Effects of Phenology, Physiology, and Gradients in Community Composition
443
indicated are those that decrease photosynthetic capacity by approximately
one half of that obtained during mid-season.
In the vegetation of Imnavait Creek watershed, deciduous shrubs are low
in stature with stems of small diameter. Chlorophyll containing tissues in
the bark of such shrubs may be important in providing for energy needs.
Gas exchange of twigs and stems may be quantified using the approach
described above for leaves. In the present version of GAS-FLUX, however,
a simpler regression model for gas exchange of stems of the alpine shrub
Rhododendron ferrugineum has been adopted from Siegwolf (1987). Stem
gas exchange is a function of incident light on the stems in each layer, the air
temperature of stems in the layer, and average stem diameter for the layer.
Photosynthetic capacity of stems decreases with age. Stem photosynthesis in
our simulations is large enough to approximately compensate for stem dark
respiration.
21.4.2 CO 2 Exchange of Poikilohydric Plants
While GAS-FLUX at present includes only Sphagnum spp. as a poikilohydric
plant element, it is constructed to simultaneously consider several poikilohydric functional groups distributed below the vascular plant canopy. Nevertheless, Sphagnum alone is an extremely important tundra ecosystem
structural component (Tenhunen et al. 1992), making up a major portion of
the living biomass (Hastings et al. 1989), modifying soil characteristics and
the accumulation of ground ice, strongly influencing habitat characteristics,
and modifying biomass allocation and production in vascular plant species
(Luken et al. 1985; Tenhunen et al. 1992). Modeling of CO2 gas exchange of
Sphagnum, as well as that of other poikilohydric plants, is made difficult by
the dynamic response to multiple factors; light, temperature, COl? and
water content. In addition, apparent long-term photo inhibitory effects can
greatly modify photosynthetic capacity (Murray et al. 1993), such that the
ecosystem role and vitality of Sphagnum is very dependent on structure
within the plant community where it occurs (Murray et al. 1989b; Tenhunen
et al. 1992).
Given the important role of Sphagnum in tundra ecosystems and recent
research emphases on climate change effects, it seemed essential that the
mechanistic approach of Farquhar et al. (1980) be utilized to describe
gas exchange response of this ecosystem component. Derivation of the
parameters in this model are based on analysis of the net photosynthesis dependency on carbon dioxide concentration. Experiments of the type shown
in Fig. 21.4B were undertaken in which the carbon dioxide response curve
of net photosynthesis was determined at a series of Sphagnum water contents. In such experiments, the Sphagnum shoots were vertically oriented
but separated to ensure uniform lighting of the tissues. Air relative hunidity
was maintained near saturation such that water content decreased slowly
443
indicated are those that decrease photosynthetic capacity by approximately
one half of that obtained during mid-season.
In the vegetation of Imnavait Creek watershed, deciduous shrubs are low
in stature with stems of small diameter. Chlorophyll containing tissues in
the bark of such shrubs may be important in providing for energy needs.
Gas exchange of twigs and stems may be quantified using the approach
described above for leaves. In the present version of GAS-FLUX, however,
a simpler regression model for gas exchange of stems of the alpine shrub
Rhododendron ferrugineum has been adopted from Siegwolf (1987). Stem
gas exchange is a function of incident light on the stems in each layer, the air
temperature of stems in the layer, and average stem diameter for the layer.
Photosynthetic capacity of stems decreases with age. Stem photosynthesis in
our simulations is large enough to approximately compensate for stem dark
respiration.
21.4.2 CO 2 Exchange of Poikilohydric Plants
While GAS-FLUX at present includes only Sphagnum spp. as a poikilohydric
plant element, it is constructed to simultaneously consider several poikilohydric functional groups distributed below the vascular plant canopy. Nevertheless, Sphagnum alone is an extremely important tundra ecosystem
structural component (Tenhunen et al. 1992), making up a major portion of
the living biomass (Hastings et al. 1989), modifying soil characteristics and
the accumulation of ground ice, strongly influencing habitat characteristics,
and modifying biomass allocation and production in vascular plant species
(Luken et al. 1985; Tenhunen et al. 1992). Modeling of CO2 gas exchange of
Sphagnum, as well as that of other poikilohydric plants, is made difficult by
the dynamic response to multiple factors; light, temperature, COl? and
water content. In addition, apparent long-term photo inhibitory effects can
greatly modify photosynthetic capacity (Murray et al. 1993), such that the
ecosystem role and vitality of Sphagnum is very dependent on structure
within the plant community where it occurs (Murray et al. 1989b; Tenhunen
et al. 1992).
Given the important role of Sphagnum in tundra ecosystems and recent
research emphases on climate change effects, it seemed essential that the
mechanistic approach of Farquhar et al. (1980) be utilized to describe
gas exchange response of this ecosystem component. Derivation of the
parameters in this model are based on analysis of the net photosynthesis dependency on carbon dioxide concentration. Experiments of the type shown
in Fig. 21.4B were undertaken in which the carbon dioxide response curve
of net photosynthesis was determined at a series of Sphagnum water contents. In such experiments, the Sphagnum shoots were vertically oriented
but separated to ensure uniform lighting of the tissues. Air relative hunidity
was maintained near saturation such that water content decreased slowly