Effects of Phenology, Physiology, and Gradients in Community Composition
439
relatively vertically oriented twigs (50°) and deciduous shrub leaves (45°). In
the subsequent sequence of alternating growth form layers, equal protions
of leaves were maintained, but leaf angle of graminoid elements increased to
85° at the bottom of the canopy, stem angle of deciduous shrub material
increased to 80° at the bottom, and SAl increased with canopy depth as
determined in stratified harvests.
Based on phenological observations at the Imnavait Creek site, we assumed that canopy development was initiated only in mid-June. Therefore,
on June 15, LAI of both deciduous shrubs and graminoids was zero in the
simulations and carbon fixation was carried out only by the evergreen shrubs
and mosses. Except for LAI of deciduous shrubs and graminoids, other
structural components were held constant, as described above. For simulations on July 1 and August 15, LAI for deciduous shrubs and graminoids
was increased to one half of that shown in Table 21.1. On the first date, this
is the result of rapid leaf expansion occurring and on the latter date due
to the onset of senescence as days become shorter. While variation in
phenological pattern occurs depending on climate conditions in any particular
year, the approach taken here adequately illustrates the critical manner in
which. phenological events influence the carbon balance of tundra ecosystems (Sect. 21.5.2; Fig. 21.8).
21.4 Ecophysiological Inputs to GAS-FLUX Along Water Gradients
in Tundra
21.4.1 CO2 Exchange of Vascular Plant Species
of Differing Growth Forms
Photosynthetic response at the individual leaf level of vascular plants of the
three growth forms discussed in Section 21.3 was described with equations
proposed by Farquhar et al. (1980), based on ribulose-1,5-bisphosphate
carboxylase-oxygenase (Rubisco) kinetics as mediated by (1) the concentrations of competing gaseous substrates, CO2 and O 2 , and (2) the ratio of
ribulose-1,5-bisphosphate (RuBP) concentration to enzyme active sites. CO 2
assimilation is integrated with an empirical description of stomatal conductance (Ball et al. 1987). The specific equations as well as methods for
application of the resulting leaf gas exchange model have been discussed in
detail by Harley and Tenhunen (1991).
In order to use the model with tundra species, leaf gas exchange of
representative deciduous shrub, graminoid, and evergreen shrub species was
monitored with a Li-Cor portable gas exchange system (LI-6200) over 24-h
periods and at a number of locations within the Imnavait Creek watershed
(Gebauer et al., in prep.). Care was taken to obtain comparative measures
of photosynthetic response to prevailing weather conditions by monitoring an
439
relatively vertically oriented twigs (50°) and deciduous shrub leaves (45°). In
the subsequent sequence of alternating growth form layers, equal protions
of leaves were maintained, but leaf angle of graminoid elements increased to
85° at the bottom of the canopy, stem angle of deciduous shrub material
increased to 80° at the bottom, and SAl increased with canopy depth as
determined in stratified harvests.
Based on phenological observations at the Imnavait Creek site, we assumed that canopy development was initiated only in mid-June. Therefore,
on June 15, LAI of both deciduous shrubs and graminoids was zero in the
simulations and carbon fixation was carried out only by the evergreen shrubs
and mosses. Except for LAI of deciduous shrubs and graminoids, other
structural components were held constant, as described above. For simulations on July 1 and August 15, LAI for deciduous shrubs and graminoids
was increased to one half of that shown in Table 21.1. On the first date, this
is the result of rapid leaf expansion occurring and on the latter date due
to the onset of senescence as days become shorter. While variation in
phenological pattern occurs depending on climate conditions in any particular
year, the approach taken here adequately illustrates the critical manner in
which. phenological events influence the carbon balance of tundra ecosystems (Sect. 21.5.2; Fig. 21.8).
21.4 Ecophysiological Inputs to GAS-FLUX Along Water Gradients
in Tundra
21.4.1 CO2 Exchange of Vascular Plant Species
of Differing Growth Forms
Photosynthetic response at the individual leaf level of vascular plants of the
three growth forms discussed in Section 21.3 was described with equations
proposed by Farquhar et al. (1980), based on ribulose-1,5-bisphosphate
carboxylase-oxygenase (Rubisco) kinetics as mediated by (1) the concentrations of competing gaseous substrates, CO2 and O 2 , and (2) the ratio of
ribulose-1,5-bisphosphate (RuBP) concentration to enzyme active sites. CO 2
assimilation is integrated with an empirical description of stomatal conductance (Ball et al. 1987). The specific equations as well as methods for
application of the resulting leaf gas exchange model have been discussed in
detail by Harley and Tenhunen (1991).
In order to use the model with tundra species, leaf gas exchange of
representative deciduous shrub, graminoid, and evergreen shrub species was
monitored with a Li-Cor portable gas exchange system (LI-6200) over 24-h
periods and at a number of locations within the Imnavait Creek watershed
(Gebauer et al., in prep.). Care was taken to obtain comparative measures
of photosynthetic response to prevailing weather conditions by monitoring an