Effects of Phenology, Physiology, and Gradients in Community Composition
441
light at the top of the plant canopy typically fluctuates between a photon flux
density (PFD) of 1000 and 2000 !lmol m -2 s -1 during the middle of clear
days, while temperature varies from near zero °C to approximately 15°C.
Nevertheless, much higher temperatures of 25 to 30 °C may occur during
warm weather periods. Transpiration rate and leaf conductance decrease to
very low values between 2200 and 0400 h primarily in response to low
incident PFD (below 1O!lmolm- 2 s- 1 ). Net photosynthesis becomes negative
during these periods. Similar data have been reported by many researchers
working in arctic regions (Stocker 1931; Kjelvik et al. 1975; Johansson and
Linder 1975). The advantage provided here is that a truly comparative data
base was constructed using plants of different growth forms growing adjacent
to one another.
As seen in Fig. 21.3, maximum leaf conductance of the graminoid is approximately twice as large during midday as that observed for the deciduous
and evergreen shrub species (300 versus 150mmolm- 2 s- 1 ). A difference in
maximum net photosynthesis rate is also apparent, although this is not as
large (10 versus 7!lmol m- 2 S-1). By pooling data from many experiments,
verification data sets are obtained for the leaf gas exchange model that
permit us to describe these individual species responses over a wide range of
conditions.
Following the procedures outlined by Harley and Tenhunen (1991),
parameter values for the gas exchange model were obtained for tundra
species as indicated in Table 21.2 (see Harley and Tenhunen 1991 or
Appendix of Tenhunen et al. 1990 for parameter definitions). Differences
among species were easily described with only small changes in C(P ml) and
C(VcmaJ, scaling parameters which describe the capacities for RuBP regeneration and enzymatic CO2 fixation respectively. Parameters determining
temperature response of photosynthesis and respiration were adjusted to
obtain the best fits to verification data sets and to obtain QlO values comparable to those generally found for arctic species (Limbach et al. 1982;
Semikhatova et al. 1992). Differences in temperature response among species
and growth forms included in the model at present are small, but this may
easily be changed as appropriate generalizations as these ecophysiological
responses are defined for particular sites.
The values shown (Table 21.2) for deciduous shrubs, graminoids, and
evergreen shrubs are compatible with measured data for (1) Betula nana, (2)
Eriophorum vagina tum and Carex bigelowii, and (3) Ledum palustre and
Vaccinium vitis-idaea. However, response of the deciduous shrub Salix
pulchra was found to be similar to the evergreen Ledum palustre with
lower photosynthetic capacity than that of Betula nana. This suggests that
generalizations based on functional groups must be made with caution. Due
to the physiological differences established for Betula and Salix, canopy
structure was described in such a manner that contributions of the two
major deciduous shrubs were independently estimated. In other words, this
is the reason for utilizing eight deciduous shrub layers above the graminoid
441
light at the top of the plant canopy typically fluctuates between a photon flux
density (PFD) of 1000 and 2000 !lmol m -2 s -1 during the middle of clear
days, while temperature varies from near zero °C to approximately 15°C.
Nevertheless, much higher temperatures of 25 to 30 °C may occur during
warm weather periods. Transpiration rate and leaf conductance decrease to
very low values between 2200 and 0400 h primarily in response to low
incident PFD (below 1O!lmolm- 2 s- 1 ). Net photosynthesis becomes negative
during these periods. Similar data have been reported by many researchers
working in arctic regions (Stocker 1931; Kjelvik et al. 1975; Johansson and
Linder 1975). The advantage provided here is that a truly comparative data
base was constructed using plants of different growth forms growing adjacent
to one another.
As seen in Fig. 21.3, maximum leaf conductance of the graminoid is approximately twice as large during midday as that observed for the deciduous
and evergreen shrub species (300 versus 150mmolm- 2 s- 1 ). A difference in
maximum net photosynthesis rate is also apparent, although this is not as
large (10 versus 7!lmol m- 2 S-1). By pooling data from many experiments,
verification data sets are obtained for the leaf gas exchange model that
permit us to describe these individual species responses over a wide range of
conditions.
Following the procedures outlined by Harley and Tenhunen (1991),
parameter values for the gas exchange model were obtained for tundra
species as indicated in Table 21.2 (see Harley and Tenhunen 1991 or
Appendix of Tenhunen et al. 1990 for parameter definitions). Differences
among species were easily described with only small changes in C(P ml) and
C(VcmaJ, scaling parameters which describe the capacities for RuBP regeneration and enzymatic CO2 fixation respectively. Parameters determining
temperature response of photosynthesis and respiration were adjusted to
obtain the best fits to verification data sets and to obtain QlO values comparable to those generally found for arctic species (Limbach et al. 1982;
Semikhatova et al. 1992). Differences in temperature response among species
and growth forms included in the model at present are small, but this may
easily be changed as appropriate generalizations as these ecophysiological
responses are defined for particular sites.
The values shown (Table 21.2) for deciduous shrubs, graminoids, and
evergreen shrubs are compatible with measured data for (1) Betula nana, (2)
Eriophorum vagina tum and Carex bigelowii, and (3) Ledum palustre and
Vaccinium vitis-idaea. However, response of the deciduous shrub Salix
pulchra was found to be similar to the evergreen Ledum palustre with
lower photosynthetic capacity than that of Betula nana. This suggests that
generalizations based on functional groups must be made with caution. Due
to the physiological differences established for Betula and Salix, canopy
structure was described in such a manner that contributions of the two
major deciduous shrubs were independently estimated. In other words, this
is the reason for utilizing eight deciduous shrub layers above the graminoid