Measuring Leaf Area Index (LI-COR LAI-2200C)

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Описание:
Leaf area index is a great way for farmers, plant scientists, and
ecologists to track the health and growth of plants over time.
Leaf area index, or LAI, is one-sided leaf surface area divided by
the ground area.
[00:17]
The gap fraction method is the most practical way to measure
leaf area without damaging the plants. Gap fraction indicates
how much of the sky is visible from beneath a plant canopy. If
you can see more sky, the gap fraction is bigger. If you see less
sky, the gap fraction is smaller. The amount of foliage can be
calculated from the gap fraction, if we assume the leaves are
positioned randomly within the canopy. Let’s see how this works.
[00:48]
How many leaves does it take to block your view of the sky? That
depends on how they are arranged. If they don’t overlap, it takes
relatively few leaves to block your view. At the other extreme, if
those same leaves are stacked on top of each other, not much of
your view is blocked. A useful assumption is that the leaves are
positioned randomly, which puts us between these extremes,
with some leaf overlap. When leaves are randomly arranged,
there is a simple exponential relationship between leaf area
index and gap fraction. This means we can compute leaf area
index from gap fraction.
[1:27]
If we measure the gap fraction at several angles of view, it turns
out we can also deduce something about the leaves’ orientation.
Are they laying flat, vertical, or something in between?
[1:41]
If all the leaves in a canopy are vertical, your view of the sky is
barely blocked looking straight up. But if you look more toward
the horizon, your view is mostly blocked.
[1:51]
With the same leaves laying flat, on the other hand, you can see
more sky looking toward the horizon. When you look straight up,
your view is mostly blocked.
[2:01]
As you can see, gap fraction information - especially as a
function of view angle - is quite powerful for indirectly estimating
canopy architecture.
[2:11]
How can we measure gap fraction? One method is photography.
A high-contrast fisheye photograph taken from below lets you
compute gap fraction as a function of the view angle. Digital
processing can automate this, but two potential limitations are
resolution, especially for small leaves in tall canopies, and
differentiating bright, sunlit leaves from the sky background.
Another method for determining gap fraction uses multiple
measurements of direct sunlight at different sun angles over
time. Gap fraction for each sun angle is determined by how much
of the sun is blocked.
[2:50]
LI-COR’s LAI-2200C Plant Canopy Analyzer doesn’t require
multiple sun angles. It uses a fisheye lens to project a nearly
hemispheric image of the canopy and sky onto a ringed detector.
This lets you quickly get gap fraction at 5 different angles by
comparing two readings: one above the canopy, with one below
the canopy. A single above-canopy reading can serve as a
reference for a number of below-canopy readings, allowing the
LAI-2200C to quickly measure leaf area index and foliage
orientation over a large area without waiting for the sun to move.
[3:30]
The LAI-2200C comes with view restricting ‘caps’ to limit the view
in small plots, or to block the operator from being viewed. Also,
some of the outer view rings can be masked, restricting the
diameter of the cone-shaped area viewed by the light sensor.
[3:48]
Some canopies don’t conform to the assumption of random leaf
location. A widely spaced row crop is a good example. A single
below-canopy measurement can’t represent the entire field. But
if you take a several measurements along two or more diagonal
transects with a narrow-view cap attached, the LAI-2200C can
account for the non-randomness in the canopy.
[4:13]
Simple gap fraction methods also generally assume leaves are
perfectly black: that they block all sunlight. In reality, some
sunlight passes through or bounces off leaves. This can cause
large errors, especially when sunlit leaves are in view. The LAI-
2200C deals with this issue in two ways. First, it uses a blue filter
that improves the contrast between leaves and sky. Also, a postprocessing
correction is provided in the support software. This
correction is based on a powerful, published light scattering
model. It uses measurements of sky conditions and foliage
properties made with the LAI-2200C.
[4:55]
Reliance on actual measurements pays off with better results.
Multiple studies have proven the accuracy of the LAI-2200C, with
results that compare favorably to destructive sampling methods.
[5:10]
What questions could you answer with the LI-COR’s plant canopy
analyzer? Contact us with your ideas and questions.