434
J.D. Tenhunen et al.
manner in which a simplified "phenomenological" or "aggregate" version of
GAS-FLUX has been derived for CO2 exchange of tundra communities
under present climate conditions, and the manner in which it may be
extended to consider spatial and temporal variation in ecosystem properties,
including those which may result due to long-term climate change.
21.2 Concept and General Structure of the Stand Model GAS-FLUX
The concept of the GAS-FLUX model is extremely simple, as depicted in
Fig. 21.1. The objective is to quantify all significant sources of respiratory
CO2 from an ecosystem, to quantify all significant CO2 sinks as a result of
carbon fixation in photosynthesis, and to obtain an estimate of ecosystem
net CO 2 exchange by summing these fluxes. In practice, the calculation of
each average flux rate shown is not simple, because these on a square meter
ground surface basis are influenced by the momentary physiological status
of many organisms as well as by microclimate conditions. Both physiological state and microclimate vary temporally, and many interactions occur,
especially with respect to physical structure and vertical microclimate gradients. In the case of tundra ecosystems, separate considerations must be
made for ecosystem components with differing physiological regulation of
CO2 flux, i.e., vascular plants (separating graminoid elements, deciduous
shrubs, and evergreen shrubs), poikilohydric plants (mosses of different
types and lichens), and soil microbes. As indicated in Fig. 21.1, verification
of calculated net ecosystem CO2 exchange is possible by comparison with
measured net exchange rates obtained in small chambers (approximately 0.1
to 1.0m 3 in size; Oechel et al. 1992) that isolate a three-dimensional block
of tundra for short periods.
In the GAS-FLUX model, the vertical structure and microclimate of a
definable area or community is considered to be relatively homogeneous. The
structure of the model is described in Fig. 21.2. The vascular plant canopy is
divided into a series of layers containing leaf, and possibly stem, material
with a single type of physiological response (representative species - see
discussion of physiological parameterization below). Consecutive layers may
have material of different species or of the same species. Aspects of light
interception, calculation of energy budget for leaves and estimation of
leaf temperature, and calculation of photosynthetic CO2 uptake as well as
stomatal conductance and water loss for leaves in all canopy layers have
been discussed previously (Caldwell et al. 1986; Reynolds et al. 1988;
Tenhunen et al. 1990). Light interception by both stems and leaves is
considered. With respect to gas exchange, stems are considered to remain at
air temperature. Light incident on the understory moss is that passing
through the lowest layer of the vascular plant canopy. Just as sunlit and
J.D. Tenhunen et al.
manner in which a simplified "phenomenological" or "aggregate" version of
GAS-FLUX has been derived for CO2 exchange of tundra communities
under present climate conditions, and the manner in which it may be
extended to consider spatial and temporal variation in ecosystem properties,
including those which may result due to long-term climate change.
21.2 Concept and General Structure of the Stand Model GAS-FLUX
The concept of the GAS-FLUX model is extremely simple, as depicted in
Fig. 21.1. The objective is to quantify all significant sources of respiratory
CO2 from an ecosystem, to quantify all significant CO2 sinks as a result of
carbon fixation in photosynthesis, and to obtain an estimate of ecosystem
net CO 2 exchange by summing these fluxes. In practice, the calculation of
each average flux rate shown is not simple, because these on a square meter
ground surface basis are influenced by the momentary physiological status
of many organisms as well as by microclimate conditions. Both physiological state and microclimate vary temporally, and many interactions occur,
especially with respect to physical structure and vertical microclimate gradients. In the case of tundra ecosystems, separate considerations must be
made for ecosystem components with differing physiological regulation of
CO2 flux, i.e., vascular plants (separating graminoid elements, deciduous
shrubs, and evergreen shrubs), poikilohydric plants (mosses of different
types and lichens), and soil microbes. As indicated in Fig. 21.1, verification
of calculated net ecosystem CO2 exchange is possible by comparison with
measured net exchange rates obtained in small chambers (approximately 0.1
to 1.0m 3 in size; Oechel et al. 1992) that isolate a three-dimensional block
of tundra for short periods.
In the GAS-FLUX model, the vertical structure and microclimate of a
definable area or community is considered to be relatively homogeneous. The
structure of the model is described in Fig. 21.2. The vascular plant canopy is
divided into a series of layers containing leaf, and possibly stem, material
with a single type of physiological response (representative species - see
discussion of physiological parameterization below). Consecutive layers may
have material of different species or of the same species. Aspects of light
interception, calculation of energy budget for leaves and estimation of
leaf temperature, and calculation of photosynthetic CO2 uptake as well as
stomatal conductance and water loss for leaves in all canopy layers have
been discussed previously (Caldwell et al. 1986; Reynolds et al. 1988;
Tenhunen et al. 1990). Light interception by both stems and leaves is
considered. With respect to gas exchange, stems are considered to remain at
air temperature. Light incident on the understory moss is that passing
through the lowest layer of the vascular plant canopy. Just as sunlit and