448
J.D. Tenhunen et al.
considerably more carbon than mosses because they have a relatively high
photosynthetic capacity and are exposed to high light. Lower light on overcast days reduces carbon fixation by leaves. Respiratory losses from twigs
are unimportant, for most twigs of tundra species are small in diameter
and fix enough carbon to compensate for their own respiration. Sphagnum
carbon dioxide exchange is strongly influenced by variation in light and
temperature during the day (at lower temperatures, moss maintained positive
photosynthesis despite lower light on the overcast day), but most strongly by
long-term changes in soil water content (not shown). Although the model
predicts ecosystem net carbon dioxide exchange (diurnal sum of all components shown in upper right of each panel) to be positive on clear days, the
carbon balance is sensitive to daily changes in environmental conditions,
with carbon fixation by both leaves and Sphagnum reduced enough on
overcast days so that the daily integrated carbon dioxide flux may be negative
(cf. Grulke et al. 1990).
"Coarse scale data", i.e., measured diurnal course data on net ecosystem
carbon dioxide exchange of the tussock tundra plant community within the
Imnavait Creek watershed assessed with small chambers as indicated in Fig.
21. 7, were gathered to examine potential "aggregation error" in the model
(cf. Rathstetter et al. 1992) during summer 1990 (Gillespie et al. 1993).
Chambers were placed in the field to observe both individual tussocks and
intertussock areas separately, and community gas exchange was observed in
both light and dark chambers. Subtraction of the measured dark CO2 efflux
from net exchange rates provided estimates of total carbon fixation or "C02
uptake". Oiurnal changes in net ecosystem CO2 uptake were strongly correlated with diurnal changes in PFD. A scatter plot of measured CO2 uptake
values versus incident PFO at the time of measurement indicates that uptake
is sensitive to PFO below 500J.lmolm- 2 s- 1 but tends toward saturation at
higher flux levels (Fig. 21.7). In agreement with differences in LAI, CO2
uptake in intertussock areas was in general lower than that of tussocks. We
compared the estimated uptake rates with the net aboveground flux rate
predicted by GAS-FLUX. As seen in Fig. 21.7, the GAS-FLUX model
similarly predicts large influences of PFO below 550J.lmolm- 2 s- 1 and
uptake fluxes on the same order of magnitude as those observed when soil
respiration is "removed". The differences in CO2 uptake observed between
tussock and intertussock areas suggest that while the model is reasonably
calibrated (sensu Rathstetter et al. 1992) for the Imnavait Creek watershed,
these independent calibration measurements are an important consideration
in each individual application of the model. Tussock density and development are factors that may deserve further consideration. While "calibration
measurements" were not undertaken in dry shrub tundra and riparian
meadow vegetation, they are highly desirable.
Billings et al. (1982) found daily net carbon dioxide exchange of coastal
tundra microcosms to vary between -2 and +lOgC0 2 m- 2 d- 1 , the highest
values being obtained with a water table near the ground surface, I.e.,
J.D. Tenhunen et al.
considerably more carbon than mosses because they have a relatively high
photosynthetic capacity and are exposed to high light. Lower light on overcast days reduces carbon fixation by leaves. Respiratory losses from twigs
are unimportant, for most twigs of tundra species are small in diameter
and fix enough carbon to compensate for their own respiration. Sphagnum
carbon dioxide exchange is strongly influenced by variation in light and
temperature during the day (at lower temperatures, moss maintained positive
photosynthesis despite lower light on the overcast day), but most strongly by
long-term changes in soil water content (not shown). Although the model
predicts ecosystem net carbon dioxide exchange (diurnal sum of all components shown in upper right of each panel) to be positive on clear days, the
carbon balance is sensitive to daily changes in environmental conditions,
with carbon fixation by both leaves and Sphagnum reduced enough on
overcast days so that the daily integrated carbon dioxide flux may be negative
(cf. Grulke et al. 1990).
"Coarse scale data", i.e., measured diurnal course data on net ecosystem
carbon dioxide exchange of the tussock tundra plant community within the
Imnavait Creek watershed assessed with small chambers as indicated in Fig.
21. 7, were gathered to examine potential "aggregation error" in the model
(cf. Rathstetter et al. 1992) during summer 1990 (Gillespie et al. 1993).
Chambers were placed in the field to observe both individual tussocks and
intertussock areas separately, and community gas exchange was observed in
both light and dark chambers. Subtraction of the measured dark CO2 efflux
from net exchange rates provided estimates of total carbon fixation or "C02
uptake". Oiurnal changes in net ecosystem CO2 uptake were strongly correlated with diurnal changes in PFD. A scatter plot of measured CO2 uptake
values versus incident PFO at the time of measurement indicates that uptake
is sensitive to PFO below 500J.lmolm- 2 s- 1 but tends toward saturation at
higher flux levels (Fig. 21.7). In agreement with differences in LAI, CO2
uptake in intertussock areas was in general lower than that of tussocks. We
compared the estimated uptake rates with the net aboveground flux rate
predicted by GAS-FLUX. As seen in Fig. 21.7, the GAS-FLUX model
similarly predicts large influences of PFO below 550J.lmolm- 2 s- 1 and
uptake fluxes on the same order of magnitude as those observed when soil
respiration is "removed". The differences in CO2 uptake observed between
tussock and intertussock areas suggest that while the model is reasonably
calibrated (sensu Rathstetter et al. 1992) for the Imnavait Creek watershed,
these independent calibration measurements are an important consideration
in each individual application of the model. Tussock density and development are factors that may deserve further consideration. While "calibration
measurements" were not undertaken in dry shrub tundra and riparian
meadow vegetation, they are highly desirable.
Billings et al. (1982) found daily net carbon dioxide exchange of coastal
tundra microcosms to vary between -2 and +lOgC0 2 m- 2 d- 1 , the highest
values being obtained with a water table near the ground surface, I.e.,