ISOTOPES AND LAND PLANT ECOLOGY C3 vs. C4 vs. CAM
Cool season grassmost trees and shrubs
Warm season grassArid adapted dicots
Cerling et al. 97Nature
δ13C
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.
Ci, δi
Inside leafCa,δa
Ca,δa
Cf,δf
φ1,δ1,εt
φ3,δ3,εt
φ2,δ2,εf
-12.4‰
-35‰
-27‰
Plant δ13C
(if δa = -8‰)
εp = εt = +4.4‰
εp = εf = +27‰
εf
0 0.5 1.0
Fraction C leaked (φ3/φ1 C∝ i/Ca)
δi
δf
δ1
εp = δa - δf = εt + (Ci/Ca)(εf-εt)
G3P
Photo-respirationMajor source of leakageIncreasingly bad with rising T or O2/CO2 ratio
Why is C3 photosynthesis so inefficient?
CO2 a
δa
φ1,δ1
φ3,δ3
δi CO2 i
(aq)
HCO3
Δi-εd/b
“Equilibrium box”
C4
PEP pyruvate
CO2 xδx
Cf
δf
φ2,δ2 ,εf
φ4,δ4,εPEP
Leakageφ5,δ5,εtw
εta
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• φ1δ1 + φ5δ5 = φ4δ4 + φ3δ3
i) 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)
Ci/Ca
In C4, L is ~ 0.3, so εp is insensitive to Ci/Ca, typically with values less than
those for εta.
εp = εta+[εPEP-7.9+L(εf-εtw)-εta](Ci/Ca)
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.
εp = 4.4+[-10.1+L(26.3)](Ci/Ca)
Environmental Controls on plant δ13C values
Temperature, water stress, light level, height in the canopy, E.T.C . . .
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soil water
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drought
normal
εp = εt + (Ci/Ca)(εf-εt)
When its dry, plants keep theirstomata shut. Drive down Ci/Ca.
C3
drywet
Much less variability in C4, except for different C4 pathways.
NADP C4 > NAD or PCK C4
Water Use Efficiency (WUE) = Assimilation rate/transpiration rate
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.
A/E = (Ca-Ci)/1.6v = Ca (( 1-Ci )/Ca) /1.6v
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salty fresh
Salinity stress = Water stress
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Diurnal variation
Light matters too
Buchman et al. (1997) Oecologia
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
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Heaton (1999) Journal of Archaeological Science
Ehleringer et al. (2002) Plant Biology
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 ‰).
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
Qu
antu
mY
ield
(mo
les
C f
ixed
per
ph
oto
ns
abso
rbed
)
Temperature (°C)
3 6 9 12 15 18 21 24 27 30
C4 plants
C3 plants
Crossover Temperature
Today (360 ppm)
What happens when pCO2 changes?
Ehleringer et al. 1997 Oecologia
C3 decreases in efficiency because of Photorespiration
QuantumYield
(moles C fixed perphoton absorbed)
Temperature (°C)
3 6 9 12 15 18 21 24 27 30
C4 plants
C3 plants
Crossover Temperature
Today (360 ppm)
LGM (180 ppm)
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)
Regression from Paruelo & Lauenroth (1996)
Different records suggest different things
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 CUse isotopes in animals and soils to track C33-to-C-to-C44 balance balance
Why Texus?Why Texus?
Climate means from 1931-1990Climate means from 1931-1990From New et al. (2000)From New et al. (2000)Archived at www.ipcc-ddc.cru.uea.ac.ukArchived at www.ipcc-ddc.cru.uea.ac.uk
106°W 104°W 102°W 100°W 98°W 96°W 94°W
26°N
28°N
30°N
32°N
OKLAHOMA
MEXICO
TEXAS
Trans PecosEdwardsPlateau
RollingPlains
S. TexasBrushland
PineyWoods
LlanoUplift
Gulf CoastMarsh&Prairies
BP
34°N
36°N
NEW MEXICO
N
HighPlains
High Plains
From Diamond et al. 1987
Texasvegetation
today
Holocene - Late Glacial
Last GlacialMaximum
Pre-LGM
Proboscideans
Holocene bison
Ingelside horses
Horses - Bison
Initial conclusions from isotope studies of Texas mammals
1) No changes in mean δ13C value through time.
1) Bison and mammoths are grazers. They can be used to monitor C3 to C4 balance on Pleistocene grasslands.
2) Mastodons are browsers. Their presence suggests tree cover.
3) 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.
Holocene0-10 Ka
Post-LGM10-15 Ka
LGM25-15 Ka
%C4 Grass from Regression Model
%C4 plants in grazer dietsMammuthus
Bison
Mammut present
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)
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