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 13 C 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 CO 2

The carbon isotope composition of plant tissues depends on  13 C a, atmospheric source a, 13 CO 2 diffusion rates relative to 12 CO 2 b, enzymatic discrimination during carboxylation c i /c a, ratio of internal to ambient CO 2  13 C leaf =  13 C a - a - (b - a)c i /c a 4.4 ‰-8 ‰27 ‰

 13 C leaf =  13 C a - a - (b - a)c i /c a cici caca This carbon isotope discrimination occurs continuously during photosynthesis and the resulting organic carbon integrates over the entire photosynthetic period.

Precipitation Stomatal Conductance Photosynthetic Capacity Leaf Ci/Ca Carbon Isotope Discrimination Soil Moisture

Water Availability Low High Leaf  13 C, per mil

Sampling Atmospheric CO 2 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)

C4C4 C3C3 Land Use Change Effects

18 O in CO 2 could be an important signal for C3-C4 vegetation conversions

The 18 O Content of Atmospheric CO 2 in terrestrial ecosystems is controlled by:  Discrimination during CO 2 Assimilation (equilibration with chloroplast water)  Release of Respiratory CO 2 from Soils (equilibration with soil water)

We expect differences between C3 and C4 plants for discrimination against C 18 O 16 O because:  Leaf Water O-18 values  Ci/Ca differences  Carbonic Anhydrase Activity

C 3 and C 4 plants contribute different  C 18 O 16 O signals

Conclusions: 1.Significant temporal variation occurs in  13 C of forest respired carbon dioxide  Associated with seasonal and interannual variation in precipitation??

Conclusions: 2.A shift occurs in the  13 C of respired CO 2 caused by forest-pasture conversion  Pastures do not have a pure C4 signal  Temporal variation is caused by C3 encroachment and pasture burning

Conclusions: O in CO 2 could be an important signal for forest-pasture conversions  Tropical pasture respired CO 2 is higher in 18 O than that from tropical forest   C 18 O 16 O is different in C3 and C4 ecosystems

Discrimination against CO 2 containing 18 O

Predicted  18 O LW and ∆C 18 O 16 O values for forests and pastures in Amazonia  18 O LW ∆C 18 O 16 O CA eq. C 3 forest-5.6 ‰2.8 ‰100 % C 4 grassland+2.3 ‰6.7 ‰ 38 %