The Role of Atmospheric CO2, Climate, and Disturbance in the Carbon Balance of the Terrestrial Biosp

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Modeling Modes of Variability in Carbon Exchange
Between High Latitude Ecosystems and the
AtmosphereDave McGuire
(UAF), Joy Clein (UAF), and Qianlai Zhuang (MBL)
Map Courtesy of Cath Copass
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Organization of Talk

• Why is understanding carbon exchange of high
  latitude ecosystems important?
• Progress in modeling
• seasonal-scale exchange
• interannual-scale exchange
• decadal-scale exchange
• century-scale exchange
• Summary

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Feedbacks of Terrestrial Ecosystems to the
Earths Climate System
Regional Climate
Global Climate
Exchange of radiatively active gases (CO2 and CH4)
Delivery of freshwater to Arctic Ocean
Water and energy exchange
Impacts
Terrestrial Ecosystems
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Strategy to Evaluate Seasonal Exchange of Carbon
Simulated by Terrestrial Biosphere Models
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Terrestrial models underestimate winter CO2
concentrations and drawdown CO2 too early in the
spring (from Dargaville et al., 2002)
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Seasonal Variability

• Winter Exchange
• - Does underestimate of atmospheric CO2
  concentrations in winter indicate that
  terrestrial ecosystem models should do a better
  job in representing controls over winter
  decomposition?
• Exchange at transition from winter to summer
• - Does early drawdown of atmospheric CO2 at the
  winter to summer transition indicate that
  terrestrial ecosystem should do a better job in
  representing consideration the effects of spring
  thaw on carbon exchange?

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(from McGuire et al., 2000)
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(from McGuire et al., 2000)
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(Running et al., EOS)
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Coupling STM to TEM
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Mean Net Carbon Exchange during the 1980s (Zhuang
et al., 2003)
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Comparison of Observed and Simulated CO2 at
Monitoring Stations
(from Zhuang et al., 2003)
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Inter-annual Variability

• Climate
• - To what extent does variability in temperature,
  precipitation, and radiation influence
  inter-annual variability in carbon exchange?
• Disturbance
• - To what extent does variability in climate vs.
  disturbance influence inter-annual variability
  in carbon exchange?

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Application of STM-TEM parameterized for black
spruce at Taiga LTER to simulate the C fluxes of
the NSA-OBS BOREAS site. Field-based estimates
are based on eddy covariance measurements. Zhuang
et al. 2002.
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(from McGuire et al., in preparation)
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Fire regime (Severity, History)
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Uptake
Release
GAC 7/29/02 Rates weighted by area
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Stations used to constrain atmospheric inversions
of high latitudes
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Comparison of process-based and atmospheric
approaches for Alaska-Canada after
consideration of fire by the process-based approa
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Alaska and Canada Carbon Flux Variability from an
Atmospheric Inversion - R. Dargaville
(from McGuire et al., In preparation)
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Decadal Variability

• Climate
• To what extent do changes in the start and length
  of the growing season influence decadal changes
  in carbon storage? (Zhuang et al., 2003)
• Disturbance
• Can responses of carbon exchange to changes in
  disturbance regime negate carbon storage that
  might be gained from longer growing seasons?
  (McGuire et al., in press).

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Courtesy of K. McDonald
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Biomass of Northern Hemisphere Ecosystems has
been Changing in Recent Decades
From Myneni et al. (2001)
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(Fire in Canada became more frequent after 1970)
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Cumulative Changes in Carbon Stocks for Canada
from 1950-1995
(from McGuire et al., in press)
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Century-Scale Variability

• Will changes in vegetation distribution
  dominate?
• (McGuire and Hobbie, 1997)
• Will directional changes in disturbance regimes
  dominate?
• (Kurz and Apps, 1999 McGuire et al., in press)
• How sensitive is the response of carbon storage
  to how fast carbon cycles through soils? (Clein
  et al., 2000)

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Soil carbon storage depends on the dynamics of
carbon and nitrogen transformations in soils.
From Clein et al. (2000)
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Different representations of soil carbon
transformations have different impacts on
century-scale responses of carbon storage.
TEM, reference
TEM, fast soil C
1985-1994
2085-2094
(from Clein et al., 2000)
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Summary

• Seasonal variation improved by representing
  insulating effects of snowpack on winter
  decomposition and representing freeze-thaw
  processes.
• Inter-annual responses to climate not well
  understood at large scales. At local scales
  responses depend on soil moisture. Need to more
  accurately represent hydrologic variability
  across the landscape.
• It is not clear whether improvement of decadal
  responses requires improved information on
  winter precipitation or requires improved
  snowmelt algorithms in models.
• Century-scale responses may be improved through
  better understanding of carbon and nitrogen
  transformations in soils.
• Disturbance can have strong effects on carbon
  storage at all time scales.

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