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

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

Cool season grass most trees and shrubs Warm season grass Arid adapted dicots Cerling et al. 97 Nature δ 13 C

ε p = δ a - δ f = ε t + (C i /C a )(ε f -ε t ) When C i ≈ C a (low rate of photosynthesis, open stomata), then ε p ≈ ε f. Large fractionation, low plant δ 13 C values. When C i << C a (high rate of photosynthesis, closed stomata), then ε p ≈ ε t. Small fractionation, high plant δ 13 C values.

C i, δ i Inside leaf C a,δ a C f,δ f φ 1,δ 1,ε t φ 3,δ 3,ε t φ 2,δ 2,ε f -12.4‰ -35‰ -27‰ Plant δ 13 C (if δ a = -8‰) ε p = ε t = +4.4‰ ε p = ε f = +27‰ εfεf 00.51.0 Fraction C leaked (φ 3 /φ 1 ∝ C i /C a ) δiδi δfδf δ1δ1 ε p = δ a - δ f = ε t + (C i /C a )(ε f -ε t )

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

G3P Photo-respiration Major source of leakage Increasingly bad with rising T or O 2 /CO 2 ratio Why is C3 photosynthesis so inefficient?

The C4 solution

CO 2 a δ a φ 1,δ 1 φ 3,δ 3 δ i CO 2 i (aq) HCO 3 Δ i -ε d/b “Equilibrium box” C4 PEPpyruvate CO 2 x δ x CfδfCfδf φ 2,δ 2,ε f φ 4,δ 4,ε PEP Leakage φ 5,δ 5,ε tw ε ta ε ta = 4.4‰ ε tw = 0.7‰ ε PEP = 2.2‰ ε f = 27‰ ε d/b = -7.9‰ @ 25°C δ 1 = δ a - ε ta δ 2 = δ x - ε f δ 3 = δ i - ε ta δ 4 = δ i + 7.9 - ε PEP δ 5 = δ x - ε tw Two branch points: i and x i)φ 1 δ 1 + φ 5 δ 5 = φ 4 δ 4 + φ 3 δ 3 x)φ 4 δ 4 = φ 5 δ 5 + φ 2 δ 2 Leakiness: L = φ 5 /φ 4 After a whole pile of substitution ε p = δ a - δ f = ε ta + [ε PEP - 7.9 + L(ε f - ε tw ) - ε ta ](C i /C a )

C i /C a In C4, L is ~ 0.3, so ε p is insensitive to C i /C a, typically with values less than those for ε ta. ε p = ε ta +[ε PEP -7.9+L(ε f -ε tw )-ε ta ](C i /C a ) Under arid conditions, succulent CAM plants use PEP to fix CO 2 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 CO 2 during the day using RUBISCO. As a consequence, they have higher, and more variable, ε p values. ε p = 4.4+[-10.1+L(26.3)](C i /C a )

Δ 13 C fraction-whole plant

Environmental Controls on plant δ 13 C values Temperature, water stress, light level, height in the canopy, E.T.C...

δ 13 C varies with environment within C3 plants C3 plants

soil water drought normal ε p = ε t + (C i /C a )(ε f -ε t ) When its dry, plants keep their stomata shut. Drive down C i /C a.

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 C i /C a and therefore negatively correlated with ε p or Δ, for a constant v (vapor pressure difference) Evergreen higher WUE than decid. A/E = (C a -C i )/1.6v = C a (( 1-C i )/C a ) /1.6v

saltyfresh Salinity stress = Water stress

CANOPY EFFECT Winner et al. (2004) Ecosystems

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

BOTTOM LINE Anything that affects stomatal conductance or carboxylation rate affects  13 C Increased light, decreased Δ, higher plant δ Increased height in canopy, decreased Δ (more light, less CO 2 ), 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

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 pCO 2 ? Does C i /C a (δ 13 C) change in C3 plants as CO 2 rises? ε p = ε t + (C i /C a )(ε f -ε t ) Experiments suggest no. What about abundance of C3 vs. C4

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

Quantum Yield (moles C fixed per photons absorbed) Temperature (°C) 36912151821242730 C4 plants C3 plants Crossover Temperature Today (360 ppm)

What happens when pCO 2 changes? Ehleringer et al. 1997 Oecologia C3 decreases in efficiency because of Photorespiration

Quantum Yield (moles C fixed per photon absorbed) Temperature (°C) 36912151821242730 C4 plants C3 plants Crossover Temperature Today (360 ppm) LGM (180 ppm)

What about glacial abundance of C3 vs. C4? Does pCO 2 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 C 4 biomass (because of lower temperatures) or more C 4 biomass (because of lower pCO 2 )? Use isotopes in animals and soils to track C 3 -to-C 4 balance

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

From Diamond et al. 1987 Texas vegetation today

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

Initial conclusions from isotope studies of Texas mammals 1)No changes in mean δ 13 C value through time. 2)Bison and mammoths are grazers. They can be used to monitor C 3 to C 4 balance on Pleistocene grasslands. 3)Mastodons are browsers. Their presence suggests tree cover. 4)Pleistocene horses ate lots of C 3 vegetation, even when bison and mammoths had ~100% C 4 diets. Horses were mixed feeders. What's next? Compare %C 4 from mammals to values simulated via modeling. 1)Use Quaternary climate model output, and estimate %C 4 biomass using the Regression Equation. 2)Use the same climate model output, but estimate %C 4 biomass as the percentage of growing season months that are above the appropriate Crossover Temperature.

Holocene 0-10 Ka Post-LGM 10-15 Ka LGM 25-15 Ka %C 4 Grass from Regression Model %C 4 plants in grazer diets Mammuthus 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)

%C 4 Grass from Crossover Temperature Model

Summary on Quaternary Prairies 1)Despite climate change, %C 4 biomass is remarkably constant through time. 2)Always lots of C 4 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 pCO 2 as well as temperature provide reasonable %C 4 estimates in parts of the Quaternary with different atmospheric compositions. Koch et al. (2004) P3

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