Fig. 1 - Basic relative LAI comparisons.
W
hat is leaf area index (LAI)? The answer is
a bit more involved and complex than it
might seem. Leaf area index is a two-dimensional
quantity that characterizes plant canopies, and
the basic definition is the ratio of total one-sided
green leaf area per unit ground area. (That might
seem a bit unreal, seeing as leaves are three, not
two, dimensional, but the measurements taken to
determine LAI are in square inches and not cubic
inches.) Mathematically, LAI is determined by
measuring one side of all the leaves of a plant,
and then dividing the sum of those surfaces by the
surface of soil that a plant occupies. The final LAI
indicates the number of times the plant’s leaves
would cover the soil. For example, if a plant has a
LAI of 3.0, its leaves would cover the soil beneath
the plant three times over. The more leaf area a
plant has in comparison to the soil area beneath it,
the higher the LAI. To attain an accurate portrayal
of a plant’s (or crop’s) LAI, one needs to take into
consideration the vertical area as well as the
horizontal area. The height of the canopy plays just
as significant a role as does the horizontal area of
that canopy.
"LAI IS DETERMINED BY MEASURING ONE SIDE OF
all the leaves of a plant,
AND THEN DIVIDING THE SUM OF THOSE SURFACES
BY THE SURFACE OF SOIL THAT A PLANT OCCUPIES."
Direct measurement is the most accurate way to
attain LAI, but it’s also the most exhausting and
usually requires some destruction. To directly
measure a deciduous plant’s LAI, one would
collect the leaves during leaf fall. Then, one would
calculate the combined area by measuring a small
sample of the leaves, figuring out the average area,
and then drying and weighing this sample. Next,
one would dry and weigh the entire collection of
leaves and interpolate the average leaf area using
the ratio obtained from the total versus sample
gross weight. This same, complicated process could
be used to calculate the LAI of an evergreen plant,
but one would have to destroy the tree to attain
the leaves. So, though considerably accurate, this
method is not very practical.
grow cycle
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