Agricultural and Forest Meteorology 100 2000 155–168
Photographic exposure affects indirect estimation of leaf area in plantations of Eucalyptus globulus Labill
Craig Macfarlane
a,∗
, Michael Coote
a
, Donald A. White
b
, Mark A. Adams
a
a
Department of Botany, Faculty of Science, The University of Western Australia, Nedlands, Western Australia 6907, Australia
b
CSIRO Forestry and Forest Products, Private Bag, PO Wembley, Western Australia 6014, Australia Received 16 February 1999; received in revised form 29 September 1999; accepted 29 September 1999
Abstract
Calibrations of indirect methods for estimating leaf area are usually based on small data sets and are often species-specific and may even be stand-specific. We used the Licor LAI-2000 plant canopy analyser PCA as a reference to calibrate leaf
area measurements based on hemispherical photography. Ten stands of 6–8 year-old, plantation grown Tasmanian bluegum Eucalyptus globulus Labill. in Western Australia were used to investigate the effects of variations in sampling position,
photographic exposure and image processing on leaf area or leaf area index, L estimated using hemispherical photography. We also compared our photographic estimates of L with those obtained via destructive sampling allometry in both evenly
spaced and highly clumped stands of E. globulus. Varying exposure by one stop affected estimated L by approximately 13 but we confirmed that correct exposure can be approximately predicted by metering exposure outside the canopy.
In situations where metering exposure outside the canopy is impractical, we recommend the use of empirically derived relationships between L estimated from photographic images made at a constant exposure and actual L derived from other
means. Mean tilt angle obtained for E. globulus from the photographic method 68.7
◦
± 2.5 s.e. agreed well with estimates for
Eucalyptus species from other studies. Owing to the non-random arrangement of crowns within plantations, sampling position
significantly affected mean tilt angle. In highly clumped stands with closely spaced double rows and wide inter-double row gaps, underestimation of L by 16–30 with the photographic method was probably the result of greater foliage clumping
at the crown level. We concluded that, in stands of E. globulus and probably other broadleaf species with evenly distributed crowns, foliage clumping at the shoot or branch level is unlikely to be a significant source of error in indirect estimates of L.
Scattering of blue light may result in large underestimates of L when using the PCA. In stands with ‘extreme’ architecture, indirect, light interception-based methods are likely to greatly underestimate L, although, positioning the sensor so as remove
large gaps from view may allow accurate estimates of L even in these stands. ©2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Blue light scattering; Foliage clumping; Hemispherical photography; LAI-2000; Leaf angle distribution; Mean tilt angle
1. Introduction
Leaf area is a key determinant of the productivity and water use of vegetation Watson, 1958; Lieth,
∗
Corresponding author. Fax: +61-8-9380-7925. E-mail address:
cmacfarlcyllene.uwa.edu.au C. Macfarlane.
1975; Waring, 1983; Gholz et al., 1990. Accurate estimation of leaf area or leaf area index, L is
thus critical to understanding and modelling ecosys- tem function. Measuring L directly is laborious and
time consuming, especially in tall vegetation such as forests and plantations, and destructive sampling and
allometry may not always be practical. L of vegeta-
0168-192300 – see front matter ©2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 3 9 - 2
156 C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168
tion can be estimated indirectly from the proportion of incident radiation transmitted through the canopy
at a given zenith angle known as the gap fraction as- suming that individual foliage elements are small, do
not transmit any light, are randomly distributed and have no ‘azimuthal preference’ Norman and Camp-
bell, 1989. The gap fraction measured at each zenith angle is dependent on the amount, spatial distribution
and orientation of foliage but is also influenced by the instruments used to measure the gap fractions and
the techniques used for their analysis. Comprehensive reviews of the theory, instruments and methods for
measurement of gap fractions and estimation of L are contained in Norman and Campbell 1989; Welles
and Cohen 1996; Chen et al. 1997.
Of all the sensors available for measuring gap fractions in tall canopies, the LAI-2000 plant canopy
analyser PCA, Licor Inc., Lincoln, NE, USA and hemispherical photography are most popular because
hemispherical sensors can simultaneously measure the canopy gap fraction at a range of zenith angles,
enabling more efficient sampling than is possible with linear sensors Welles and Norman, 1991. Hemi-
spherical photography is a relatively cheap alternative to the PCA for measurement of gap fractions but
there are several potential sources of error associated with both techniques, in particular, foliage clump-
ing. Consistent underestimation of L by the PCA in stands of both broadleaf and conifer species has been
attributed to clumping of foliage at the shoot level; invalidating the assumption of random distribution of
foliage Gower and Norman, 1991; Stenberg, 1996. Generally, clumping of foliage at the shoot level is
less in broadleaf than in coniferous species Chen et al., 1997. Foliage can also be clumped at the
branch or crown scale Chen and Cihlar, 1996.
Leaf area estimated from hemispherical photogra- phy is sensitive to photographic exposure Chen et al.,
1991 but indicated exposure may vary among cam- eras and light meters Chen et al., 1991; Wagner, 1998
and exposure may be metered either outside or be- low the canopy by different operators Canham et al.,
1990; ter Steege, 1994. The ‘correct’ exposure for hemispherical photographs is that required to obtain
good contrast between foliage and sky Rich, 1990; Chen et al., 1991 such that when the grayscale im-
ages are converted to black and white, the sky appears white and the foliage black. The effect of exposure
on leaf area can be reduced by varying the threshold at which the grayscale images are converted to black
and white during digitisation Rich et al., 1995 but this introduces subjectivity into the analysis Martens
et al., 1993. Chen et al. 1991 proposed that the cor- rect exposure is achieved when the exposure settings
shutter speed and lens aperture are selected to make the sky appear white. They suggested that 1–2 stops of
overexposure relative to the automatic exposure me- tered outside the canopy should produce this outcome.
However, metering exposure outside the canopy is not always possible owing to the rapid changes in light
conditions at sunrise and sunset, and the time required to travel from outside the canopy to sample areas be-
neath the canopy. We note also that this theory was only tested on a single canopy.
Scattering of blue light, especially at large zenith an- gles, invalidates the assumption that the foliage trans-
mits and reflects no radiation. This is a more impor- tant source of error for the PCA than hemispherical
photographs because the PCA overestimates the gap fraction at large zenith angles resulting in underes-
timates of leaf area. In contrast, hemispherical pho- tography can underestimate the gap fraction at large
zenith angles as a result of insufficient angular resolu- tion Chen et al., 1997 and darker sky near the hori-
zon Wagner, 1998 which together cause small gaps at large zenith angles to disappear during image pro-
cessing. Scattering of light from well-lit foliage at the top of the canopy can result in underestimation of the
gap fraction from photographs at small zenith angles. This problem is minimal if photographs are obtained
in even lighting conditions, before sunrise or after sun- set Norman and Campbell, 1989.
Rich 1990 observed that techniques for hemi- spherical photography are not sufficiently standard-
ised to enable results from different sites to be com- pared owing to the lack of rigorous assessment of
the significance of errors introduced at each stage of the process. Furthermore, calibrations of indirect
methods for estimating leaf area are usually based on small data sets and may be stand-specific.
We calibrated the hemispherical photography tech- nique for estimating L using the PCA as a reference in
a large number of stands of plantation grown Eucalyp- tus globulus
Labill. The PCA has previously been cal- ibrated in plantations of E. globulus Hingston et al.,
1998 and other Eucalyptus species with similar leaf
C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168 157
morphology and stand structure E. nitens Deane and Maiden Maiden and E. grandis W. Hill ex Maiden;
Dye, 1993; Battaglia et al., 1998. We also compared our photographic technique with destructive sampling
allometry in plantations of E. globulus with different canopy structures. Our objectives were:
1. To investigate the effect of photographic exposure and image processing on estimates of L in plan-
tations of E. globulus. 2. To examine the effect of stand structure and sam-
pling position on L estimated from hemispherical photography.
2. Theory
