Ecology and Soil Moisture:
Using an Oak-Grass Savanna as a Model System for Studying
the Effects of Soil Moisture Dynamics on Water and Carbon
Exchange
Dennis Baldocchi
Siyan Ma, Naama Raz-Yaseef, Laurie Koteen, Joe Verfaillie, Trenton Franz
ESPM
UC Berkeley
COSMOS Workshop
Tucson, AZ, Dec 2012
Oak-Savanna Model System for Studying the Effect of Soil
Moisture on Ecosystem Ecology
• Structure/Function
– Oak and grasses provide contrasting life forms, woody/herbaceous,
perennial/annual
– The Canopy is open and heterogeneous, giving us a opportunity to test
the applicability of ecosystem and biogeophysical models, mostly
developed for ideal and closed canopies
• Environmental Biology
– The Mediterranean climate provides distinct wet/ cool and dry/hot
seasons to examine the ecosystem response (photosynthesis,
transpiration, respiration, stomatal conductance) to a spectrum of soil
moisture and temperature conditions
• Global Change
– The Mediterranean climate experiences great extremes in inter-annual
variability in rainfall; we experience a wider range in ppt over a few
years than long-term predicted changes.
Tonzi Ranch Flux Tower
Oak-Grass Savanna: A Two Layer System
Winter:
Trees deciduous; grass green
Spring:
Trees green;grass green
Summer:
Trees green; grass dead
Oak Savanna consists of Heterogeneous and
Open Canopy with Low LAI (< 2.0)
Objectives
• Examine fluxes of water and carbon with changes
in soil moisture
– Role of Seasonal Moisture Deficits
– Role of Rain Pulses
• Explore spatial/temporal variation of soil
moisture
–
–
–
–
Temporal variation with TDR
Depth of soil with GPR
Root distribution with GPR
Soil moisture spatial patterns with EMI
O a k S a va n n a W o o d la n d , Io n e , C A
14
12
A Decade of
Evaporation
Measurements
8
6
4
2
0
150
200
250
300
A n n u a l G ra s s la n d
350
D ay
12
10
8
-1
100
d )
50
-2
0
L E (M J m
L E (M J m
-2
-1
d )
10
6
4
2
0
0
50
100
150
200
D ay
250
300
350
InterAnnual Variation in Water Balance
Ppt: 350 to 900 mm
ET savannna: 380 to 520 mm
ET grass: 280 to 520 mm
Io n e , C A
1000
900
ET, O ak Savanna
E T , a n n u a l g ra s s la n d
W a te r F lu x (m m /y)
800
p re c ip ita tio n
700
600
500
400
300
200
2000
2002
2004
2006
E n d o f H yd ro lo g ic a l Y e a r
2008
2010
In Semi-Arid Regions, ET is Conservative:
The Most ET lost, scales with Precipitation during the
Drier Years
1000
g ra s s la n d : E T + /- 8 7 m m /y; p p t + /- 1 7 0 m m /y
o a k s a va n n a : E T + /- 6 1 m m /y
E T (m m /y)
800
600
400
200
200
400
600
800
1000
p p t (m m /y)
‘Mediterranean Ecosystems Don’t
evaporate more than 500 mm per year’, Serge Rambal
Evaporation from an Oak Savanna > Annual Grassland
How and Why?
10
g ra s s la n d
S a va n n a
6
LE, M J m
-2
d
-1
8
4
2
0
0
50
100
150
200
Day
250
300
350
0 .5
o a k s a va n n a
g ra s s la n d
0 .4
3
-3
vo lu m e tric so il m o istu re (cm cm ): 0 to 0 .6 0 m
Soil Moisture Dynamics at Oak Savanna Differ
from Near by Annual Grassland
0 .3
0 .2
0 .1
0 .0
-5 0
0
50
100
150
200
250
300
350
D a ys A fte r Ja n . 1 , 2 0 0 2
Smaller Water Reservoir Contributes to Lack of Trees
E/Eeq
Eco-hydrology:
ET, Functional Type, Physiological Capacity and Drought
?
?

?
Effects of Functional Types and Rsfc on Normalized Evaporation
2 .0 0
1 .7 5
w heat
c o rn
ja c k p in e
1 .5 0
o a k -s a va n n a
 E / E e q
1 .2 5
1 .0 0
0 .7 5
0 .5 0
0 .2 5
0 .0 0
10
100
1000
10000
-1
R c a n o p y (s m )
Rc is a f(LAI, N, soil moisture, Ps Pathway)
1 .2 5
1 .0 0
s u m m e r ra in
 E / E e q
ET and Soil Water Deficits:
Root-Weighted Soil Moisture
0 .7 5
0 .5 0
0 .2 5
0 .0 0
0 .0 0
0 .0 5
0 .1 0
0 .1 5
0 .2 0
0 .2 5
3
0 .3 0
0 .3 5
0 .4 0
-3
 w e ig h te d b y ro o ts (cm cm )
G ra ssla n d
1 .0
O a k S a va n n a
1 .2 5
0 .8
1 .0 0
 E / E e q
 E / E e q
s u m m e r ra in
0 .7 5
0 .6
0 .4
0 .5 0
0 .2
0 .2 5
0 .0
0 .0 0
0 .0 0
0 .0 5
0 .1 0
0 .1 5
0 .2 0
0 .2 5
0 .3 0
0 .3 5
0 .4 0
0 .0 0
0 .0 5
0 .1 0
0 .1 5
0 .2 0
3
3
-3
 w e ig h te d b y ro o ts (cm cm )
1 .0
O a k S a va n n a
0 .8
Baldocchi
et al., 2004 AgForMet
0 .2 5
-3
 w e ig h te d b y ro o ts (cm cm )
0 .3 0
Measuring Spatial/Temporal Variation in Soil Moisture
Hourly Sampling, Few points and Depths, Theta Probe
Poor Vertical and Horizontal Sampling
T o n zi
0 .5
S o il M o istu re
0 .4
0 .3
0 .2
0 .1
0 .0
0
100
200
Day
300
How Deep is the Soil?
Trenton Franz, EMI
ESI, Moisture Point
Many Locations, Discrete Depths, Bi-Weekly, Manual Sampling
T o n zi R a n ch , 2 0 0 9
V o lu m e tric S o il M o istu re (cm 3 cm -3 )
50
0 -1 5 c m
1 5 -3 0 c m
3 0 -4 5 c m
4 5 -6 0 c m
40
30
20
10
0
0
50
100
150
200
Day
250
300
350
Marry the Two Sensor Types
Calibrate Theta-Probe with Moisture Point
Better Spatio-Temporal Resolution
0 .5
D a ily S a m p lin g
W e e k ly S a m p lin g
3
-3
S o il m o is tu re (c m c m )
0 .4
0 .3
0 .2
0 .1
0 .0
0
50
100
150
200
day
250
300
350
Where are the Roots and How Many?
Remote Sensing Coarse
Roots with GPR
Ground Truth with Soil Pits
Vertical Distribution of Roots with Ground
Penetrating Radar
Raz-Yaseef et al. JGR Biogeosciences, in press
Radial Distribution of Coarse Roots, with Ground
Penetrating Radar
Raz-Yaseef et al. JGR Biogeosciences, in press
ET and Soil Water Deficits:
How do Trees stay Alive with such Low Water Potentials?
oak savanna
1 .0
p re d a w n w a te r p o te n tia l
s o il w a te r p o te n tia l
 E / E e q
0 .8
0 .6
0 .4
0 .2
0 .0
-5
-4
-3
-2
-1
0
so il w a te r p o te n tia l (M P a )
1 .2 5
Baldocchi et al., 2004 AgForMet a n n u a l g ra s s la n d
Root-Weighted Soil Moisture Matches
Pre-Dawn Water Potential
But, Oak Trees Seem to Tap Ground Water
groundwater elevation at Tonzi
ground water elevation [m]
158.2
158.18
158.16
158.14
158.12
158.1
158.08
158.06
158.04
158.02
158
156
158
G. Miller, X Chen, Y. Rubin, D. Baldocchi WRR 2010
160
DOY
162
164
166
Pre-Dawn Water Potential Represents Mix of Dry Soil and Water Table
During Summer MidDay Water Potential is Less Negative
than Shallow Soil Water Potential, Evidence the Trees are tapping Groundwater
Miller et al WRR, 2010
‘Soil Moisture’ Maps with EMI
Trenton Franz, U Arizona
Water Fluxes are Coupled with Carbon Fluxes
O a k S a va n n a , 2 0 0 1 -2 0 1 1
C a n o p y P h o to syn th e sis (g C m
-2
-1
d )
2
0
-2
-4
-6
0
50
100
150
200
Day of Year
250
300
350
Interannual Variation in Net Carbon Exchange:
Grassland is Carbon Neutral; Savanna is a C Sink
Io n e , C A
400
300
O a k S a va n n a
A n n u a l G ra s s la n d
N E E (g C m
-2
-1
y )
200
100
0
-1 0 0
-2 0 0
-3 0 0
0 0 -0 1 0 1 -0 1 0 2 -0 3 0 3 -0 4 0 4 -0 5 0 5 -0 6 0 6 -0 7 0 7 -0 8 0 8 -0 9 0 9 -1 0 1 0 -1 1
H yd ro lo g ica l Y e a r
Carbon Fluxes Scale with Spring Rainfall
O pen G rassland
S avanna
A n n u al F lu x (g C m -2 )
1200
1000
800
GPP
R eco
600
NEE
400
200
0
-2 0 0
0
50
100
150
200
P P T 3-6 (m m )
Ma et al, 2007 AgForMet
250
300 0
50
100
150
200
P P T 3-6 (m m )
250
300
Soil Moisture Controls on Respiration
2 .0
F ast g ro w th
p erio d d ata
R eco /R ref
1 .5
R ain p u lse
1 .0
0 .5
0 .0
0 .0
0 .1
0 .2
0 .3
0 .4
-3
S o il v o lu m etric w ater co n ten t (m 3 m )
Xu + Baldocchi, AgForMet 2004
Sustained and Elevated Respiration after Fall Rain
Vaira 2008
9
8
-2 -1
NEE [  mol m s ]
7
6
5
4
3
2
1
0
-1
274
276
278
280
282
DOY
284
286
288
Impact of Rain Pulse, Timing of Rain and
Photodegradation on Ecosystem Respiration
6
u n d e rs to ry
o p e n g ra s s la n d
4
( m o l m
3
Fc
-2
-1
s )
5
2
1
0
150
200
250
D ay
Baldocchi et al, JGR, Biogeosciences, 2006
300
350
Synthesis and Conclusion
CO2
CO2
200
180
a:  Photosynthesis >
Respiration
160
e: Constraints from Water
Budget force the Ecosystem
to produce a Sparse Canopy
with Limited Leaf Area and
Reduced ET
140
120
100
80
10
60
40
b:
Photosynthesis
scales with
Water Use
-2 -1
GPP (gC m d )
8
20
6
20
0
1
2
3
ET (mm d )
120
140
160
180
-2 -1
s )
120
0.6
0.05 m
0.50 m
0.5
0.4
f: Photosynthetic Capacity
must be Great, for a Short
Period when soil water is
ample, to Facilitate High Rates
of Photosynthesis
100
Vcmax (mol m
Volumetric Water Content
100
4
-1
80
60
40
0.3
20
0.2
0
0.1
100
150
200
250
300
350
DOY
0.0
0
50
100
150
200
250
300
350
Broadleaved, Deciduous Trees
Day
300
700
250
600
Amass (nmol g-1 s-1)
Cumulative Water Flux, mm
80
140
0
d: Stomatal
Closure Occurs
and Roots tap
Ground water to
Ensure
Evaporation <
Precipitation
60
Quercus alba (Wilson et al)
Quercus douglasii (Xu and Baldocchi)
2
c: Seasonal Water
Deficits Occur &
Shorten the Growing
Season
40
4
500
400
300
200
ET
ppt
g: Leaf N and Leaf
Thickness must be
adequate (high) to support
these demands by the
Photosynthetic Machinery
200
Quercus douglasii
150
100
50
100
0
0
0
50
100
150
200
Day
250
300
350
60
80
100
120
140
160
Specific Leaf Area (cm 2 g-1)
180
200
Conclusions
• Savanna woodlands need about 80 mm more water to
function than nearby grasslands
– Trees tap ground-water to sustain themselves during the summer
• Year to year variability in Carbon Uptake is due to length of
wet season.
– Oaks are risk adverse and experience less inter-annual variability in
NEE than grasslands
• Photosynthesis and Respiration are tightly linked
– Oaks need high N levels to attain sufficient rates of carbon
assimilation for the short growing season
• Oaks are darker, warmer and use more water than
grasslands
Biometeorology Team
Funding: US DOE/TCP; NASA;
WESTGEC; Kearney; Ca Ag Expt Station