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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:
.
Manaus
1. Large seasonal
changes in
precipitation and
associated seasonal
drought
Precipitation, mm month
-1
300
200
100
0
2
4
6
8
10
Time, Month of Year
12
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 ambient CO2
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
Water Availability
High
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)
Santarem Km 83
September 2000
40
New Leaves
Old Leaves
Height, m
30
20
10
0
-38
-36
-34
13
-32
Leaf d C, ‰
-30
-28
.
Santarem Km 83
35
September 2000
9 am
12 noon
4 pm
9 am
12 noon
4 pm
30
Height, m
25
20
15
10
5
0
360
380
400
420
440
CO2 Concentration, mol mol -1
-12
-11
-10
13
d C, ‰
-9
-8
13
Ecosystem Respiration d C, ‰
-25
Manaus
Santarem
-26
-27
-28
-29
-30
2
4
6 8 10 12 2
1999
4
6 8 10 12
2000
Time, Month of Year
C13 Ecosystem Respiration
(per mil)
Santarem
y = -0.0112x - 25.699
2
R = 0.8626
-25
-26
-27
-28
-29
-30
0
100
200
300
400
Monthly Precipitation (mm)
500
Land Use Change Effects
C
3
C
4
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)
.
40
Ecosystem Respiration
20
Pasture
Forest
10
Forest
Pasture
18
d O, ‰
30
0
-10
-20
Stem Water
2
4
6 8 10 12 2
1999
4
6 8 10 12
2000
Time, Month of Year
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 DC18O16O signals
Conclusions:
1. Significant temporal variation occurs in
d13C of forest respired carbon dioxide

Associated with seasonal and interannual
variation in precipitation??
Conclusions:
2. 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:
3.
18O

Tropical pasture respired CO2 is higher
in 18O than that from tropical forest
DC18O16O is different in C3 and C4
ecosystems

in CO2 could be an important signal
for forest-pasture conversions
Discrimination against CO2 containing 18O
Predicted d18OLW and ∆C18O16O values
for forests and pastures in Amazonia
d18OLW
∆C18O16O CA eq.
-5.6 ‰
2.8 ‰
100 %
C4 grassland +2.3 ‰ 6.7 ‰
38 %
C3 forest