446
J.D. Tenhunen et al.
value of light, temperature, [C02], and water content. Community flux rate
for the moss component was obtained by multiplying by the bulk density
and ground area covered by Sphagnum (40, 50, and 60% cover in dry shrub,
tussock, and riparian tundra respectively). This was done for layers 1 cm in
thickness, determining the average light and temperature values for each
layer as described in Section 21.2 and calculating rates for sunlit and shaded
Sphagnum separately. Water content of the moss was varied from 750, to
500, and to 250% of dry weight in dry shrub tundra for wet, normal, and dry
conditions based on field observations. The corresponding values for tussock
tundra and riparian meadow were 1100, 750, and 500% of dry weight.
Gradients in [C02] within moss cushions were not considered, rather [C0 2 ]
was maintained in all simulations at ambient concentration (340 ppm).
21.4.3 CO2 Exchange of the Soil
The integrated response of soil microbial and root respiration is approximately equal in magnitude to total canopy carbon fixation. Since one objective of the GAS-FLUX model in applications for tundra ecosystems is to
understand controls on overall ecosystem carbon balance, relatively extensive monitoring studies were undertaken to examine carbon dioxide flux
rates from soils of different communities and to correlate these flux rates
with soil environmental factors (Oberbauer et al. 1991, 1992). The results of
these studies indicated that CO2 efflux from tundra communities with deep
organic soils, such as those for which simulations have been conducted,
varies primarily in response to changes in the depth to water table and/or
soil moisture and changes in soil temperature. Presumably the mechanism
controlling CO2 loss in response to depth to water table is related to oxygen
diffusion limitation. Correlations between soil moisture and CO2 efflux were
very high, but explained less of the site variability in CO2 effluxes than
depth to water table. Soil temperature is a factor which modifies the respiration rates permitted by prevailing water table or thaw depth. Since seasonal
increases in mean soil temperature are small, the primary effect of temperature is seen in diurnal changes in CO2 efflux.
The CO 2 efflux data were fit to an empirical model incorporating the
Arrhenius function for temperature and an asymptotic function for water
table depth or thaw depth (Oberbauer et al. 1992). An asymptotic model
was chosen based on preliminary examination of the data, which suggested
that at depths greater than lOcm, depth to water table had little effect. This
simple mathematical description for CO2 efflux in response to soil environmental factors was included as a subroutine of GAS-FLUX. In simulations,
soil water content to 10 cm depth was assumed to be the same as that of
Sphagnum (See Sect. 21.4.2 - only very small differences were found in the
field). Permafrost at shallow depth that might reduce respiring soil volume
occurs only very early in June, and thus was not included here as a factor
J.D. Tenhunen et al.
value of light, temperature, [C02], and water content. Community flux rate
for the moss component was obtained by multiplying by the bulk density
and ground area covered by Sphagnum (40, 50, and 60% cover in dry shrub,
tussock, and riparian tundra respectively). This was done for layers 1 cm in
thickness, determining the average light and temperature values for each
layer as described in Section 21.2 and calculating rates for sunlit and shaded
Sphagnum separately. Water content of the moss was varied from 750, to
500, and to 250% of dry weight in dry shrub tundra for wet, normal, and dry
conditions based on field observations. The corresponding values for tussock
tundra and riparian meadow were 1100, 750, and 500% of dry weight.
Gradients in [C02] within moss cushions were not considered, rather [C0 2 ]
was maintained in all simulations at ambient concentration (340 ppm).
21.4.3 CO2 Exchange of the Soil
The integrated response of soil microbial and root respiration is approximately equal in magnitude to total canopy carbon fixation. Since one objective of the GAS-FLUX model in applications for tundra ecosystems is to
understand controls on overall ecosystem carbon balance, relatively extensive monitoring studies were undertaken to examine carbon dioxide flux
rates from soils of different communities and to correlate these flux rates
with soil environmental factors (Oberbauer et al. 1991, 1992). The results of
these studies indicated that CO2 efflux from tundra communities with deep
organic soils, such as those for which simulations have been conducted,
varies primarily in response to changes in the depth to water table and/or
soil moisture and changes in soil temperature. Presumably the mechanism
controlling CO2 loss in response to depth to water table is related to oxygen
diffusion limitation. Correlations between soil moisture and CO2 efflux were
very high, but explained less of the site variability in CO2 effluxes than
depth to water table. Soil temperature is a factor which modifies the respiration rates permitted by prevailing water table or thaw depth. Since seasonal
increases in mean soil temperature are small, the primary effect of temperature is seen in diurnal changes in CO2 efflux.
The CO 2 efflux data were fit to an empirical model incorporating the
Arrhenius function for temperature and an asymptotic function for water
table depth or thaw depth (Oberbauer et al. 1992). An asymptotic model
was chosen based on preliminary examination of the data, which suggested
that at depths greater than lOcm, depth to water table had little effect. This
simple mathematical description for CO2 efflux in response to soil environmental factors was included as a subroutine of GAS-FLUX. In simulations,
soil water content to 10 cm depth was assumed to be the same as that of
Sphagnum (See Sect. 21.4.2 - only very small differences were found in the
field). Permafrost at shallow depth that might reduce respiring soil volume
occurs only very early in June, and thus was not included here as a factor