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
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])
Rationale for Expected Environmental Effects on Forest Physiology:
1. Large seasonal changes in precipitation and associated seasonal drought
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Time, Month of Year
Pre
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Rationale for Expected Environmental Effects on Forest Physiology:
2. El Nino/La Nina can cause substantial interannual variation in precipitation
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
The carbon isotope composition of plant tissues depends on
• 13Ca, atmospheric source • a, 13CO2 diffusion rates relative to 12CO2 • b, enzymatic discrimination during carboxylation • ci/ca, ratio of internal to ambient CO2
13Cleaf = 13Ca - a - (b - a)•ci/ca
4.4 ‰-8 ‰ 27 ‰ 0.4 - 0.9
13Cleaf = 13Ca - a - (b - a)•ci/ca
ci
ca
This carbon isotopediscrimination occurscontinuously duringphotosynthesis andthe resulting organiccarbon integrates overthe entire photosyntheticperiod.
Precipitation
StomatalConductance
Photosynthetic Capacity
Leaf Ci/Ca
Carbon Isotope Discrimination
Soil Moisture
Water Availability
Low High
-25
-35
Leaf 13C, per mil
Sampling Atmospheric CO2 Stable Isotope Ratios
Increases the spatial integration of
Eco-Physiological information obtained
A Keeling Plot
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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New LeavesOld Leaves
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Leaf C, ‰13
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CO Concentration, mol mol2-1 C, ‰13
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Time, Month of Year
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Santaremy = -0.0112x - 25.699
R2 = 0.8626
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Monthly Precipitation (mm)
C1
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C4
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Land Use Change Effects
18O in CO2 could be an important signal for C3-C4 vegetation conversions
The 18O Content of Atmospheric CO2 in terrestrial ecosystems is controlled by:
Discrimination during CO2 Assimilation(equilibration with chloroplast water)
Release of Respiratory CO2 from Soils (equilibration with soil water)
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PastureForest
ForestPasture
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Time, Month of Year
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8Ecosystem Respiration
Stem Water
We expect differences between C3 and C4 plants for discrimination against C18O16O because:
Leaf Water O-18 values
Ci/Ca differences
Carbonic Anhydrase Activity
C3 and C4 plants contribute different C18O16O signals
Conclusions:1. Significant temporal variation occurs in
13C of forest respired carbon dioxide
Associated with seasonal and interannual variation in precipitation??
Conclusions:2. A shift occurs in the 13C 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
Conclusions:3. 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 C18O16O is different in C3 and C4
ecosystems
Discrimination against CO2 containing 18O
Predicted 18OLW and ∆C18O16O valuesfor forests and pastures in Amazonia
18OLW ∆C18O16O CA eq.
C3 forest -5.6 ‰ 2.8 ‰ 100 %
C4 grassland +2.3 ‰ 6.7 ‰ 38 %