Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 239

green

visible

1.0 0.0

0.8 0.2

transmissivity

0.6 0.4

absorptivity

0.4 0.6

reflectivity

0.2 0.8

0.0 1.0

0. 5 1 .0 1. 5 2.0 2.5

Wavelength (µm)

Figure 6.21 Spectral properties of a soybean leaf: relectivity (bottom line), trans-
missivity (top line, with axis in opposite direction) and absorptivity (shaded area).
(Data from Jacquemoud and Baret, 1990 .)

This has given rise to a number of alternative vegetation indices (e.g., Enhanced Veg-
etation Index [EVI]; see Huete et al., 2002 ).
The attenuation of the total amount of short wave radiation can be approximated
by Lambert-Beer's law:

Kz Ke aA z

↓ ↓ − ()

() = (6.39)

where the K ↓ (z) is the shortwave radiation at a given height, A(z) is the cumulative leaf
area index (accumulated starting at the top, see, e.g., Figure 6.22 ) and a is an extinc-
tion coeficient. The latter depends not only on the canopy characteristics, but also on
the characteristics of the radiation, in particular on the fraction of diffuse radiation
and on the solar zenith angle.
Because leaves are not necessarily oriented horizontally, relection of radiation
will take place not only upward (out of the canopy) but also sideways and downward
into the canopy. As a result, the proportion of diffuse radiation will be higher at the
bottom of the canopy than at the top. Furthermore, if the incident radiation is mostly
diffuse (on cloudy days) the radiation can penetrate deeper into the canopy because
there will always be pathways that are not blocked by leaves. This can be seen in Fig-
ure 6.22 , where on a clear day the radiation decreases sharply below 10 cm, whereas
on an overcast day the decrease is more gradual. Of course on overcast days the total
amount of radiation will be less than on clear days.
The vertical attenuation of PAR has a direct inluence on the rate of photosyn-
thesis in each layer. Here it should be noted that the light-use eficiency (amount of
ixed carbon per amount of absorbed PAR) of leaves decreases above a certain radia-
tion level (decreasing slopes in Figure 6.18 ). This implies that although on clear days
the top of a vegetation receives a large amount of radiation, not all of this radiation