Stable Isotope Analyses of Carbon Dioxide Exchange in Forest and Pasture Ecosystems L. Flanagan, J. Ometto, T. Domingues, L. Martinelli, J. Ehleringer.

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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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

Santarem Km 83 September 2000

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CO Concentration, mol mol2-1 C, ‰13

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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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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 %