Data Analysis and Modeling with Stable Isotope Ratios. Chun-Ta Lai San Diego State University June 2008

Transcription

1 Data Analysis and Modeling with Stable Isotope Ratios Chun-Ta Lai San Diego State University June 2008

2 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

3 Oxygen-18 ratios of water in a forest canopy Lai et al. (2006) Plant Cell & Environment 29:77-94

4 Modeling leaf water 18 O enrichment - Dissect biotic and abiotic influences Craig-Gordon model Péclet effect Effect of water vapor Steady state v.s. non-steady state

5 Measured leaf water 18 O enrichment (Δ o ) is expressed relative to the source water: Δ o = ( δ δ ) (1 + δ L s s 1000) δ L : 18 O of bulk leaf water δ s : 18 O of stem water

6 Modeling leaf water 18 O enrichment A modified Craig-Gordon formulation (Craig and Gordon 1965) Δ es = ε * + ε k + (Δ v ε k ) e e a i Δ es : steady-state leaf water 18 O enrichment at the sites of evaporation ε * : equilibrium fractionation factor (see Barbour et al. 2004) ε k : kinetic fractionation factor Δ v : water vapor 18 O enrichment relative to the source water e a /e i : ratio of vapor pressure in the air to leaf intercellular space Farquhar et al. (1989)

7 CG model overestimates observed δ 18 O of leaf water - the Péclet effect Lai et al. PC&E (2006) 29: 77-94

8 Observed leaf water 18 O enrichment

9 The Péclet effect describes the convection of non-fractionated water to the site of evaporation opposed by the diffusion of the enriched water Δ Ls = ( P 1 e ) P Δ es P is the Péclet number Farquhar and Lloyd (1993)

10 Modeling leaf water 18 O enrichment A modified Craig-Gordon formulation (Craig and Gordon 1965) Δ es = ε * + ε k + (Δ v ε k ) e e a i Δ v : water vapor 18 O enrichment relative to the source water Δv needs to be known but is rarely measured in the field Farquhar et al. (1989)

11 Oecologia DOI /s PHYSIOLOGICAL ECOLOGY - ORIGINAL PAPER Life form-specific variations in leaf water oxygen- 18 enrichment in Amazonian vegetation Lai et al. (2008) Lai et al. Oecologia: in press

12 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!

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

14 Modeling leaf water 18 O enrichment A modified Craig-Gordon formulation (Craig and Gordon 1965) Δ es = ε * + ε k + (Δ v ε k ) e e a i Δ v : water vapor 18 O enrichment relative to the source water Δv needs to be known but is rarely measured in the field Farquhar et al. (1989)

15 18 O exchange between water vapor and leaf water Δ es = ε * + ε k + (Δ v ε k ) e e a 1 i When e a /e i = 1 (i.e. RH=100%, nighttime) es * Δ = ε + Δ v Leaf water is in isotopic equilibrium with water vapor!

16 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)

17 When do measured δ 18 O of bulk leaf water resemble Δ es? Cernusak et al PCE 25:893

18 Observed leaf water 18 O enrichment

19 Observed leaf water 18 O enrichment

20 Observed leaf water 18 O enrichment Mid-canopy & understory species

21 A plant-based approach to estimate δ 18 O of water vapor 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

22 Plant-based estimates forest 13.4 ± 0.8 pasture 14.9 ± 1.1 forest 7.1 ± 1.1 pasture 11.2 ± 0.2 Rain-based estimates Comparison between measured and estimated δ 18 O of water vapor for the two study periods

23 A plant-based approach to estimate δ 18 O of water vapor δ v = δ L s ( δ + 1) ε * This approach takes into account the contribution of local vegetation (via transpiration) to water vapor contents in the atmosphere!!! Lai et al. (2008) Oecologia

24 wet dry

25 Steady-state vs Non-steady state τ = * Wα kα gw i τ: turnover time of leaf water α k α * 1 g : stomatal conductance w : leaf water content Dongmann et al δ en ( t) = δ ( t) [ δ ( t) δ e e en ( t Δt 1)]exp( τς )

26 Plants subject to drought are more likely to transpire at isotopic non-steady state Lai et al. PC&E (2006) 29: 77-94

27 Model results are greatly improved after considering NSS and Péclet effects Lai et al. (2008) Oecologia