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ISOTOPES AND LAND PLANT ECOLOGY C3 vs. C4 vs. CAM

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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ISOTOPES AND LAND PLANT ECOLOGY C3 vs. C4 vs. CAM

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  1. ISOTOPES AND LAND PLANT ECOLOGY C3 vs. C4 vs. CAM

  2. Cerling et al. 97 Nature δ13C Warm season grass Arid adapted dicots Cool season grass most trees and shrubs

  3. 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.

  4. 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

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

  6. G3P Why is C3 photosynthesis so inefficient? Photo-respiration Major source of leakage Increasingly bad with rising T or O2/CO2 ratio

  7. The C4 solution

  8. “Equilibrium box” 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)

  9. 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.

  10. Δ13C fraction-whole plant

  11. Environmental Controls on plant δ13C values Temperature, water stress, light level, height in the canopy, E.T.C . . .

  12. δ13C varies with environment within C3 plants C3 plants

  13. drought normal soil water When its dry, plants keep their stomata shut. Drive down Ci/Ca. εp = εt + (Ci/Ca)(εf-εt)

  14. C3 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

  15. Salinity stress = Water stress salty fresh

  16. CANOPY EFFECT Winner et al. (2004) Ecosystems

  17. Diurnal variation Buchman et al. (1997) Oecologia Light matters too

  18. 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

  19. Generates variation within C3 ecosystems Brooks et al. (1997) Oecologia

  20. Heaton (1999) Journal of Archaeological Science

  21. 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

  22. 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

  23. Tieszen et al. Ecol. Appl. (1997) Tieszen et al. Oecologia (1979)

  24. C3 plants 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)

  25. What happens when pCO2 changes? C3 decreases in efficiency because of Photorespiration Ehleringer et al. 1997 Oecologia

  26. LGM (180 ppm) 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)

  27. %C4 = -0.9837 + 0.000594 (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

  28. 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

  29. Why Texus? Climate means from 1931-1990 From New et al. (2000) Archived at www.ipcc-ddc.cru.uea.ac.uk

  30. From Diamond et al. 1987 Texas vegetation today

  31. Horses - Bison Holocene bison Ingelside horses Proboscideans Holocene - Late Glacial Last Glacial Maximum Pre-LGM

  32. Initial conclusions from isotope studies of Texas mammals 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.

  33. Mammuthus 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)

  34. %C4 Grass from Crossover Temperature Model

  35. Summary on Quaternary Prairies 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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