Use of Stable Oxygen Isotopes in Studies of Forest-Atmospheric CO 2 & H 2 O Exchange. Chun-Ta Lai San Diego State University

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1 Use of Stable Oxygen Isotopes in Studies of Forest-Atmospheric CO 2 & H 2 O Exchange Chun-Ta Lai San Diego State University June 2008

2 Atmospheric Composition Change Increasing CO 2 Increasing CH 4 Decreasing O 3 Climate Change Warming Precipitation change Extreme weather events Biological Extinctions Global Changes Biological Invasions Land-use changes Deforestation Agriculture Urbanization Water-use changes Irrigation Diversion Impoundment

3 Global Carbon Budget Emissions (ff + cement) +6.3± ±0.4 Atmosphere Increase +3.2± ±0.1 Net ocean-to-atmosphere flux -2.2± ±0.5 Net land-to-atmosphere flux -1.0± ±0.6 Land use change flux +1.6± ±0.5 Residual terrestrial sink IPCC 2007 revised; Canadell et al. 2007, submitted

4 Evidence for N Hemisphere terrestrial carbon sink N-S hemispheric gradient in [CO 2 ] 13 C/ 12 C and 18 O/ 16 O ratios of atmospheric CO 2 O 2 /N 2 ratios increase in amplitude of seasonal cycle of CO 2 in N hemisphere oceanic inventories, especially 14 C, tritium, CFCs, DIC climate variability (especially El Niño) and CO 2 increase rate forestry and ecological inventories direct eddy covariance measurements of net ecosystem CO 2 flux Tans and White (1998)

5 13 C/ 12 C and 18 O/ 16 O ratios of atmospheric CO 2

6 Amplitudes of CO 2 and 13 C/ 12 C ratio above a forest canopy are greater than that observed in marine boundary layer

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8 O 2 in the atmosphere is decreasing as CO 2 increases

9 Understanding biological feedbacks is critical to projecting future climate + Temperature (global warming) Soil decomposition Atmospheric CO 2

10 Understanding land-atmosphere interactions using oxygen isotope ratios of leaf water

11 Oxygen isotopes link carbon and water transfer through fundamental processes in terrestrial plants Picture of a stoma C 18 OO C 18 OO Hydration reaction H 2 18 O + C 18 OO H 2 18 O + C 18 OO

12 C 16 OO C 16 OO C 16 OO C 18 OO H 2 18 O leaf Transpiration Respiration Photosynthesis air C 18 OO C 18 OO H 2 18 O 18 O/ 16 O ratio links carbon and water transfer through gasexchange in terrestrial plants

13 We recognize a breathing attributed to shifts in the balance of photosynthesis and respiration across very large geographic scales Plant physiology matters!!

14 Leaf water 18 O enrichment 18 O/ 16 δ 18 Osample O = O/ 16 Ostandard Units = parts per thousand or per mille ( ) Δ 18 leaf = δ 18 (1+ O δ leaf 18 O δ stem 18 O stem /1000) δ 18 O leaf > δ 18 O stem Δ 18 leaf > 0

15 How do we measure stable isotope ratios? Mass spectrometry A gas-phase technique that separates ions based on their mass-to-charge ratios Basic idea for how an isotope ratio mass spectrometer (IRMS) works

16 Isotope ratio mass spectrometer for C and O isotope analyses of atmospheric CO 2

17 Optical instruments are becoming available for stable isotope ratio determination Liquid-water isotope analyzer Las Gatos Research, CA, USA

18 Modeling leaf water 18 O enrichment (Δ) - Dissecting biotic and abiotic influences on Δ Craig-Gordon model (abiotic) Péclet effect (biotic) Effect of water vapor (abiotic) Steady state v.s. non-steady state (abiotic & biotic)

19 Leaf water is 18 O-enriched via transpiration δ 18 O vapor : -12 H 2 16 O H 2 18 O δ 18 O leaf : +8 δ 18 O source : -2 No change in δ 18 O during plant uptake

20 Research questions: - What are the processes that influence temporal and individual variability of Δ 18 leaf among species growing in the same environment? - What are the factors contributing to these differences? Environmental versus biotic effects

21 Two distinct seasons in the Amazonia basin resulting from differences in precipitation inputs.

22 Leaf, non-green stem and water vapor samples were collected for 18O and 2H isotope analyses 6 species in the forest: 2 overstory trees, 2 lianas and 2 understory species 2 species in the pasture: 1 C4 grass and 1 C3 sapling Manilkara huberi Brachiaria brizantha Abuta rufescens Prionostemma aspera

23 Observed leaf water 18 O enrichment Mid-canopy & understory species Lai et al. (2008) Oecologia

24 18 O effects of water vapor on leaf water δ 18 O vapor : -12 H 2 16 O H 2 18 O δ 18 O leaf : +8 Leaf water is in isotopic equilibrium with water vapor!

25 Diel changes of leaf water 18 O enrichment in the understory of tropical forests Mid-canopy & understory species

26 18 O effects of water vapor on Δ leaf Δ leaf ( ) Lai et al. (2008) Oecologia

27 Diel changes of leaf water 18 O enrichment Péclet effect + non-ss non-ss Steady state

28 Summary: Factors that influence leaf water 18 O enrichment Environmental factors: Relative humidity (+++) Temperature (+) δ 18 O of water vapor (+++ at high RH) Biological factors: Stomatal conductance (Transpiration) Leaf water turnover time (Non-steady state) Effective path length (Péclet effect)

29 Estimating δ 18 O of water vapor from leaf water δ 18 O measurements When RH = 100% Δ vapor = Δ es ε * Can we use measured δ 18 O values of bulk leaf water as a proxy to estimate δ 18 O of water vapor on daily time scales? Requirements: Δ es : steady state Δ es : sites of evaporation (vs bulk leaf)

30 A plant-based approach to estimate δ 18 O of water vapor (tomorrow s talk) δ v = δ L s ( δ + 1) ε * δ v : 18 O of water vapor δl: 18 O of bulk leaf water δs: 18 O of stem water ε*: equilibrium fractionation factor Lai et al. (2008) Oecologia

31 Global water cycle starts from the ocean that supplies moisture to the atmosphere, which distributes it around the globe by precipitation Physical processes involved in evaporation, precipitation and cloud formation determine water isotope ratios Starting point

32 Continental Effect Due to Rayleigh distillation, rainfall signatures become more negative as the storm moves across the landscape This rain signal is then picked up and recorded by biological organisms

33 Stable Isotope Composition of Water Vapor in Coniferous Forests of the Pacific Northwest, USA Wind River Canopy Crane

34 Wind River Canopy Crane, Southern Washington Shaw et al. (2004) Ecosystems 7:427

35 Water vapor collection Leaf and stem sample collection

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37 (2006) 29: 77-94

38 Oxygen-18 ratios of water in a forest canopy Lai et al. (2006) PC&E

39 Backward trajectory calculations indicate the airmass originates from the ocean Produced with HYSPLIT from the NOAA ARL Website (

40 Mass conservation of water vapor and H 2 18 O in a plant canopy For total H 2 O dv dt c M c = F+ F + FT + F E For H 2 18 O dr v dt R F c c M c = T T + R F R F R E F E M c v c R c F : number of moles of air : mole fraction of water vapor : δ 18 O of water vapor : fluxes Subscript : + downdraft - updraft T transpiration E evaporation

41 Mass conservation of H 2 18 O in a plant canopy M c v c dδ c dt = δ + δ c ) F+ ( + ( δ δ ) F + ( δ δ ) F T c T E c E Isoflux atmosphere above Isoflux transpiration Isoflux soil evaporation A positive isoflux increases the δ 18 O ratio of water vapor in the atmosphere

42 Lai et al. (2006) PC&E

43 Lai et al. (2006) PC&E

44 Summary Oxygen-18 is a powerful biological tracer to study land-atmosphere interactions Evapotranspiration affects water isotopes of atmospheric vapor within forest canopies; the local signals are clearly detectable