450
J.D. Tenhunen et al.
feedback effects, i.e., changes in nutrient availability, and structural change,
i.e., phenology, that influence carbon dioxide uptake and growth occur
on such a time scale (Reynolds and Leadley 1992). It is essential in the
aggregate model to separate carbon fixation from CO2 efflux, since the
former is directly related to the growth process. At present the entire
community net aboveground carbon dioxide flux is treated as a lumped
variable (comparable to the data in Fig. 21.6). Including carbon dioxide
efflux from the soil is important, since there is much interest in the potentials
for carbon storage in tundra soils, especially as related to climate change.
An aggregate "semi-mechanistic" model such as the aggregate GAS-FLUX
may be very useful in examining climate effects on tundra carbon balance.
Finally, the separation of above- and below ground carbon dioxide flux rates
is essential in the aggregate GAS-FLUX, since they are sensitive to different
microclimatic driving variables.
The response of integrated diurnal aboveground net carbon dioxide exchange (labeled as uptake, since adequate light and photosynthetic capacity
are available on all days to permit aboveground net uptake) to integrated
diurnal PFD input is shown for the three vegetation communities in Fig.
21.8. Simulations conducted for differing stages of phenological development
are indicated with different symbols. Thirty-six symbols of each of five types
(five different dates) are plotted in each panel of Fig. 21.8. The 36 diurnal
simulations extensively sample the range of environmental conditions occurring in the Imnavait Creek watershed. From Fig. 21.8, the major factors
influencing aboveground CO2 uptake are easily recognized. At any point
in time, diurnal CO2 uptake (U) in response to integrated PFD may be
described with a relatively simple hyperbolic function of the type described
by Smith (1937):
(1)
where I is integrated incident photosynthetic photon flux density, s is the
initial slope of the curve relating CO2 uptake to irradiance, and Umax is the
maximum potential uptake rate of the community for a particular stage in
phenological development (point in time). Umax may not actually be attainable, since radiation input is limited by daylength and sun angle.
Along with radiation input, phenological development of the vegetation
strongly affects CO 2 uptake. Three light response curves are in general
evident in each panel of Fig. 21.8 which are the result of both structural and
physiological aspects of phenology included in the model (Sects. 21.3 and
21.4). Lowest carbon fixation potential occurs before leaves expand and the
moss cushions grow. Carbon fixation potential on July 1 and August 15 are
approximately equal due in the first case to changes in the developing
canopy and later to the onset of senescence. During a short period at midseason, carbon fixation potential remains high. Scatter of the symbols of
each type at a constant light value is the result of aboveground temperature
J.D. Tenhunen et al.
feedback effects, i.e., changes in nutrient availability, and structural change,
i.e., phenology, that influence carbon dioxide uptake and growth occur
on such a time scale (Reynolds and Leadley 1992). It is essential in the
aggregate model to separate carbon fixation from CO2 efflux, since the
former is directly related to the growth process. At present the entire
community net aboveground carbon dioxide flux is treated as a lumped
variable (comparable to the data in Fig. 21.6). Including carbon dioxide
efflux from the soil is important, since there is much interest in the potentials
for carbon storage in tundra soils, especially as related to climate change.
An aggregate "semi-mechanistic" model such as the aggregate GAS-FLUX
may be very useful in examining climate effects on tundra carbon balance.
Finally, the separation of above- and below ground carbon dioxide flux rates
is essential in the aggregate GAS-FLUX, since they are sensitive to different
microclimatic driving variables.
The response of integrated diurnal aboveground net carbon dioxide exchange (labeled as uptake, since adequate light and photosynthetic capacity
are available on all days to permit aboveground net uptake) to integrated
diurnal PFD input is shown for the three vegetation communities in Fig.
21.8. Simulations conducted for differing stages of phenological development
are indicated with different symbols. Thirty-six symbols of each of five types
(five different dates) are plotted in each panel of Fig. 21.8. The 36 diurnal
simulations extensively sample the range of environmental conditions occurring in the Imnavait Creek watershed. From Fig. 21.8, the major factors
influencing aboveground CO2 uptake are easily recognized. At any point
in time, diurnal CO2 uptake (U) in response to integrated PFD may be
described with a relatively simple hyperbolic function of the type described
by Smith (1937):
(1)
where I is integrated incident photosynthetic photon flux density, s is the
initial slope of the curve relating CO2 uptake to irradiance, and Umax is the
maximum potential uptake rate of the community for a particular stage in
phenological development (point in time). Umax may not actually be attainable, since radiation input is limited by daylength and sun angle.
Along with radiation input, phenological development of the vegetation
strongly affects CO 2 uptake. Three light response curves are in general
evident in each panel of Fig. 21.8 which are the result of both structural and
physiological aspects of phenology included in the model (Sects. 21.3 and
21.4). Lowest carbon fixation potential occurs before leaves expand and the
moss cushions grow. Carbon fixation potential on July 1 and August 15 are
approximately equal due in the first case to changes in the developing
canopy and later to the onset of senescence. During a short period at midseason, carbon fixation potential remains high. Scatter of the symbols of
each type at a constant light value is the result of aboveground temperature