knyazikhin Y, et al. (2013) Hyperspectral remote sensing of foliar
nitrogen content. Proc Natl Acad Sci USA 110(3):E185–E192.
4Fisher JB (2009) Canopy nitrogen and albedo from remote sensing:
What exactly are we seeing? Proc Natl Acad Sci USA 106(7):E16–E16,
author reply E17.
5Ustin SL (2013) Remote sensing of canopy chemistry. Proc Natl
Acad Sci USA 110(3):804–805.
Author contributions: Y.K., P.L., M.I.D., P.S., M.M., M.R., R.K.K.,
A.M., M.A.S., P.L.C., V.V., A.B.D., F.B., S.J., A.L., Y.Y., and R.B.M.
wrote the paper.
The authors declare no conflict of interest.
1
To whom correspondence should be addressed. E-mail: jknjazi@
bu.edu.
www.pnas.org/cgi/doi/10.1073/pnas.1301247110 PNAS Early Edition
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LETTER
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