ISOTOPES AND LAND PLANT ECOLOGY
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ISOTOPES AND LAND PLANT ECOLOGY C3 vs. C4 vs. CAM. Cerling et al. 97 Nature. δ 13 C. Warm season grass Arid adapted dicots. Cool season grass most trees and shrubs. ε p = δ a - δ f = ε t + (C i /C a )(ε f -ε t ).

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Cerling et al. 97

Nature

δ13C

Warm season grass

Arid adapted dicots

Cool season grass

most trees and shrubs


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

1.0

Fraction C leaked (φ3/φ1 ∝ Ci/Ca)

εp = δa - δf = εt + (Ci/Ca)(εf-εt)

φ3,δ3,εt

φ1,δ1,εt

Ca,δa

Ci, δi

Inside leaf

Ca,δa

Cf,δf

φ2,δ2,εf


(Relative to preceding slide, note that the Y axis is reversed, so that εp increases up the scale)


G3P reversed, so that ε

Why is C3 photosynthesis so inefficient?

Photo-respiration

Major source of leakage

Increasingly bad with rising T or O2/CO2 ratio


The C4 solution reversed, so that ε


“Equilibrium box” reversed, so that ε

PEP

pyruvate

φ1,δ1

φ2,δ2 ,εf

δi

CO2 i

(aq)

HCO3

Δi-εd/b

CO2x

δx

Cf

δf

CO2 a

δa

C4

εta

φ4,δ4,εPEP

φ3,δ3

Leakage

φ5,δ5,εtw

δ1 = δa - εta

δ2 = δx - εf

δ3 = δi - εta

δ4 = δi + 7.9 - εPEP

δ5 = δx - εtw

εta = 4.4‰

εtw = 0.7‰

εPEP = 2.2‰

εf = 27‰

εd/b = -7.9‰ @ 25°C

Two branch points: i and x

φ1δ1 + φ5δ5 = φ4δ4 + φ3δ3

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)


ε reversed, so that εp = εta+[εPEP-7.9+L(εf-εtw)-εta](Ci/Ca)

εp = 4.4+[-10.1+L(26.3)](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.

Ci/Ca

In C4, L is ~ 0.3, so εp is insensitive to Ci/Ca, typically with values less than those for εta.


Δ reversed, so that ε13C fraction-whole plant


Environmental Controls on plant δ reversed, so that ε13C values

Temperature, water stress, light level, height in the canopy, E.T.C . . .


δ reversed, so that ε13C varies with environment within C3 plants

C3 plants


drought reversed, so that ε

normal

soil water

When its dry, plants keep their

stomata shut. Drive down Ci/Ca.

εp = εt + (Ci/Ca)(εf-εt)


C3 reversed, so that ε

wet

dry

Water Use Efficiency (WUE) = Assimilation rate/transpiration rate

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.

NADP C4 > NAD or PCK C4


Salinity stress = Water stress reversed, so that ε

salty

fresh


CANOPY EFFECT reversed, so that ε

Winner et al. (2004) Ecosystems


Diurnal variation reversed, so that ε

Buchman et al. (1997) Oecologia

Light matters too


BOTTOM LINE reversed, so that ε

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 reversed, so that ε

Brooks et al. (1997) Oecologia



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 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 ‰).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. 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 ‰).

Ecol. Appl. (1997)

Tieszen et al. Oecologia (1979)


C3 plants 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 ‰).

Crossover Temperature

Quantum

Yield

(moles C fixed per

photons absorbed)

C4 plants

Today (360 ppm)

3

6

9

12

15

18

21

24

27

30

Temperature (°C)


What happens when pCO 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 ‰).2 changes?

C3 decreases in efficiency because of Photorespiration

Ehleringer et al. 1997 Oecologia


LGM (180 ppm) 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 ‰).

C3 plants

Crossover Temperature

Quantum

Yield

(moles C fixed per

photon absorbed)

C4 plants

Today (360 ppm)

3

6

9

12

15

18

21

24

27

30

Temperature (°C)


%C4 = -0.9837 + 0.000594 ( 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 ‰).MAP) + 1.3528(JJA/MAP) + 0.2710 (lnMAT)

Regression from Paruelo & Lauenroth (1996)

What about glacial abundance of C3 vs. C4?

Does pCO2 or WUE win out?

And does WUE matter at the ecosystem scale?

Different records suggest different things


Two questions about Great Plains ecosystems 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 ‰).

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? 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 ‰).

Climate means from 1931-1990

From New et al. (2000)

Archived at www.ipcc-ddc.cru.uea.ac.uk


From Diamond et al. 1987 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 ‰).

Texas

vegetation

today


Horses - Bison 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 ‰).

Holocene bison

Ingelside horses

Proboscideans

Holocene -

Late Glacial

Last Glacial

Maximum

Pre-LGM


Initial conclusions from isotope studies of Texas mammals 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 ‰).

No changes in mean δ13C value through time.

Bison and mammoths are grazers. They can be used to monitor C3 to C4 balance on Pleistocene grasslands.

Mastodons are browsers. Their presence suggests tree cover.

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.

    • 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.


Mammuthus 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 ‰).

Bison

Mammut present

%C4 Grass from Regression Model

Holocene

0-10 Ka

Post-LGM

10-15 Ka

%C4 plants in grazer diets

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)


%C 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 ‰).4 Grass from Crossover Temperature Model


Summary on Quaternary Prairies 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 ‰).

Despite climate change, %C4 biomass is remarkably constant through time.

Always lots of C4 biomass on plains and plateaus and no mastodons. No LGM boreal forest in the region.

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


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