Principles of terrestrial ecosystem ecology.pdf

temperature than are the biophysically controlled
light-harvesting reactions. Carbon
fixation reactions therefore tend to limit photosynthesis
at low temperature. Plants adapted
to cold climates compensate for this by producing
leaves with high concentrations of leaf
nitrogen and photosynthetic enzymes, which
enable carboxylation to keep pace with the
energy supply from the light-harvesting reactions
(Berry and Björkman 1980). This explains
why arctic and alpine plants typically have
high leaf nitrogen concentrations despite low
soil nitrogen availability (Körner and Larcher
1988). Plants in cold environments also have
hairs and other morphological traits that
raise leaf temperature above air temperature
(Körner 1999). In hot environments with an
adequate water supply, plants produce leaves
with high photosynthetic rates. The associated
high transpiration rate can cool the leaf, so leaf
temperatures are much lower than air temperatures.
In hot, dry environments, plants close
stomata to conserve water, and the cooling
effect of transpiration is reduced. Plants in
these environments often produce small leaves
that shed heat effectively and maintain temperatures
close to air temperature (see Chapter
4). In summary, despite the sensitivity of photosynthesis
to short-term variation in temperature,
leaf properties minimize the differences in
leaf temperature among ecosystems, and plants
acclimate and adapt so there is no clear relationship
between temperature and average
photosynthetic rate in the field, when ecosystems
are compared.
Pollutants
Pollutants reduce carbon gain primarily by
reducing leaf area or photosynthetic capacity.
Many pollutants, such as sulfur dioxide (SO2)
and ozone, reduce photosynthesis through their
effects on growth and the production of leaf
area. Pollutants also directly reduce photosynthesis
by entering the stomata and damaging
the photosynthetic machinery, thereby reducing
photosynthetic capacity (Winner et al.
1985). Plants then reduce stomatal conductance
to balance CO2 uptake with the reduced capacity
for carbon fixation.This reduces the entry of
Gross Primary Production 115
pollutants into the leaf, reducing the vulnerability
of the leaf to further injury. Plants
growing in low-fertility or dry conditions are
preadapted to pollutant stress because their
low stomatal conductance minimizes the quantity
of pollutants entering leaves. These plants
are therefore less affected by pollutants than
are rapidly growing crops and other plants with
high stomatal conductance.
Gross Primary Production
Gross primary production is the sum of the
photosynthesis by all leaves measured at the
ecosystem scale. It is typically integrated over
time periods of days to a year (gCm -2 of
ground yr -1 ) and is the process by which carbon
and energy enter ecosystems. GPP is generally
estimated from simulation models rather than
measured directly, because it is impossible to
measure the net carbon exchange of all the
leaves in the canopy in isolation from other
ecosystem components (e.g., respiration by
stems and soil) and without modifying the vertical
gradient in environment. The results of
these modeling studies suggest that most conclusions
derived from leaf-level measurements
of net photosynthesis can be extended to the
ecosystem scale, when the vertical profiles of
photosynthetic capacity and environment are
considered. Measurement of whole-ecosystem
carbon exchange provides another way to estimate
GPP (see Chapter 6).
Canopy Processes
The vertical profile of leaf photosynthetic
properties in a canopy maximizes GPP. In most
closed-canopy ecosystems, photosynthetic
capacity decreases exponentially through the
canopy in parallel with the exponential decline
in irradiance (Eq. 5.1) (Hirose and Werger
1987).This matching of photosynthetic capacity
to light availability is the response we would
expect from individual leaves within the
canopy, because it maintains the co-limitation
of photosynthesis by diffusion and biochemical
processes in each leaf. It also serves to maximize
GPP in closed-canopy ecosystems. The