436
J.D. Tenhunen et al.
shaded leaf area is calculated in each canopy layer, sunlit and shaded
portions of the ground surface are calculated. Considering the area of
ground covered by Sphagnum, average sunlit and shaded gas exchange rates
for each moss layer are determined in response to light intensity within
the layer estimated from extinction coefficients for compact Sphagnum
(Clymo and Hayward 1982; their Fig. 8.30 unshaded natural stand-extinction
coefficient = -0.75). An average layer rate is obtained by weighting the two
fluxes according to sunlit and shaded area. Soil respiratory flux is determined from empirical regression formulations established in field studies
(Oberbauer et al. 1991, 1992). Temperature in the moss and upper soil
layers is included in the factors influencing gas flux rates and is considered to
decrease linearly with depth. Net ecosystem gas exchange is obtained by
combining flux rates from all layers of the model. Input variables to the
model for each diurnal simulation, for determining layered structure, and as
hourly drivers are listed together with output variables of major importance
in the Appendix.
The size of a "homogeneous" area to which the model may be applied
will vary depending on particular applications and on the type of input
data available to support model parameterization. In the application discussed here, structural homogeneity is assumed to occur over the entire area
mapped for a particular tundra plant community within the Imnavait Creek
watershed near Toolik Lake, Alaska (cf. vegetation map in Walker et al.
1989). Structural as well as functional homogeneity, which depends on
momentary stand microclimate, is considered to occur for a ground surface
area (pixel size) of at least 10 x 10m. At such a scale, spatial patterning in
variables, such as remotely sensed normalized difference vegetation index
(Stow et al. 1989) and water table depth determined by hydrological models,
may eventually be used to derive landscape patterns in tundra ecosystem
CO2 exchange (see below).
Simulations of diurnal courses of CO2 exchange of three tundra communities in the Imnavait Creek watershed were conducted for clear and
overcast days that might occur during the summer on June 15, July 1, July
15, August 1, and August 15. Three vegetation communities were considered, dry shrub tundra, tussock tundra, and wet sedge (riparian) meadow
with a high presence of deciduous shrubs. These communities cover on
the order of 85% of the surface within the watershed and exist as broad
bands or zones along the topographic and water-availability gradients from
the ridgetops to streamside within the basin. General structural and microclimatic gradients along this toposequence have been described (Walker
et al. 1989; Tenhunen et aI.1992, see also below). Not considered here are
water track communities (cf. Hastings et al. 1989) and a series of heath
types occurring on the ridges (Hahn 1991).
Input weather conditions were based on extensive monitoring of stand
microclimate carried out between 1985 and 1989 and included variation
in above- and below ground temperature profiles within the natural range
J.D. Tenhunen et al.
shaded leaf area is calculated in each canopy layer, sunlit and shaded
portions of the ground surface are calculated. Considering the area of
ground covered by Sphagnum, average sunlit and shaded gas exchange rates
for each moss layer are determined in response to light intensity within
the layer estimated from extinction coefficients for compact Sphagnum
(Clymo and Hayward 1982; their Fig. 8.30 unshaded natural stand-extinction
coefficient = -0.75). An average layer rate is obtained by weighting the two
fluxes according to sunlit and shaded area. Soil respiratory flux is determined from empirical regression formulations established in field studies
(Oberbauer et al. 1991, 1992). Temperature in the moss and upper soil
layers is included in the factors influencing gas flux rates and is considered to
decrease linearly with depth. Net ecosystem gas exchange is obtained by
combining flux rates from all layers of the model. Input variables to the
model for each diurnal simulation, for determining layered structure, and as
hourly drivers are listed together with output variables of major importance
in the Appendix.
The size of a "homogeneous" area to which the model may be applied
will vary depending on particular applications and on the type of input
data available to support model parameterization. In the application discussed here, structural homogeneity is assumed to occur over the entire area
mapped for a particular tundra plant community within the Imnavait Creek
watershed near Toolik Lake, Alaska (cf. vegetation map in Walker et al.
1989). Structural as well as functional homogeneity, which depends on
momentary stand microclimate, is considered to occur for a ground surface
area (pixel size) of at least 10 x 10m. At such a scale, spatial patterning in
variables, such as remotely sensed normalized difference vegetation index
(Stow et al. 1989) and water table depth determined by hydrological models,
may eventually be used to derive landscape patterns in tundra ecosystem
CO2 exchange (see below).
Simulations of diurnal courses of CO2 exchange of three tundra communities in the Imnavait Creek watershed were conducted for clear and
overcast days that might occur during the summer on June 15, July 1, July
15, August 1, and August 15. Three vegetation communities were considered, dry shrub tundra, tussock tundra, and wet sedge (riparian) meadow
with a high presence of deciduous shrubs. These communities cover on
the order of 85% of the surface within the watershed and exist as broad
bands or zones along the topographic and water-availability gradients from
the ridgetops to streamside within the basin. General structural and microclimatic gradients along this toposequence have been described (Walker
et al. 1989; Tenhunen et aI.1992, see also below). Not considered here are
water track communities (cf. Hastings et al. 1989) and a series of heath
types occurring on the ridges (Hahn 1991).
Input weather conditions were based on extensive monitoring of stand
microclimate carried out between 1985 and 1989 and included variation
in above- and below ground temperature profiles within the natural range