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
Chen et al. 1991 suggested that photographs should be overexposed by 1–2 stops relative to the
brightness of the sky outside the canopy to obtain accurate estimates of L
e
from hemispherical photog- raphy. Exposure is the amount of light acting on the
emulsion of the film or paper and is determined by the lens aperture f number and shutter speed Grimm
and Grimm, 1997. Built-in light camera meters mea- sure the brightness or luminance of the subject being
photographed and the camera calculates ‘automatic’ exposure settings assuming the light comes from a
mid-gray surface 18 visible reflectivity; Unwin, 1980. The degree of overexposure or underexpo-
sure of a photo image can be expressed simply by the relative exposure value EV
R
where EV
R
= 0 is
‘automatic’ exposure, EV
R
= 1 is one stop of over-
exposure and EV
R
= − 1 is 1 stop of underexposure
Unwin, 1980. A change in EV
R
of 1 stop represents a halving or doubling of the amount of light reaching
the film. Therefore, to make an unobscured overcast sky 18 visible reflectivity appear completely white
100 visible reflectivity should require 2.5 stops of overexposure EV
R
= 2.5. Overexposing the image
also increases the uniformity of the sky brightness Wagner, 1998.
However, digital grayscale images are typically converted to black and white prior to analysis using
a threshold algorithm which classifies pixels as black or white based on their brightness. In this process, not
only completely ‘white’ pixels are classified as sky but any pixel with a brightness value above a ’threshold’
value. If a constant threshold value of 50 brightness is used, then only 1.5 stops of overexposure should
be required to make an unobscured overcast sky appear completely white 50 visible reflectivity.
This agrees well with the 1–2 stops of over-exposure suggested by Chen et al. 1991.
Assuming that foliage is completely black, the ‘cor- rect’ EV
R
metered below the canopy should decrease below 1.5 as the proportion of light penetrating be-
low the canopy decreases below 100 and could be derived from Eq. 1, where I
D
is the fraction of light transmitted beneath the canopy. For example, beneath
a canopy through which 18 of the light above the canopy penetrated, EV
R
= − 1 should be required.
‘Automatic’ exposure would be correct for a canopy through which 36 of the light was transmitted.
EV
R
= log
2
I
D
0.36 1
In this study, the diffuse non-interceptance of light τ ; Welles and Norman, 1991, calculated using the soft-
ware from the PCA Licor, 1991;see Section 3, was used as an estimate of I
D
to predict EV
R
from Eq. 1.
3. Materials and methods
3.1. Site descriptions In Spring 1997, L was estimated in 10 stands
from nine plantations of 6–8 year-old E. globulus using a Licor LAI-2000 plant canopy analyser PCA
and hemispherical photography. Measurements were mostly made close to sunrise or sunset, with occa-
sional measurements under uniform overcast condi- tions during the day, to obtain even sky illumination
Rich, 1990. Stands were typically 15–20 m tall and had closed or nearly closed canopies. The plan-
tations covered the range of climatic variation in south-western Australia: annual rainfall and poten-
tial evaporation ranged from 669 to 1336 and from 1240 to 1729 mm, respectively Table 1. Within each
plantation, measurement sites were located on level ground. The plantations were established with a max-
imum of 2 m spacing within rows and 4 m spacing between rows. As a result of death of trees, actual
stand density at the time of sampling ranged from 836 to 1578 stems per hectare Table 1.
158 C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168
Table 1 Location, climate, age and stocking rate SR of the research stands of E. globulus. Rainfall is the long term median rainfall recorded at
the nearest Bureau of Meteorology recording station. Site
Longitude E Latitude S
Elevation m Rainfall
PE SR
Age years mm per year
mm per year stems per hectare
Bunbury 115
◦
43
′
33
◦
09
′
10 791
1692 836
7.2 Busselton
115
◦
15
′
33
◦
43
′
17 807
1408 1425
7.2 Collie
116
◦
14
′
33
◦
19
′
250 926
1533 1175
7.2 Cowaramup
115
◦
05
′
33
◦
51
′
125 1161
1319 1078
7.2 Cundinup
115
◦
49
′
33
◦
50
′
270 820
1280 1100
7.2 Grimwade
116
◦
01
′
33
◦
36
′
240 804
1426 1578
7.2 Mandurah
115
◦
49
′
32
◦
29
′
10 879
1729 1000
6.2 Northcliffe
116
◦
08
′
34
◦
42
′
75 1336
1237 1250
8.1 Scott River
115
◦
25
′
34
◦
17
′
20 923
1277 1525
6.2
3.2. Indirect estimation of L
e
using the plant canopy analyser PCA
The PCA was operated in two sensor mode Licor, 1991. The two sensors were cross calibrated under
field conditions prior to measurements being taken. The reference sensor was positioned above the canopy
using a mast, which could be raised to a maximum height of 20 m, while the measuring sensor was po-
sitioned below the canopy on a tripod 1.3 m above ground level. Both sensors were levelled and oriented
in the same direction. At Northcliffe, the trees were taller than the mast which was then positioned outside
the plot and a 180
◦
view restrictor used on both sen- sors to obscure the trees from the external sensor. Two
measurements were made at each of three randomly selected positions Fig. 1a where each position was:
1 between trees within rows, 2 between two trees between rows, 3 diagonally between four trees be-
tween rows. The software accompanying the PCA was used to calculate L and mean tilt angle ¯α after Lang
1986.
Interception of light by woody elements of vegeta- tion, clumping of foliage and scattering of blue light
at large zenith angles are all potential sources of error in the raw reading from the PCA denoted here as L
p
. L was calculated from L
p
using the relationship derived by Hingston et al. 1998; Eq. 2, R
2
= 0.88 from
E. globulus stands with L ranging from 1 to 6. Identi-
cal relationships between L and L
p
were obtained for E. nitens
and E. grandis Dye, 1993; Battaglia et al., 1998.
L = 1.51L
p
2 Assuming 15 of the underestimation of L by the
PCA is the result of scattering of blue light Chen et al., 1997 we calculated the effective plant area
Fig. 1. Indicative sampling positions for a plant canopy anal- yser and hemispherical photography measurements in stands of E.
globulus with single rows b hemispherical photography measure-
ments in stands of E. globulus with closely spaced double-rows.
C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168 159
index, L
e
, for our stands from Eq. 3. The clumping index is the ratio of L
e
to L Black et al., 1991. L
e
= 1.15L
p
3 3.3. Indirect estimation of L using hemispherical
photography Exposures were taken at a height of 1.7 m above
ground level with a Nikon F90s camera equipped with a databack Nikon MF-25, remote control shutter
module Nikon ML-3 and Sigma 8 mm, F4, fisheye lens with a clear internal filter. The focus ring was set
to infinity and taped in place. The camera was mounted in a self-levelling bracket Rich, 1989 and aligned to
magnetic north. Some photographs were overexposed for use as a template to locate the boundary of the
circular image on canopy photographs. Within the 10 E. globulus
stands, three exposures were taken at ran- domly selected positions corresponding to positions
1, 2 and 3 Fig. 1a. For the stands at Mandurah low L
and Collie high L photographs were taken over a greater range of EV
R
within rows position 1, Fig. 1a to investigate the relationship between EV
R
and L and ¯α in greater detail. No photographs were taken at
position 2 Fig. 1a at Collie. Initially, luminance beneath the canopy was me-
tered with a handheld light meter Capital DB3. The camera was operated in manual mode with three lens
apertures f number 5.6, 8.0, 11.0 and a constant shut- ter speed selected to obtain EV
R
of approximately −1, −
2 and −3 relative to the handheld light meter. We later concluded that it was preferable to use the inter-
nal light meter of the camera rather than the handheld light meter. The camera light meter was sensitive to
smaller values of luminance, was more convenient to use and there was a strong linear relationship between
exposure metered with the camera and with the hand- held meter R
2
= 0.94 which was used to derive EV
R
relative to the camera’s light meter. All subsequent references to EV
R
in this paper are relative to the cam- era’s light meter. The consistency of estimation of L
obtained in variable light conditions by metering ex- posure with the camera light meter with the fisheye
lens attached was tested by photographing two sites at EV
R
= 0 from sunset until the camera’s light meter in-
dicated a low reading and shutter speeds exceeded 4 s. Exposures taken using Ilford XP2 ASA 400 film
suitable for the automated C41 development process were developed by a commercial photography service.
A comparison between Ilford XP2 ASA 400, Kodak T400CN ASA 400 black and white chromogenic film
and Ilford Delta Professional ASA 400 pan chromatic black and white film confirmed that film type did not
affect estimates of L. In our experience, there is no improvement in contrast from using ASA 50 or ASA
100 film. ASA 5 film improves contrast but does not enable fast enough shutter speeds to freeze foliage
movement due to wind Pearcy, 1989.
Photographic negatives were scanned at 400 × 600 pixels as 16 tone grayscale negatives with a Nikon
LS-1000 35 mm Film Scanner. Adobe PhotoShop Ver- sion 3.0 was used for image processing. The over-
exposed template was used to identify the boundary of the circular canopy images. The grayscale images
were converted to black and white bitmap images at 50 threshold, cropped to 400 × 400 pixels and saved
in PCX format suitable for use in HEMIPHOT ter Steege, 1994. A second copy of each image was twice
sharpened during image processing, using the sharpen filter prior to converting to black and white. Sharpen-
ing is an image compositing technique that increases the contrast between adjacent pixels.
HEMIPHOT was used to estimate L from the bitmap images after Lang 1986 based on the gap fraction at
the same five zenith angles used by the PCA 7, 23, 38, 53 and 68
◦
; Licor, 1991. The subscript h is used to denote L and ¯α estimated from hemispherical photog-
raphy and p to denote those estimated from the PCA. A preliminary study confirmed that there was negligi-
ble difference between L
h
estimated using the method of Lang 1986 and two other methods Campbell,
1986; Lang, 1987. The gap fractions at the five zenith angles calculated in HEMIPHOT were recorded and
used to estimate ¯α
h
Welles and Norman, 1991. 3.4. Estimation of diffuse light penetration beneath
canopies The diffuse non-interceptance of light τ ; Welles
and Norman, 1991, calculated using the software from the PCA Licor, 1991, was used as an esti-
mate of I
D
to predict EV
R
from Eq. 1. This value combines the effects of foliage clumping at all scales
160 C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168
and light scattering, assuming that light scattering for all wavelengths of visible light λ = 400–700 nm is
similar to that for blue light λ 490 nm.
3.5. Statistical treatment of data Variation of L
h
and ¯α
h
owing to sampling position, EV
R
and sharpening of images, was tested using analy- sis of covariance where EV
R
was the covariate. Linear regression was used to develop relationships between
EV
R
, L
h
and ¯α
h
for each stand. Separate relationships were developed for sharpened and unsharpened im-
ages. These relationships were used to determine, iter- atively, the EV
R
that gave agreement between L
h
esti- mated using hemispherical photography, and L and L
e
calculated from L
p
using Eqs. 2 and 3. The ‘correct’ EV
R
estimated by this method was compared to that predicted from Eq. 1 using linear regression. ¯α
h
esti- mated from sharpened and unsharpened photographs
at ‘correct’ EV
R
was compared with a paired t-test. 3.6. Comparison of hemispherical photography and
direct measurement of L After calibrating the hemispherical photography
method against the PCA, the method was tested against a direct estimate of L obtained by destructive
sampling and allometry in four stands of E. globulus located on the Water Authority of Western Australia
effluent disposal treefarm 10 km north of Albany, Western Australia. Three stands were established
with a 2 m spacing between trees within rows which were alternately 2 comprising a ‘double row’ and
5 m apart to give an initial stand density of 1500 stems per hectare Fig. 1b. Trees in these stands
were about 13 m tall. The fourth stand was of similar spacing and density to the stands used to calibrate the
hemispherical photography technique, and trees were about 11.5 m tall.
Three trees within each stand were selected to cover the range of diameters at breast height 1.3 m over
bark D
bh
and felled. For each of the 12 trees, all live branches were removed and stratified into five
groups on the basis of branch diameter 11; 11–16; 16–22; 22–28; 28 mm. The total mass of branches
in each group was measured to the nearest 0.1 kg and two sample branches were selected from each group.
These branches were immediately stripped of leaves and the wood and leaf components weighed. The area
of a 200–250 g sub-sample of leaves from each branch was measured with a calibrated leaf area meter. The
mean ratio of leaf area to total fresh branch mass for all branches from all sample trees in each size class was
calculated and used to estimate the total leaf area of each sample tree. A logarithmic regression was devel-
oped to predict total tree leaf area from tree diameter D
bh
and used to calculate the total leaf area of each stand. The regression was corrected for proportional
bias using Snowden 1991 ratio estimator for bias correction and tested for homogeneity of slope and in-
tercept between stands using analysis of covariance. L for each stand was calculated as the total area of leaves
for the stand divided by the total area of the stand.
3.6.1. Indirect estimation of L by hemispherical photography
Within each of the four stands, three exposures were taken within rows or within the 2 m spaced double
rows; position 1; Fig. 1a, b and between rows posi- tion 2, 3 at EV
R
= − 0.3. Photographic negatives were
scanned and processed as described earlier and all digitised images were sharpened twice. L
h
was esti- mated in HEMIPHOT and the gap fractions at the five
zenith angles used to estimate ¯α
h
after Lang 1986. L
was derived from L
h
using a relationship developed from the other stands. Analysis of variance was used
to test the effect of sampling position within the three double-row stands on L estimated from photography.
A paired t-test was used to compare L estimated from photography and that from allometry within the three
double-row stands.
4. Results
