Principles of terrestrial ecosystem ecology.pdf

106 5. Carbon Input to Terrestrial Ecosystems
different light environments. Leaves experience
large fluctuations (10- to 1000-fold) in
incident light due to changes in sun angle,
cloudiness, and the location of sunflecks
(patches of direct sunlight that penetrate a
plant canopy) (Fig. 5.7). Leaf chloroplasts
Irradiance (µmol m -2 s -1 )
1200
1200
1200
0
0
0
Sunfleck
Cloud
June July
Above
canopy Midcanopy
0
11:00 11:05 11:10
Dawn
Frontal system
12:00 24:00
Time
Clouds
Dusk
Figure 5.7. Hypothetical time course of photosynthetically
active radiation above and below the
canopy of a temperate forest over minutes, hours,
and months. Over the course of a few minutes, light
at the top of the canopy varies with cloudiness.
Below the canopy, light also varies due to the presence
of sunflecks of direct irradiance, which can last
tenths of seconds to minutes. During a day, there are
large changes in light due to changes in solar angle,
with smaller fluctuations caused by passing clouds.
Convective activity often increases cloudiness in the
afternoon. During the growing season, the major
causes of variation in light are seasonal changes in
the solar angle and the passage of frontal systems.
Some times of year have greater frequency of cloudiness
than others due to changes in directions of the
prevailing winds and the passage of frontal systems.
Net photosynthesis (µmol m -2 s -1 )
0
Light
limitation
Light
compensation
point
Light
saturation
Photooxidation
Irradiance (µmol m -2 s -1 1000 2000
)
Figure 5.8. Relationship of net photosynthetic
rate to photosynthetically active radiation and the
processes that limit photosynthesis at different
irradiance. The linear increase in photosynthesis
in response to increased light (in the range of light
limitation) indicates relatively constant light use
efficiency.
respond to changes in light availability over
minutes by changing both the levels of metabolites,
which influence the activity of photosynthetic
enzymes, and the stomatal conductance,
which influences CO2 supply and water loss
(Pearcy 1990, Chazdon and Pearcy 1991). Stomatal
conductance increases in high light, when
CO2 demand is high, and decreases in low light,
when photosynthetic demand for CO2 is low.
These stomatal adjustments result in a relatively
constant CO2 concentration inside the
leaf, as expected from our hypothesis of colimitation
of photosynthesis by biochemistry
and diffusion. It allows plants to conserve water
under low light and to maximize carbon uptake
at high light.
At low light, when the supply of ATP and
NADPH from the light-harvesting reactions
limits the rate of carbon fixation, net photosynthesis
increases linearly with increasing light
(Fig. 5.8). The slope of this line (the quantum
yield of photosynthesis) is a measure of the efficiency
with which plants use absorbed light to
produce sugars. The quantum yield is similar
among all C3 plants at low light in the absence
of environmental stress. In other words, all C3
plants have a relatively constant photosynthetic
light use efficiency (LUE) (about 6%) of converting
absorbed visible light (photosynthetically
active radiation, PAR) into chemical