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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. Research Objectives: To study effects of:.
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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
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
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 • d13Ca, atmospheric source • a, 13CO2 diffusion rates relative to 12CO2 • b, enzymatic discrimination during carboxylation • ci/ca, ratio of internal to ambientCO2 d13Cleaf = d13Ca - a - (b - a)•ci/ca -8 ‰ 4.4 ‰ 27 ‰ 0.4 - 0.9
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
Precipitation Soil Moisture Stomatal Conductance Photosynthetic Capacity Leaf Ci/Ca Carbon Isotope Discrimination
-25 Leaf d13C, per mil -35 Low High Water Availability
Sampling Atmospheric CO2 Stable Isotope Ratios • Increases the spatial integration of Eco-Physiological information obtained
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)
Land Use Change Effects C3 C4
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)
We expect differences between C3 and C4 plants for discrimination against C18O16O because: • Leaf Water O-18 values • Ci/Ca differences • Carbonic Anhydrase Activity
Conclusions: • Significant temporal variation occurs in d13C of forest respired carbon dioxide • Associated with seasonal and interannual variation in precipitation??
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
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
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 %