Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 238

daytime

400 W m -2 1 m s -1

3 K2 hPa 10 ppmv

nighttime

Foliage
density

Radiation Wind speed Temperature Humidity CO 2

Figure 6.20 Canopy proiles of foliage density, net radiation, wind speed, tempera-
ture, humidity and CO 2 concentration during daytime (top) and night time (bottom).
The arrows indicate the direction of transport. Note that in the daytime igure the in-
canopy arrows are deliberately omitted owing to the possibility of counter-gradient
transport (see later). (After Monteith and Unsworth, 2008 .)

6.6.1 Radiation
Shortwave Radiation
Leaves absorb radiation, but they relect and transmit radiation as well. The optical
properties depend on the wavelength of the light, as can be seen in Figure 6.21 . The
strong absorption in the visible part is due to chlorophyll, with a small dip around
520 μ m, which corresponds to green (thus leaves relect green light effectively, hence
their green colour). Absorption at wavelengths around 2 μ m is due to absorption by
water. The spectral properties of the leaves cause a change in the spectral composi-
tion of the light. Because, in the visible part of the spectrum, green is least absorbed,
the light down in the canopy will be enriched with green, and will contain relatively
little radiation that can be used for photosynthesis. The difference in relectivity in the
visible ( r VIS ) and near-infrared ( r NIR ) wavelength regions, which is speciic for green
vegetation, is exploited in the normalized difference vegetation index (NDVI):

r −

+

r

NDVI =

NIR IS

(6.38)

r r

NIR IS

The NDVI can be determined from remote sensing observations of relectivity in
the two wavelength bands and it can be linked to a number of vegetation properties,
such as LAI and vegetation cover (see, e.g., Carlson and Ripley, 1997 ). In the case
of sparse vegetation, the spectral properties of the underlying soil become important.