Principles of terrestrial ecosystem ecology.pdf

118 5. Carbon Input to Terrestrial Ecosystems
Net ecosystem exchange (µmol m -2 s -1 )
50
40
30
20
10
-10
0
0 500 1000 1500 2000 0 500 1000 1500 2000
Figure 5.18. Effect of vegetation type and irradiance
on net ecosystem exchange in forests (A) and
crops (B). Forests maintain a relatively constant
LUE up to 30 to 50% of full sun, although there is
synthetic properties through a canopy extend
the range of irradiance over which the LUE
remains constant. (3) Light use efficiency is
reduced primarily by short-term environmental
stresses that cause plants to reduce stomatal
conductance. Over the long term, however,
plants respond to such stresses by reducing
the concentrations of photosynthetic pigments
and enzymes so photosynthetic capacity
matches stomatal conductance and by reducing
leaf area. In other words, plants in low-resource
environments reduce the amount of light
absorbed more strongly than they reduce the
efficiency with which absorbed light is
converted to carbohydrates. Modeling studies
suggest that light use efficiency varies
about twofold among ecosystems (Field 1991),
although this is difficult to demonstrate
conclusively because GPP cannot be directly
measured.
If LUE is indeed similar among ecosystems,
GPP could be estimated by determining the
quantity of light absorbed by ecosystems, which
can be measured from satellites. Leaves at the
top of the canopy have a disproportionately
large effect on the light that is both absorbed
Irradiance (µmol m -2 s -1 )
A B
considerable variability. Crops maintain a constant
light LUE over the entire range of naturally occurring
irradiance. (Redrawn with permission from
Advances in Ecological Research; Ruimy et al. 1996.)
and reflected by the ecosystem. The reflected
radiation can also be measured by satellites.
This similarity in bias between the vertical distribution
of absorbed and reflected radiation
makes satellites an ideal tool for estimating
canopy photosynthesis.The challenge, however,
is to estimate the fraction of absorbed radiation
that has been absorbed by leaves rather than by
soil or other nonphotosynthetic surfaces. Vegetation
has a different spectrum of absorbed and
reflected radiation than does the atmosphere,
water, clouds, and bare soil.This occurs because
chlorophyll and associated light-harvesting pigments
or accessory pigments, which are concentrated
at the canopy surface, absorb visible
light (VIS) effectively. The optical properties
that result from the cellular structure of leaves,
however, makes them highly reflective in the
near infrared (NIR) range. Ecologists have
used these unique properties of vegetation to
generate an index of vegetation “greenness”:
the normalized difference vegetation index
(NDVI).
( NIR - VIS)
NDVI =
( NIR + VIS)
(5.3)