21 Effects of Phenology, Physiology,
and Gradients in Community Composition,
Structure, and Microclimate
on Tundra Ecosystem CO 2 Exchange
J.D. Tenhunen, R.T.W. Siegwolf, and S.F. Oberbauer
21.1 "Phenomenological" or "Aggregate" Models of Ecosystem CO2 Flux
Recently, attention among scientists has been focused on potential global
climate change as well as on the deposition of pollutants and their impacts.
These perspectives emphasize the role of ecosystems as exchange surfaces
between atmosphere and vegetation and between vegetation and groundwater (Dickenson 1988; Bolin 1988; Ulrich 1987). Particularly with respect
to northern taiga and tundra regions, it is important to determine whether
climate change may have already altered or may in the future alter rates
(positive or negative) of ecosystem carbon storage (Oberbauer et al. 1992;
Oechel and Billings 1992). Furthermore, it is important to understand
environmental controls on carbon fluxes and carbon storage, because the
gradients in soil temperature, water availability, and available light energy
in the Arctic are large and these will strongly affect the integrated values of
net carbon dioxide (Tenhunen et al. 1992) and methane exchange (Whalen
and Reeburgh 1988, 1990) in polar regions. Even when viewed simplistically
and at the regional scale, temporal and spatial variation in ecosystem material exchange characteristics must be considered when estimating carbon
balances (Miller et al. 1983). At smaller scales such as the watershed,
temporal and spatial variation in ecosystem structure, species composition,
physiology, and environmental conditions determine momentary net gas
exchange rates, but also provide clues concerning the manner in which
ecosystem properties may be shifted regionally in a future climate (Chapin
et al. 1992).
Despite considerable effort having been devoted to the examination of
carbon dioxide exchange in tundra regions (Kjelvik et al. 1975; Coyne and
Kelley 1978; Billings et al. 1982, 1983; Eckardt 1982; Luken and Billings
1985; Hilbert et al. 1987; Grulke et al. 1990; Oberbauer et al. 1991, 1992),
beginning with investigations near Barrow, Alaska, during the International
Biological Program, we need to refine our understanding of the relationships
between dynamic features of ecosystem structure and function and transfers
at the atmosphere/vegetation interface. Limitations imposed by the small
scale at which ecosystem-level experiments may usually be conducted and
the non acceptability of large-scale ecosystem manipulations force us to rely
on "bottom-up" approaches when formulating models that will be useful
and Gradients in Community Composition,
Structure, and Microclimate
on Tundra Ecosystem CO 2 Exchange
J.D. Tenhunen, R.T.W. Siegwolf, and S.F. Oberbauer
21.1 "Phenomenological" or "Aggregate" Models of Ecosystem CO2 Flux
Recently, attention among scientists has been focused on potential global
climate change as well as on the deposition of pollutants and their impacts.
These perspectives emphasize the role of ecosystems as exchange surfaces
between atmosphere and vegetation and between vegetation and groundwater (Dickenson 1988; Bolin 1988; Ulrich 1987). Particularly with respect
to northern taiga and tundra regions, it is important to determine whether
climate change may have already altered or may in the future alter rates
(positive or negative) of ecosystem carbon storage (Oberbauer et al. 1992;
Oechel and Billings 1992). Furthermore, it is important to understand
environmental controls on carbon fluxes and carbon storage, because the
gradients in soil temperature, water availability, and available light energy
in the Arctic are large and these will strongly affect the integrated values of
net carbon dioxide (Tenhunen et al. 1992) and methane exchange (Whalen
and Reeburgh 1988, 1990) in polar regions. Even when viewed simplistically
and at the regional scale, temporal and spatial variation in ecosystem material exchange characteristics must be considered when estimating carbon
balances (Miller et al. 1983). At smaller scales such as the watershed,
temporal and spatial variation in ecosystem structure, species composition,
physiology, and environmental conditions determine momentary net gas
exchange rates, but also provide clues concerning the manner in which
ecosystem properties may be shifted regionally in a future climate (Chapin
et al. 1992).
Despite considerable effort having been devoted to the examination of
carbon dioxide exchange in tundra regions (Kjelvik et al. 1975; Coyne and
Kelley 1978; Billings et al. 1982, 1983; Eckardt 1982; Luken and Billings
1985; Hilbert et al. 1987; Grulke et al. 1990; Oberbauer et al. 1991, 1992),
beginning with investigations near Barrow, Alaska, during the International
Biological Program, we need to refine our understanding of the relationships
between dynamic features of ecosystem structure and function and transfers
at the atmosphere/vegetation interface. Limitations imposed by the small
scale at which ecosystem-level experiments may usually be conducted and
the non acceptability of large-scale ecosystem manipulations force us to rely
on "bottom-up" approaches when formulating models that will be useful