164 C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168
Table 5 Leaf area index L measured by allometry and hemispherical photography mean ± s.e. and clumping index of four stands of E.
globulus near Albany, Western Australia. Three stands were planted in closely spaced double-rows and one in evenly spaced single rows.
The positions refer to Fig. 1a, b. is calculated as the ratio of mean L
h
to L from allometry Observations
L
h
Position 1 L
h
Position 2 L
h
Mean L
Allometry
3 3
6 Double row
1 3.53 ± 0.06
2.14 ± 0.11 2.83 ± 0.32
3.37 0.84
2 4.17 ± 0.10
2.16 ± 0.05 3.16 ± 0.45
4.54 0.70
3 4.63 ± 0.11
3.11 ± 0.06 3.87 ± 0.34
4.87 0.79
Single row 4
3.32 ± 0.20 2.97 ± 0.18
3.14 ± 0.14 3.11
1.01
the stands planted in double rows Fig. 1b; stands 1–3, L
h
underestimated L by 16–30 Table 5. L
h
mea- sured beneath the double rows position 1; Fig. 1b
was 1.5–2 times greater than that measured between the double rows position 2; Fig. 1b; P 0.001. L
h
measured only beneath the double rows agreed well with directly measured L.
5. Discussion
This study revealed that hemispherical photography, a technique which is markedly cheaper than alterna-
tives, can be used to obtain estimates of leaf area in- dex L as reliable as those from the LAI-2000 plant
canopy analyser in plantations of E. globulus. L
h
is influenced by photographic exposure and stand struc-
ture but consistent measurements of L
h
can be ob- tained under varying light conditions by metering light
with a camera’s light meter with the fisheye lens at- tached. L
h
estimated at a constant photographic ex- posure is strongly correlated with L determined from
other methods such that, once calibrated, L
h
can be used to predict L.
L
h
is larger at smaller EV
R
because the calculated gap fraction decreases as images become darker with
greater underexposure. The rates of increase of L
h
with decreasing EV
R
in this study 0.3–0.7 were similar to that found in P. menziesii c. 0.6; Chen et al., 1991
and indicates that variations of photographic exposure can greatly affect estimates of L. The rate of increase
of L
h
with EV
R
was proportional to L such that a 1 stop change of exposure changed L
h
by approximately 13 of L. The rate of change of L
h
with photographic exposure as a proportion of L was so consistent that
it may be possible to use this relationship to predict L
. For such a relationship to be generally applicable it would need to be tested in vegetation where the value
of L
e
is more certain because the effect of exposure on L
h
is related to L
e
, rather than L. The variation of L
h
that can be expected from varying exposure is of sim- ilar magnitude to the total accumulated errors associ-
ated with both indirect 10–30 and direct 25 methods Chen et al., 1997.
Strong correlation between the predicted and mea- sured exposure required to correctly estimate L or L
e
from hemispherical photographs supports the proposal of Chen et al. 1991 that the ‘correct’ exposure to ob-
tain good contrast between sky and foliage decreases with increasing canopy density. However, the ‘cor-
rect’ exposure to obtain estimates of L
e
predicted from Eq. 1 underestimated the measured exposure by 2.5
stops. This may result in part from variation between light meters Wagner, 1998 but the much better agree-
ment between predicted exposure and that required to estimate L from photographs suggests instead that L
e
may be more similar to L than indicated in Eq. 3. This implies that much of the underestimation of L by
the PCA in evenly spaced stands of E. globulus results from scattering of blue light rather than foliage clump-
ing. Using Eq. 1 to predict correct exposure would have resulted in an over-estimation of L by up to 10
because of the 0.8 stop difference between predicted and actual correct exposure.
Scattering of light generally results in 8–19 underestimation by the PCA of L
e
in conifers but scattering coefficients of leaves of deciduous plants
are larger than those of conifer foliage Chen, 1996. Eucalypt forest has a larger albedo than coniferous
forest Dunin and Mackay, 1982 suggesting that
C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168 165
eucalypt foliage would also have larger scattering coefficients than conifers. This could result in greater
underestimation of L
e
in eucalypt and deciduous for- est by the PCA. Thomas and Barber 1974 measured
reflectance of photosynthetically active radiation PAR of leaves of E. urnigera. Reflectance for PAR
ranged from 13 for glossy green leaves, similar to adult leaves of E. globulus, up to 30 for glaucous
leaves, similar to juvenile foliage of E. globulus. In comparison, reflectance of PAR by old foliage of P.
menziesii
ranged from 6 to 10 Jarvis et al., 1976. The size of the PCA correction factors for species
such as E. globulus, E. grandis and E. nitens Dye, 1993; Battaglia et al., 1998; Hingston et al., 1998
may be increased by the glossy adult foliage of these three species and, except for E. grandis, any glaucous
juvenile foliage that may be present Brooker and Kleinig, 1996. Numerous other studies have found
underestimation of L
e
by the PCA of 10–40 in ev- ergreen and deciduous broadleaf species which are
not noted for foliage clumping e.g.Wang et al., 1992; Martens et al., 1993; Eschenbach and Kappen, 1996;
Strachan and McCaughey, 1996. Large underestima- tion of L
e
by the PCA might also be expected for stands of Picea spp. with large reflectance in the blue
waveband Jarvis et al., 1976. If the objective of sampling is solely to obtain an
estimate of L and not an accurate distribution of gap fractions then as an alternative to attempting to
predict ‘correct’ exposure, L can be estimated from empirical relationships between L
h
measured at the same EV
R
beneath the canopy, regardless of canopy density. This approach is convenient as it does not
require the operator to return outside the canopy to determine the correct exposure under changing light
conditions. In climates, where clear skies are com- mon and most measurements are made at sunrise
and sunset, light conditions change rapidly and it is impractical to regularly re-meter exposure outside
the canopy. The time required for metering expo- sure also reduces the number of stands that can be
sampled. Allometry or another calibrated indirect method is necessary to determine the relationship
between L
h
and L, but even if the ‘correct’ expo- sure were used, the relationship between L
e
and L
would need to be determined by similar meth- ods, assuming that the clumping index is unknown.
The need for destructive sampling appears to negate one of the main advantages of indirect methods for
estimating L. However, indirect methods still have one great advantage over allometric methods in that indi-
rect measurements can be made and their relationship to L determined either retrospectively when the op-
portunity and resources for destructive sampling are available, or in similar vegetation to that being studied.
Once calibrated for a vegetation type, indirect methods should provide reliable results whereas allometric
methods often remain site-and age-specific. In cli- mates, where uniform overcast skies persist for much
of the day and light conditions change slowly, meter- ing of exposure outside the canopy may be practical.
The difference between the gap fraction measured at large zenith angles by the PCA and that measured with
photographs is also consistent with more scattering of blue light near the horizon and is further evidence that
much of the underestimation of L in E. globulus by the PCA may result from light scattering rather than
foliage clumping. Evenly spaced crowns in a closed or nearly closed canopy generally meet the assumption of
randomly distributed foliage Chen and Cihlar, 1996, hence, it is likely that there was little foliage clumping
in the stands measured with the PCA. Agreement of L
measured between and within rows of these stands further suggests that the foliage was not significantly
clumped at the crown level. The small gap fractions derived from photographic
images at large zenith angles compared to the PCA may also result from the loss of small gaps when the
images were converted from grayscale to black and white. We attempted to reduce this effect by sharp-
ening the images during processing. Sharpening of images increased the overall gap fraction by increas-
ing the size of gaps in foliage and preventing small gaps at large zenith angles from disappearing when
images were converted from grayscale to black and white Fig. 3. Similarly, Chen et al. 1991 found
that increasing the contrast during processing of im- ages from ASA 100 film gave similar results to those
obtained from high contrast ASA 5 film. The greater sensitivity of L
h
to exposure in unsharpened images is further argument for sharpening of images prior to
analysis. However, even if photographic images are correctly
overexposed relative to overcast sky and negatives are scanned at higher resolution e.g. 512 × 512 pixels,
it is still possible that the amount of sky will be
166 C. Macfarlane et al. Agricultural and Forest Meteorology 100 2000 155–168
underestimated at large zenith angles owing to re- duced sky brightness. Wagner 1998 observed that,
even when overexposed by 3 stops, sky brightness in photographs decreased rapidly at zenith angles larger
than 70
◦
. Fournier et al. 1997 observed that simu- lated gap fractions in the BOREAS study sites agreed
well with those obtained from photographs between zenith angles of 20–70
◦
. The larger gap fraction at the 7
◦
zenith angle obtained from photography in our study probably indicates some scattering of light
from high foliage, even under relatively uniform, dif- fuse light conditions. This phenomenon would also
explain the poor agreement at zenith angles less than 20
◦
observed in the BOREAS sites Fournier et al., 1997. However, this effect is small compared to
large differences observed at large zenith angles. It may be preferable to avoid measuring gap fractions
at zenith angles larger than 70
◦
with hemispherical photography and at zenith angles larger than 50
◦
with the PCA. If much of the underestimation of L by the
PCA results from blue light scattering then the gap fraction ratios Fig. 6 are overestimates because they
are based on photographs that agreed with L
e
. Using photographs that agreed with L would have resulted
in ratios closer to one for the smaller zenith angles but increased the difference between gap fractions
from photographs and the PCA at large zenith angles.
Good agreement between ¯α
h
for the 10 E. globulus stands 68.7
◦
± 2.5 s.e. with those obtained from di-
rect measurements for another species noted for near vertical foliage, E. regnans 68
◦
± 10 s.d., from a
range of Eucalyptus saplings 70–77
◦
and with ¯α
h
obtained for a range of eucalypt forests in eastern Australia 60–80
◦
suggests that the gap fraction ob- tained from photographs is more realistic than that ob-
tained from the PCA Ashton, 1976; Anderson, 1981; King, 1997. Chen et al. 1991 concluded that the gap
fraction distribution from photographs was unrealistic but their conclusion assumed that blue light scatter-
ing made a negligible contribution to the gap fraction distribution obtained from the PCA. The difference
between ¯α
h
estimated at different sampling positions within stands and at different stands indicates that the
structure of the stand influences ¯α
h
independently of the foliage angle distribution, and that estimates of ¯α
derived from any zenith angle range, with either the PCA or hemispherical photography, should be inter-
preted with caution. In plantations with closely spaced double rows and
large inter-row gaps Fig. 1b, foliage clumping at the crown level appeared to cause significant underesti-
mation of L: ranged from 0.70 to 0.84 for these stands Table 5. Chen and Cihlar 1996 found that
for a stand obtained from indirect light interception methods was influenced by stand density, L
e
and tree height. As tree height or canopy depth increases, and
as gap size decreases, the zenith angle at which large gaps disappear decreases. Large errors in estimation
of L may result from optical methods in stands with ‘extreme’ architecture. In the double-row stands, the
trees were several metres shorter than in the stands where the photographic technique was calibrated and
the gaps were much larger owing to the spacing of the rows. was probably larger in these stands than
in even the sparsest single row stands. This resulted in significant differences between L
h
measured within and between double rows. L
h
measured only beneath the double rows was within 5–8 of L derived from
allometry. This could be the result of the large gaps between rows being removed from view by position-
ing the camera directly beneath the rows of trees and suggests that even in stands with ‘extreme’ architec-
ture L could be indirectly estimated with reasonable accuracy if the camera can be positioned to remove
large gaps from view.
6. Conclusions
