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material is collected for biomass determination (section 5.4). Because of the temporal changes in
biomass and LAI, both should be determined for each season. This method yields exact
measurements of the leaf area but suffers from two drawbacks. First, the measurements of the
specific leaf area have to be extrapolated to obtain a measure of the LAI of a plant or stand. Second,
the method is destructive and labour-intensive. Consequently, it is mostly suitable on a limited scale
as reference method for the indirect determination of the LAI from radiation measurements.
The indirect measurement of the LAI uses optical devices (photo-sensitive cells etc.) to capture the
incoming radiation below the canopy. Such techniques use relationships like the Beer-Lambert law to
estimate the LAI from measurements of incoming global radiation (LI-COR, 1990). To calculate the
interception of the radiation by the canopy, incoming radiation is measured at locations under the
canopy compared to measurements above the canopy. The following description is based on the LICOR LAI 2000 device which allows for a rapid assessment of canopy structure and is available at the
University of Amsterdam (LI-COR, 1990, Welles & Norrman, 1991). However, other methods and
devices may be equally suitable if the sampling method is designed to restrict the influence of the
underlying simplifying assumptions (see below).
The indirect method by means of optical devices has the advantage that the method is time-efficient
and non-destructive. It can be applied to any vegetation type below which the optical device can be
placed, the leaf size is large compared to the sensor and a realistic number of measurements suffices
to capture the variability in canopy density. It is, however, based on a second-order relationship and
the results, therefore, approximate only. The main assumption is that the leaves are randomly
arranged in the canopy. As a result, the cover is assumed to be uniform over the entire area under
consideration whereas in reality leaves will cluster around the stem and branches. Under this
assumption, also the apparent angle of the leaves (mean tilt angle) is calculated from which shoot and
leaf arrangement can be induced.
Deviations from the assumed randomness may result in erroneous values of evapotranspiration and
interception on a local scale. The estimated LAI may also be biased because of the underlying
empirical relationships. The solution would be to replace the general relationships for more detailed,
local relations. However, this would involve destructive sampling and this is not always feasible or
desirable.
Destructive sample is the only possible technique to determine the LAI when an optical device can not
be used (e.g. herbaceous vegetation covers). In theory, the method could provide exact estimates of
the leaf area and that of the projected canopy. However, it is doubtful that, with the tedious labour
involved, the results are more accurate than that of the indirect sampling with optical devices. It
should be noted that neither method is capable to take the arrangement of leaves in the canopy into
account. This error is more serious in the case of deciduous and broad leaf vegetation of which the
arrangement is more prone to rain and wind and the cover in many cases changes over the year. The
arrangement of coniferous vegetation is less prone these disturbance but suffers more from the fact
that the method measures predominantly the projected area of shoots than that of the needles
(Gower and Norman, 1991). Gower & Norman (ibid.) suggest a method to eliminate this bias in the
measurement of the LAI of conifer stands with the LI-COR LAI 2000.
5.3a LAI determination of trees or forest canopies
Comment The non-destructive sampling with the LI-COR LAI 2000 device provides adequate
estimates of the LAI. The device uses a wide-angled fisheye lens (zenith cut off angle74°) to measure
the distribution of light and shade. The view is divided into five concentric sensors that each sample
an area with a different angle and pathway along which the incoming radiation falls through the
canopy. Because the large angle over which the canopy is sampled, a single measurement at one
location suffices to integrate the spatial arrangement of leaves and in addition to the LAI, the
orientation of the foliage can be estimated.
To eliminate the effect of reflectance or transmittance by leaves, only light with a wavelength between
320 and 420 nm is passed to the sensor for which both values are minimal in comparison to the direct
incoming light. Sampling under overcast or clouded skies can further reduce the influence of
reflectance and transmittance. Consequently, the foliage is assumed to be black and measurements
above and below the canopy can be directly related to estimate the fraction of intercepted light. For
the analysis of the data, the following additional assumptions are made (LI-COR, 1990)
· The foliage elements are small compared to the area of view;
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