ISOTOPES AND LAND PLANT ECOLOGY
C3 vs. C4 vs. CAM
Cerling et al. 97
Nature
δ13C
Cool season grass
most trees and shrubs
Warm season grass
Arid adapted dicots
εp = δa - δf = εt + (Ci/Ca)(εf-εt)
When Ci ≈ Ca (low rate of photosynthesis, open stomata), then
εp ≈ εf. Large fractionation, low plant δ13C values.
When Ci << Ca (high rate of photosynthesis, closed stomata),
then εp ≈ εt. Small fractionation, high plant δ13C values.
Plant δ13C
(if δa = -8‰)
δi
εf
εp = εt = +4.4‰
δ1
-12.4‰
δf
-27‰
εp = εf = +27‰
-35‰
0
0.5
Fraction C leaked (φ3/φ1 ∝ Ci/Ca)
1.0
εp = δa - δf = εt + (Ci/Ca)(εf-εt)
Ca,δa
φ1,δ1,εt
φ3,δ3,εt
Ci, δi
Inside leaf
φ2,δ2,εf
Ca,δa
Cf,δf
(Relative to preceding slide, note that the Y axis is reversed, so that ε p increases up the scale)
Why is C3 photosynthesis
so inefficient?
Photo-respiration
Major source of leakage
Increasingly bad with
rising T or O2/CO2 ratio
G3P
The C4 solution
“Equilibrium box”
φ1,δ1
CO2 a
δa
εta
δi
CO2 i
(aq)
PEP
HCO3
Δi-εd/b
φ3,δ3
pyruvate
C4
φ4,δ4,εPEP
φ2,δ2 ,εf
CO2 x
δx
Cf
δf
Leakage
φ5,δ5,εtw
εta = 4.4‰
εtw = 0.7‰
εPEP = 2.2‰
εf = 27‰
εd/b = -7.9‰ @ 25°C
δ1 = δa - εta
δ2 = δx - εf
δ3 = δi - εta
δ4 = δi + 7.9 - εPEP
δ5 = δx - εtw
Two branch points: i and x
i) φ1δ1 + φ5δ5 = φ4δ4 + φ3δ3
x) φ4δ4 = φ5δ5 + φ2δ2
Leakiness: L = φ5/φ4
After a whole pile of substitution
εp = δa - δf = εta + [εPEP - 7.9 + L(εf - εtw) - εta](Ci/Ca)
εp = εta+[εPEP-7.9+L(εf-εtw)-εta](Ci/Ca)
εp = 4.4+[-10.1+L(26.3)](Ci/Ca)
Ci/Ca
In C4, L is ~ 0.3, so εp is insensitive to
Ci/Ca, typically with values less than
those for εta.
Under arid conditions, succulent CAM
plants use PEP to fix CO2 to malate at
night and then use RUBISCO for final C
fixation during the daytime. The L value
for this is typically higher than 0.38.
Under more humid conditions, they will
directly fix CO2 during the day using
RUBISCO. As a consequence, they have
higher, and more variable, εp values.
Δ13C fraction-whole plant
Environmental Controls on plant δ13C values
Temperature, water stress, light level, height in the
canopy, E.T.C . . .
δ13C varies with environment within C3 plants
C3 plants
When its dry, plants keep their
stomata shut. Drive down Ci/Ca.
drought
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
normal
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
soil water
εp = εt + (Ci/Ca)(εf-εt)
Water Use Efficiency (WUE) =
Assimilation rate/transpiration rate
C3
A/E = (Ca-Ci)/1.6v = Ca (( 1-Ci )/Ca) /1.6v
WUE is negatively correlated with
Ci/Ca and therefore negatively
correlated with εp or Δ, for a constant v
(vapor pressure difference)
Evergreen higher WUE than decid.
Much less variability in C4,
except for different C4 pathways.
wet
dry
NADP C4 > NAD or PCK C4
Salinity stress = Water stress
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
salty
fresh
CANOPY EFFECT
Winner et al. (2004) Ecosystems
Diurnal variation
Buchman et al. (1997) Oecologia
Light matters too
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
BOTTOM LINE
Anything that affects stomatal conductance or carboxylation rate affects 13C
Increased light, decreased Δ, higher plant δ
Increased height in canopy, decreased Δ (more light, less CO2), higher plant δ
Increased salinity, decreased Δ, higher plant δ
Increased water availability, increased Δ, lower plant δ
Increased leaf thickness/cuticle, decreased Δ, higher plant δ
Generates variation within C3 ecosystems
Brooks et al. (1997) Oecologia
QuickTime™ and a
decompressor
are needed to see this picture.
Heaton (1999) Journal of Archaeological Science
Respired carbon dioxide from canopy vegetation and soils is mixed by
turbulence within the canopy air space. As the concentration of carbon
dioxide increase within the canopy, there is also a change in the isotopic
composition of that air. By plotting these relationships (known as a Keeling
plot), the intercept gives us the integrated isotope ratio of the ecosystem
respiration (-25.0 ‰).
Ehleringer et al. (2002) Plant Biology
What about pCO2?
Does Ci/Ca (δ13C) change in C3 plants as CO2 rises?
εp = εt + (Ci/Ca)(εf-εt)
Experiments suggest no.
What about abundance of C3 vs. C4
Tieszen et al.
Ecol. Appl. (1997)
Tieszen et al.
Oecologia (1979)
C3 plants
Quantum
Yield
(moles C fixed per
photons absorbed)
Crossover Temperature
C4 plants
Today (360 ppm)
3
6
9
12
15
18
21
Temperature (°C)
24
27
30
What happens when pCO2 changes?
C3 decreases in efficiency because of Photorespiration
Ehleringer et al. 1997 Oecologia
C3 plants
Crossover Temperature
Quantum
Yield
C4 plants
Today (360 ppm)
(moles C fixed per
photon absorbed)
LGM (180 ppm)
3
6
9
12
15
18
21
24
27
Temperature (°C)
30
What about glacial abundance of C3 vs. C4?
Does pCO2 or WUE win out?
And does WUE matter at the ecosystem scale?
%C4 = -0.9837 + 0.000594 (MAP) +
1.3528(JJA/MAP) + 0.2710 (lnMAT)
Different records suggest different things
Regression from Paruelo & Lauenroth (1996)
Two questions about Great Plains ecosystems
At the LGM, was there less C4 biomass (because of lower
temperatures) or more C4 biomass (because of lower pCO2)?
Use isotopes in animals and soils to track C3-to-C4 balance
Why Texus?
Climate means from 1931-1990
From New et al. (2000)
Archived at www.ipcc-ddc.cru.uea.ac.uk
N
High Plains
36¡N
OKLAHOMA
34¡N
Texas
vegetation
today
NEW MEXICO
High
Plains
Rolling
Plains
TEXAS
32¡N
Piney
Woods
Llano
Edw ards Uplift
Plateau
Trans Pecos
30¡N
28¡N
BP
S. Texas
Brushland
MEXICO
Gulf Coas t
Mars h&Prairie s
26¡N
From Diamond et al. 1987
106¡W
104¡W
102¡W
100¡W
98¡W
96¡W
94¡W
Proboscideans
Horses - Bison
Holocene Late Glacial
Holocene bison
Last Glacial
Maximum
Ingelside horses
Pre-LGM
Initial conclusions from isotope studies of Texas mammals
1) No changes in mean δ13C value through time.
2) Bison and mammoths are grazers. They can be used to monitor C3
to C4 balance on Pleistocene grasslands.
3) Mastodons are browsers. Their presence suggests tree cover.
4) Pleistocene horses ate lots of C3 vegetation, even when bison and
mammoths had ~100% C4 diets. Horses were mixed feeders.
What's next?
Compare %C4 from mammals to values simulated via modeling.
1) Use Quaternary climate model output, and estimate %C4 biomass
using the Regression Equation.
2) Use the same climate model output, but estimate %C4 biomass as
the percentage of growing season months that are above the
appropriate Crossover Temperature.
%C4 Grass from Regression Model
Holocene
0-10 Ka
Post-LGM
10-15 Ka
%C4 plants in grazer diets
Mammuthus
Bison
Mammut present
LGM
25-15 Ka
Holocene model driven by modern climate data from New
et al. (2000). LGM and Post-LGM models driven by GCM
output from Kutzbach et al. (1996)
(archived at www.ngdc.noaa.gov/paleo/paleo.html)
%C4 Grass from Crossover Temperature Model
Summary on Quaternary Prairies
1) Despite climate change, %C4 biomass is remarkably
constant through time.
2) Always lots of C4 biomass on plains and plateaus and
no mastodons. No LGM boreal forest in the region.
3) Only climate-vegetation models that account for
changes in pCO2 as well as temperature provide
reasonable %C4 estimates in parts of the Quaternary
with different atmospheric compositions.
Koch et al. (2004) P3