Effects of Phenology, Physiology, and Gradients in Community Composition
437
experienced. Humidity profile data were adjusted to be appropriate for the
air temperature range imposed during a particular simulation run. Soil
temperature profiles were adjusted such that they were compatible with soil
moisture conditions in the simulations. Wind speed at the top of the canopy
varied over the course of the day, as determined in field studies, and was
assumed to decrease exponentially with canopy depth, thus decreasing leaf
boundary layer conductance. The exponential decrease was adjusted to be
compatible with measured wind profiles between 1 and 200 cm height above
ground. Thirty-six sets of input weather data were utilized on each date,
corresponding to three belowground moisture conditions (dry, moist, wet),
three aboveground weather conditions (clear cold, clear warm, and overcast
cold), and four levels of incident radiation. The 180 simulations for each of
the three plant communities adequately revealed seasonal trends in potential
canopy gas exchange. A 1-h time step was used, although other intervals are
accommodated by the model. Iterative solutions were obtained for steadystate gas exchange and canopy microclimate at each hour, and these were
integrated separately for sunlit and shaded leaves, for stem respiration, for
aboveground vascular plant layers, for moss layers, for the soil compartment, and for the entire community (above- and belowground).
21.3 Structural Inputs to GAS-FLUX Along Water Gradients in Tundra
Biomass harvests were carried out in midsummer (1986 through 1989) after
complete development of the vascular plant canopy in nine major vegetation
types within the Imnavait Creek watershed (Hastings et al. 1989; Hahn
1991). General trends in the composition of vegetation along the water availability gradient from ridgetop to valley bottom were described by Tenhunen
et al. (1992). In several cases, stratified harvests were conducted to determine the vertical distribution of stem and leaf biomass and to sample leaf
angles. Based on these field studies, differences in canopy structure between
the vegetation types simulated were summarized as shown in Table 21.1 for
the mid-season or mature stage of phenological development. Vascular plant
biomass is distributed among three functional groups or physiological types,
deciduous shrubs, graminoids, and evergreen shrubs. Deciduous shrubs and
graminoids occurred in significant quantities in all three vegetation zones,
with lowest LAI in tussock tundra at mid-slope and highest LAI in riparian
meadows of the lower slope. Stem area index (SAl) of the deciduous shrub
component plays a role in light interception in all three communities, but
decreases in riparian meadow "aboveground", apparently due to more rapid
moss growth. Evergreen shrubs were equally present in upper and mid-slope
locations but disappear in moist meadow sites.
Dry shrub and tussock tundra were considered to be very similar in
structure with the main difference being that total LAI for deciduous shrubs
437
experienced. Humidity profile data were adjusted to be appropriate for the
air temperature range imposed during a particular simulation run. Soil
temperature profiles were adjusted such that they were compatible with soil
moisture conditions in the simulations. Wind speed at the top of the canopy
varied over the course of the day, as determined in field studies, and was
assumed to decrease exponentially with canopy depth, thus decreasing leaf
boundary layer conductance. The exponential decrease was adjusted to be
compatible with measured wind profiles between 1 and 200 cm height above
ground. Thirty-six sets of input weather data were utilized on each date,
corresponding to three belowground moisture conditions (dry, moist, wet),
three aboveground weather conditions (clear cold, clear warm, and overcast
cold), and four levels of incident radiation. The 180 simulations for each of
the three plant communities adequately revealed seasonal trends in potential
canopy gas exchange. A 1-h time step was used, although other intervals are
accommodated by the model. Iterative solutions were obtained for steadystate gas exchange and canopy microclimate at each hour, and these were
integrated separately for sunlit and shaded leaves, for stem respiration, for
aboveground vascular plant layers, for moss layers, for the soil compartment, and for the entire community (above- and belowground).
21.3 Structural Inputs to GAS-FLUX Along Water Gradients in Tundra
Biomass harvests were carried out in midsummer (1986 through 1989) after
complete development of the vascular plant canopy in nine major vegetation
types within the Imnavait Creek watershed (Hastings et al. 1989; Hahn
1991). General trends in the composition of vegetation along the water availability gradient from ridgetop to valley bottom were described by Tenhunen
et al. (1992). In several cases, stratified harvests were conducted to determine the vertical distribution of stem and leaf biomass and to sample leaf
angles. Based on these field studies, differences in canopy structure between
the vegetation types simulated were summarized as shown in Table 21.1 for
the mid-season or mature stage of phenological development. Vascular plant
biomass is distributed among three functional groups or physiological types,
deciduous shrubs, graminoids, and evergreen shrubs. Deciduous shrubs and
graminoids occurred in significant quantities in all three vegetation zones,
with lowest LAI in tussock tundra at mid-slope and highest LAI in riparian
meadows of the lower slope. Stem area index (SAl) of the deciduous shrub
component plays a role in light interception in all three communities, but
decreases in riparian meadow "aboveground", apparently due to more rapid
moss growth. Evergreen shrubs were equally present in upper and mid-slope
locations but disappear in moist meadow sites.
Dry shrub and tussock tundra were considered to be very similar in
structure with the main difference being that total LAI for deciduous shrubs