SRS Sensors 3 THEORY
Robinson et al. (2012) suggest changes in LAI are difficult to detect
when LAI is much greater than 3 m
2
m
−2
. This should not be surpris-
ing considering the spectral measurement being made. NDVI mea-
surements rely on reflected light from leaf surfaces. As the canopy
fills and upper leaves begin to cover lower leaves, the leaf area will
continue to increase without making a further contribution to re-
flected radiation. Furthermore, foliar chlorophyll is a very efficient
absorber of radiation in red wavelengths so that reflectance from
leaves is typically very low in the red region (Figure 3). Therefore,
increasing LAI, and thus canopy chlorophyll content does not sub-
stantially change red reflectance beyond a certain point. Thus, NDVI
has limited predictive ability in canopies with high LAI. For some
applications NDVI saturation at high LAI may not be as important
as it would appear.
Although the technique may give poor estimates of LAI at high LAI,
shaded leaves tend to have much less impact on resource capture
compared to sunlit leaves, and therefore contribute proportionally
less to canopy productivity. As a general modeling parameter, an
estimate of sunlit leaves may be adequate for estimating photosyn-
thesis and biomass accumulation (i.e., carbon uptake) for some ap-
plications.Monteith (1977) proposed the now well-known relationship
between biomass accumulation and radiation capture seen in equa-
tion 2.
A
n
, canopy = f
s
S
t
(2)
In equation 2, A
n,canopy
is the biomass accumulation or carbon assim-
ilation and is a conversion efficiency often referred to as light use ef-
ficiency (LUE). The LUE depends on a variety of factors such as pho-
tosynthetic acclimation, physiological stress level, and plant species.
f
s
is the fractional interception of radiation by the canopy, and S
t
is the total incident radiation. The relationship between NDVI and
LAI in Nguy-Robinson et al. (2012) and the relation between frac-
tional interception and LAI (Campbell and Norman, 1998) show that
NDVI and fractional interception are approximately related linearly
(Figure 5). NDVI can provide a good estimate of the fractional in-
terception by green leaves in a canopy; a value that is critical for
carbon assimilation models.
9
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