Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems

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Stable Isotope Analyses of Carbon Dioxide
Exchange in Forest and Pasture Ecosystems

• L. Flanagan, J. Ometto, T. Domingues,
• L. Martinelli, J. Ehleringer
• Atlanta LBA Ecology, February 12-14, 2001

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Research Objectives To study effects of

• Environmental variation on forest carbon dioxide
  and water vapor exchange
• (Using C stable isotope measurements)
• Land-use change on ecosystem stable isotope
  discrimination
• (Forest C3 conversion to Pasture C4)

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Rationale for Expected Environmental Effects on
Forest Physiology

• 1. Large seasonal changes in precipitation and
  associated seasonal drought

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Rationale for Expected Environmental Effects on
Forest Physiology

• 2. El Nino/La Nina can cause substantial
  interannual variation in precipitation

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Stable Isotopes Provide Integrated
Eco-physiological Measurements

• 13C measurements represent changes in the ratio
  of stomatal conductance to photosynthetic
  capacity
• Spatial and temporal integration depends on the
  nature of the measurements
• Single leaves
• Tree rings
• Atmospheric CO2

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The carbon isotope composition of plant tissues
depends on d13Ca, atmospheric source
a, 13CO2 diffusion rates relative to 12CO2
b, enzymatic discrimination during
carboxylation ci/ca, ratio of internal to
ambient CO2
d13Cleaf d13Ca - a - (b - a)ci/ca
4.4
-8
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0.4 - 0.9
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d13Cleaf d13Ca - a - (b - a)ci/ca
This carbon isotope discrimination
occurs continuously during photosynthesis and the
resulting organic carbon integrates over the
entire photosynthetic period.
ci
ca
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Precipitation
Soil Moisture
Stomatal Conductance
Photosynthetic Capacity
Leaf Ci/Ca
Carbon Isotope Discrimination
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-25
Leaf d13C, per mil
-35
Low High
Water Availability
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Sampling Atmospheric CO2 Stable Isotope
Ratios

• Increases the spatial integration of
• Eco-Physiological information obtained

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A Keeling Plot
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Keeling Plot Technique Provides an estimate
of

• Spatially integrated changes in the ratio of
  stomatal conductance to photosynthetic capacity
• Spatial integration similar to E.C. footprint
• Temporal integration Days Week
• (primarily represents recently fixed carbon)

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Land Use Change Effects
C3
C4
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18O in CO2 could be an important signal for
C3-C4 vegetation conversions
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• The 18O Content of Atmospheric CO2 in terrestrial
  ecosystems is controlled by
• Discrimination during CO2 Assimilation (equilibra
  tion with chloroplast water)
• Release of Respiratory CO2 from Soils
  (equilibration with soil water)

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• We expect differences between C3 and C4 plants
  for discrimination against C18O16O because
• Leaf Water O-18 values
• Ci/Ca differences
• Carbonic Anhydrase Activity

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C3 and C4 plants contribute different DC18O16O
signals
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Conclusions

• Significant temporal variation occurs in d13C of
  forest respired carbon dioxide
• Associated with seasonal and interannual
  variation in precipitation??

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Conclusions

• A shift occurs in the d13C of respired CO2 caused
  by forest-pasture conversion
• Pastures do not have a pure C4 signal
• Temporal variation is caused by C3 encroachment
  and pasture burning

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Conclusions

• 18O in CO2 could be an important signal for
  forest-pasture conversions
• Tropical pasture respired CO2 is higher
• in 18O than that from tropical forest
• DC18O16O is different in C3 and C4 ecosystems

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Discrimination against CO2 containing 18O
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Predicted d18OLW and ?C18O16O values for forests
and pastures in Amazonia
d18OLW ?C18O16O CA eq. C3 forest -5.6
2.8 100 C4 grassland 2.3 6.7 38