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Hydrogen & Oxygen in Plants: Applications

Hydrogen & Oxygen in Plants: Applications. Primary focus of studies: Tracing water uptake sources The Canopy Effect Tree-leaf Temperature. Modified by Guangsheng Zhuang Feb. 8, 2010. Outline. Water Uptake – Hydrogen (Dawson, 1993) Mixing model Case studies Forest Communities

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Hydrogen & Oxygen in Plants: Applications

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  1. Hydrogen & Oxygen in Plants: Applications • Primary focus of studies: • Tracing water uptake sources • The Canopy Effect • Tree-leaf Temperature Modified by Guangsheng Zhuang Feb. 8, 2010

  2. Outline • Water Uptake – Hydrogen (Dawson, 1993) • Mixing model • Case studies • Forest Communities • Riparian Communities • Desert Communities • Coastal Communities • Plant-Plant interactions • Canopy Effect – Oxygen • Relative humidity: Sternberg, L. et al. 1989. • Leaf Temperature – Oxygen • Helliker and Richter, 2008; Woodward, 2008.

  3. A brief word about mixing models… • No fractionation from water uptake to plant • Plants take water from many sources • How do you recognize the isotopic signals from different water sources? • Mixing Models! • A simple, two-ended linear model allows for calculations of the fraction of each source in the plant • These case studies rely on the capability of mixing models

  4. Common water sources Fig. 3 in Dawson, 1989

  5. Mixing Models δDsap: the δD value of the xylem sap; δDGW: the δD value of the groundwater; δDR: the δD value of the rain; d=decay time; t = time, in days after the rain storm event; X: a function of the site hydrology Dawson, 1989 can be expanded to accommodate two or more rainfall events, but a simple two end-member model

  6. Forest communities Upper panel (bald cypress): using complete groundwater Lower panel (white pine): dry site – almost entirely rainfall for 5 days; wet and intermediate sites combined waters sources Fig. 5 in Dawson, 1989

  7. Riparian Communities: Are streamside trees too good for streamside water? Where do trees get their water? • Setup: • Western riparian community: water-stressed, large gradient in water availability farther from streams • D ratios from xylem water analyzed to compare with D of stream water and D of groundwater

  8. Results: expected & unexpected • Small Trees: • Non-adjacent looked like soil water • Adjacent looked like stream water • Big Trees: • ALL trees looked like groundwater! Fig. 2: Dawson & Ehleringer, 1991

  9. Conclusions • Older trees take water from deep source • Trees need stable source of water • In a water-stressed environment, the most stable source is groundwater, so trees primarily draw from there

  10. Implications • Assumption that proximity implies a source is not necessarily true • Availability of groundwater can allow for drought-intolerant species in water-stressed ecosystems • Stream management practices need to be rethought? (e.g. stream flow diversion)

  11. Desert Communities:Winter vs. Summer precipitation dependence • Lateral root distribution species depended more on summer precipitation • Deep root species depended on groundwater • Summer precipitation dependence correlated with greater overall water stress & more WUE Ehleringer et al., 1991

  12. Implications • Different strengths related to use of water sources impacts coexistence, competition and community composition • ie, drought periods vs. rainy summers - who wins? • Regarding global climate change (GCM predictions) • CO2 , T’s mean more summer precipitation • This change will favor perennial species with widely distributed roots over deeper-rooted species

  13. Coastal Communities • Plant type limited by salinity tolerance • Change in the ratio of seawater to freshwater will have a large impact on ecosystem • e.g., natural disasters, runoff diversions, human consumption Fig. 11, Dawson, 1993 • Interesting application: FOG as a water source • Prevalent in coastal areas • Isotopically, much different than other source of surface water for vegetation • e.g., Coastal Redwood in California

  14. Hardwood Hammock http://sofia.usgs.gov/virtual_tour/ecosystems/index.html

  15. Plant-Plant interactionsHydraulic Lift: the plant version of a squirrel’s life… • Soil water absorbed at night is deposited in upper soil layers • Enables plant to “squirrel” away water for use during the summer drought, but at a cost… • Lost through evaporation; • Mooching neighbors will steal the water! • D values can show what fraction of “lifted” water is taken by neighboring plants

  16. Mechanics of Hydraulic Lift • Past 2.5 m, plants can’t access “lifted” water • If plants use “lifted” water, D of the plant will look like D of groundwater • If they do not use “lifted” water, D will look like D of precipitation Dawson, 1993

  17. Implications • “Lifted” water is important for neighboring plants during droughts • In some situations, close proximity may be a competitive advantage instead of a disadvantage

  18. Long-term studies: tree rings Main Goal: to reconstruct the long-term record of patterns of source water variation and plant water use; Tool: the analysis of δD and δ18O in tree rings; Basis: A linear relationship between the δD in cellulose nitrate and that of source waters Fig. 14, Dawson, 1993

  19. Tree Growth • 1st 20-25 years: • Ring width indicates growth is erratic • D values similar to D of summer precipitation • 25+ years: • Growth stabilizes • D looks like D of groundwater Implication: Young trees are restricted to surface waters, so growth is limited by availability & therefore erratic. Older trees access groundwater, so growth is more stable Fig. 15, Dawson, 1993

  20. Outline • Water Uptake – Hydrogen (Dawson, 1993) • Mixing model • Case studies • Forest Communities • Riparian Communities • Desert Communities • Coastal Communities • Plant-Plant interactions • Canopy Effect – Oxygen • Relative humidity: Sternberg, L. et al. 1989. • Leaf Temperature – Oxygen • Helliker and Richter, 2008; Woodward, 2008.

  21. The Canopy Effect • 13C gradient from the forest floor to the canopy is well documented, and provides insight to CO2 gradients under the canopy. • What about relative humidity? • Humidity gradients from the floor to the top of the canopy well documented but 13C does not provide much insight to the effects this has on plants • 18O however is more directly influenced by changes in humidity • Motivation: Can 18O be used to find relative humidity gradient from floor to canopy?

  22. Nuts & Bolts • Three Sources of Oxygen: • CO2, H2O - affect 18O of carbohydrates during photosynthesis • O2(atm) - affect 18O of carbohydrates during photorespiration • For this study: • H2O considered to be the primary labeling agent • 18O of the cellulose is 27‰ enriched with respect to the leaf water: • 18Ocell = 18Olw + 27‰

  23. Equation Breakdown Relative humidity Ambient vapor 18Ocell = 18Olw + 27‰ 18Olw = 18Os(1-h) + h 18Oamb + * + k(1-h) 18Os = 18Or { Leaf water Soil or stem Equilibrium & Kinetic fractionation factors Yearly average rainfall 18Oamb: mixture of 2 pools - source of rain & evapotranspiration ie., 18Oatm & 18Os 18Oatm = 18Or - * = 18Os - * So, 18Oamb = h 18Os -h’ * h Bottom line: it may be possible to approximate relative humidity with oxygen isotopes from soil water & tree cellulose After a little rearranging…. 18Ocell - 27‰ - 18Os - *(1-h’) h = 1- k

  24. Results • Leaf cellulose isotopic values from 1m were lower than samples from 9m • Values from the irrigated plots showed a greater isotopic gradient than the control plots

  25. Conclusions • Covariance of 18O and 13C for irrigation plots: • Low sites: • light intensity , humidity = low 13C and 18O • (ie. 13C discrimination and evaporative regime) • High sites: • light intensity , humidity = high 18O and 13C • Weak correlation observed at control plots? • stomatal opening variability • Stomates in irrigated plots controlled by humidity while in control plots, other factors like root or leaf water potential apply

  26. Outline • Water Uptake – Hydrogen (Dawson, 1993) • Mixing model • Case studies • Forest Communities • Riparian Communities • Desert Communities • Coastal Communities • Plant-Plant interactions • Canopy Effect – Oxygen • Relative humidity: Sternberg, L. et al. 1989. • Leaf Temperature – Oxygen • Helliker and Richter, 2008; Woodward, 2008.

  27. Leaf Temperature • (from last part)Canopy effect: • δ18Ocellrelative humidity • Factors determining the 18O:16O • ratio in wood cellulose • Differential discrimination; • Isotopic composition of water; Woodward, 2008

  28. Why it is not what we see? --T-dependent Humidity 18Ocell = 18Olw + 27‰ 18Olw = 18Os(1-h) + h 18Oamb + * + k(1-h) 18Os = 18Or

  29. Leaf Temperature ei-saturation vapor pressure; It can be calculated by isotope data and determine the relative humidity Helliker and Richter, 2008

  30. Relative humidity Humidity is related to ea/ei Helliker and Richter, 2008

  31. Implications • Effect on real and modeled water loss from boreal ecosystems; • False assumption: leaf temperatures are the same as ambient temperatures; • Humidity reconstructions – will yield much lower values for cooler climates and higher values for warmer climates than expected • Architectural controls of branches on leaf T

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