Assessing Douglas-fir water-use history using
stable isotope (13C and 18O) in tree rings:
principles and potential
J. Renée Brooks
Western Ecology Division, Corvallis OR
Environmental Protection Agency
Stable Isotopes in Tree rings

Isotopes indicate the magnitude of key
ecological processes
d13C – intrinsic water-use efficiency
 d18O – RH, stomatal conductance


Isotopes record these responses to
changing environmental condition.


Tree rings are formed incrementally creating a
record over time.
Isotopes integrate ecological processes
over time

An annual ring integrates over the year.
Carbon isotope discrimination
and its relationship to leaf physiology
d Cair  d Cleaf

13
1  d Cleaf
13
13
 ci 
  a  (b  a ) 
 ca 
Where a = 4.4 (diffusion of CO2); b = ~27 (enzymatic
fractionation), ci internal [CO2], ca = ambient [CO2]
water stress
transpiration
rate
humidity
leaf
conductance
photon flux
CO2

ci
ca
Nitrogen
photosynthetic rate
productivity
Growth,
reproductive output
canopy
leaf area
Carbon Isotope Discrimination
a measure of Intrinsic Water-Use Efficiency
 ci 
  a  (b  a) 
 ca 
WUE
intrinsic

(
C

C
)
A i a
g
16
.
Where a = 4.4 (diffusion of CO2); b = ~27 (enzymatic
fractionation), ci internal [CO2], ca = ambient [CO2]
Interpreting 13C
A
Decreased 13C Value
Increased 13C Value
gs
Interpreting d13C and d18O
power of dual isotopes
Grams et al. 2007 PCE, Scheidegger et al. 2000 Oecol.
Oxygen isotopes
in plant tissues
What happens to leaf water?
ea
 Oe    k  ( Ov  k )
ei
18

18
18Oe enrichment of leaf water (above the source)
ea/ei Atmosphere - leaf vapor gradient
18Ov Water vapor
+ Equilibrium fractionation
k Kinetic fractionation
Craig Gordon (1965), Farquhar and Lloyd
(1993)
Péclet Effect
Transpiration
Leaf surface
(preferential loss of H216O)
18O
Mass flow of leaf water
enrichment according to
Craig-Gordon Model (1965)
Back diffusion of
18O enriched water
d18O [per mil]
Stronger Transpiration
 less 18O in leaf water
Stomatal conductance, gH2O [mmol m-2 s-1]
Bulk Water vs. site of evaporation
the Péclet effect

e 1  e
L 



LE

CD
Where C = molar density of water,
D = diffusivity of H218O in water,
E = transpiration rate
L = effective path length
Barbour et al. (2007)
Model for Cellulose d18O
d Ocx  f o d Owx   o   1  f o d Owl   o 
18





18
18
ƒo = fraction exchanged with xylem water
wl = leaf water
wx = xylem water
cx = xylem cellulose
εo = fractionation factor (+27 ‰)
Roden et al. 2000
Isotopic applications
to field studies
d13C (‰)
Effects of soil Water
Relative Extractable Water (%)
Dupouey et al. 1993 PCE
d13C (‰)
Transpiration
Livingston and Spittlehouse 1993
Effects of Thinning
2
Basal Area Increment (cm )
80
70
a
200 year-old Ponderosa Pine
Thinned
60
50
40
30
20
Control
10
0
17.5
Discrimination (‰)
b
17.0
16.5
16.0
15.5
15.0
14.5
1980
1985
Thinning
1990
Year
1995
2000
McDowell et al. 2003 PCE
Effects of Thinning
12
-2 -1
A ( mol m s )
a
11
10
9
-2 -1
g (mol m s )
0.12
b
0.11
0.10
0.09
and g after thinning
0.08
0.07
30
1980
c
20
McDowell et al. 2003 PCE
10
1985
1990
1995
2000
Effects of Fertilization
Basal Area Increment (mm2)
Wind River Fertilization Experiment
Control
157 Kg/ha
314 Kg/ha
471 Kg/ha
3000
2000
1000
0
1940
1945
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
Year
Brooks & Coulombe in review
Fertilization effects on 
Early Wood
21.0
20.5
20.0
19.5
19.0
13C (‰)
18.5
N addition
18.0
Late Wood
21.0
20.5
20.0
19.5
19.0
18.5
18.0
Control
157 kg/ha
314 kg/ha
471 kg/ha
1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974
Year
Brooks &
Coulombe
in review
Leaf Gas-Exchange
Estimated from Late Wood
Change in A/gs (mol mol-1)
relative to controls
15
Control
157 kg/ha
314 kg/ha
471 kg/ha
10
5
0
-5
N addition
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
Brooks & Coulombe in review
d18O response to Fertilizer
Late Wood
Control
157 kg/ha
314 kg/ha
471 kg/ha
2
1
18
d O (‰)
Normalized for pretreat means
3
0
13C
effect
-1
N addition
Leaf Area
effect
-2
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
Year
Brooks & Coulombe in review
gs (%) change from controls
A () change from controls
40
Late Season Changes in Gas-exchange
20
0
-20
157 kg/ha
314 kg/ha
471 kg/ha
-40
-60
20
0
-20
-40
-60
1954
1956
1958
1960
1962
1964
Year
1966
1968
1970
1972
1974
Late Season Changes in growth and Leaf Area
4.5
157 kg/ha
314 kg/ha
471 kg/ha
Late wood BAI
(Prop. of control)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Leaf Area
(prop. of control)
6
5
4
3
2
1
0
1954
1956
1958
1960
1962
1964
Year
1966
1968
1970
1972
1974
N Fertilization Created
Hydraulic Imbalance





Leaf area increased
Roots and sapwood insufficient to support
increased leaf area
Fertilized trees experience drought at the
end of summer.
Increase in leaf area offset decrease in
leaf gas-exchange – Growth increased.
Hydraulic imbalance lasted 10 years
Basal Area Increment (mm2)
Multiple Fertilizer Applications
6000
Bald Hill Site
Fertilized
Control
5000
4000
3000
2000
1000
0
20
13
 C
19
18
17
16
1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004
Year
Unresponsive Site
8000
Ostrander Site
2
BAI (mm )
6000
4000
2000
0
20
13C
19
18
17
16
1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004
Year
Tree rings records



Extent and duration of growth response is
recorded in ring width data.
d13C and d18O allow for understanding the
leaf physiology and whole tree hydraulics.
Control trees necessary for separating
management treatments from climate
signals.
Tree rings provide added insights
into long-term experiments.