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
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. 1989.
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
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
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
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
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)
•CO2 , 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
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
Hardwood Hammock
http://sofia.usgs.gov/virtual_tour/ec
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
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
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 δ18O 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 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:
Fig. 15, Dawson, 1993
Young trees are restricted to surface waters, so growth is
limited by availability & therefore erratic. Older trees
access groundwater, so growth is more stable
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. 1989.
Leaf Temperature – Oxygen
•Helliker and Richter, 2008; Woodward, 2008.
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?
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‰
Equation Breakdown
Relative humidity
Ambient vapor
{
18Ocell = 18Olw + 27‰
18Olw = 18Os(1-h) + h 18Oamb + * + k(1-h)
Leaf water
18Os = 18Or
Soil or stem
Yearly average rainfall
Equilibrium & Kinetic
fractionation factors
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
After a little rearranging….
relative humidity with
18Ocell - 27‰ - 18Os - *(1-h’)
oxygen isotopes from soil
h = 1water & tree cellulose
k
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 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
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. 1989.
Leaf Temperature – Oxygen
•Helliker and Richter, 2008; Woodward, 2008.
Leaf Temperature
•(from last part)Canopy effect:
δ18Ocell
relative humidity
•Factors determining the 18O:16O
ratio in wood cellulose
1.Differential discrimination;
2.Isotopic composition of water;
Woodward, 2008
Why it is not what we see?
--T-dependent Humidity
18Ocell = 18Olw + 27‰
18Olw = 18Os(1-h) + h 18Oamb + * + k(1-h)
18Os = 18Or
Leaf Temperature
ei-saturation vapor pressure;
It can be calculated by
isotope data and determine
the relative humidity
Helliker and Richter, 2008
Relative humidity
Humidity is
related to
ea/ei
Helliker and
Richter, 2008
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