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

Outline Water Uptake – Hydrogen (Dawson, 1993) Mixing model Case studies 1.Forest Communities 2.Riparian Communities 3.Desert Communities 4.Coastal Communities 5.Plant-Plant interactions Canopy Effect – Oxygen Relative humidity: Sternberg, L. et al Leaf Temperature – Oxygen Helliker and Richter, 2008; Woodward, 2008.

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

Common water sources Fig. 3 in Dawson, 1989

δD sap : the δD value of the xylem sap; δD GW : the δD value of the groundwater; δD R : 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 Mixing Models

Forest communities Fig. 5 in Dawson, 1989 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

Riparian Communities: Are streamside trees too good for streamside 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 Where do trees get their water?

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

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

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)

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

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) CO 2, T’s mean more summer precipitation This change will favor perennial species with widely distributed roots over deeper-rooted species

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 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 Fig. 11, Dawson, 1993

Hardwood Hammock osystems/index.html

Plant-Plant interactions Hydraulic 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

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 Mechanics of Hydraulic Lift Dawson, 1993

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

Long-term studies: tree rings 1.Main Goal: to reconstruct the long-term record of patterns of source water variation and plant water use; 2.Tool: the analysis of δD and δ 18 O in tree rings; 3.Basis: A linear relationship between the δD in cellulose nitrate and that of source waters Fig. 14, Dawson, 1993

Tree Growth 1st 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

Outline Water Uptake – Hydrogen (Dawson, 1993) Mixing model Case studies 1.Forest Communities 2.Riparian Communities 3.Desert Communities 4.Coastal Communities 5.Plant-Plant interactions Canopy Effect – Oxygen Relative humidity: Sternberg, L. et al Leaf Temperature – Oxygen Helliker and Richter, 2008; Woodward, 2008.

The Canopy Effect  13 C gradient from the forest floor to the canopy is well documented, and provides insight to CO 2 gradients under the canopy. What about relative humidity? Humidity gradients from the floor to the top of the canopy well documented but  13 C does not provide much insight to the effects this has on plants  18 O however is more directly influenced by changes in humidity Motivation: Can  18 O be used to find relative humidity gradient from floor to canopy?

Three Sources of Oxygen: CO 2, H 2 O - affect  18 O of carbohydrates during photosynthesis O 2(atm) - affect  18 O of carbohydrates during photorespiration For this study: H 2 O considered to be the primary labeling agent  18 O of the cellulose is 27‰ enriched with respect to the leaf water:  18 O cell =  18 O lw + 27‰ Nuts & Bolts

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

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

Conclusions Covariance of  18 O and  13 C for irrigation plots: Low sites: light intensity, humidity = low  13 C and  18 O (ie. 13C discrimination and evaporative regime) High sites: light intensity, humidity = high  18 O and  13 C 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

Outline Water Uptake – Hydrogen (Dawson, 1993) Mixing model Case studies 1.Forest Communities 2.Riparian Communities 3.Desert Communities 4.Coastal Communities 5.Plant-Plant interactions Canopy Effect – Oxygen Relative humidity: Sternberg, L. et al Leaf Temperature – Oxygen Helliker and Richter, 2008; Woodward, 2008.

Leaf Temperature (from last part)Canopy effect: δ 18 O cell relative humidity Factors determining the 18 O: 16 O ratio in wood cellulose 1.Differential discrimination; 2.Isotopic composition of water; Woodward, 2008

 18 O cell =  18 O lw + 27‰  18 O lw =  18 O s (1-h) + h  18 O amb +  * +  k (1-h)  18 O s =  18 O r Why it is not what we see? --T-dependent Humidity

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

Relative humidity Helliker and Richter, 2008 Humidity is related to e a /e i

Implications Effect on real and modeled water loss from boreal ecosystems; 1.False assumption: leaf temperatures are the same as ambient temperatures; 2.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