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Ecology and Soil MoistureUsing 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 FranzESPM
UC Berkeley
COSMOS WorkshopTucson, 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
Summer:Trees green; grass dead
Spring:Trees green;grass green
Winter:Trees deciduous; grass green
Objectives
• Examine fluxes of water and carbon with changes in soil moisture– Role of 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
Oak Savanna consists of Heterogeneous and Open Canopy with Low LAI (< 2.0)
Annual Grassland
Day
0 50 100 150 200 250 300 350
LE (
MJ
m-2
d-1
)
0
2
4
6
8
10
12
Oak Savanna Woodland, Ione, CA
Day
0 50 100 150 200 250 300 350
LE (
MJ
m-2
d-1
)
0
2
4
6
8
10
12
14
A Decade of Evaporation Measurements
Day
0 50 100 150 200 250 300 350
LE,
MJ
m-2
d-1
0
2
4
6
8
10
grasslandSavanna
Evaporation from an Oak Savanna > Annual GrasslandHow and Why?
Rcanopy (s m-1)
10 100 1000 10000
E/
Ee
q
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
wheatcornjack pineoak-savanna
Effects of Functional Types and Rsfc on Normalized Evaporation
Rc is a f(LAI, N, soil moisture, Ps Pathway)
Eco-hydrology:ET, Functional Type, Physiological Capacity and Drought
?
?
?
E/
Eeq
Measuring Spatial/Temporal Variation in Soil Moisture
Tonzi
Day
0 100 200 300
Soi
l Moi
stur
e
0.0
0.1
0.2
0.3
0.4
0.5
Hourly Sampling, Few points and Depths, Theta ProbePoor Vertical and Horizontal Sampling
Tonzi Ranch, 2009
Day
0 50 100 150 200 250 300 350
Vol
umet
ric S
oil M
oist
ure
(cm
3 cm
-3)
0
10
20
30
40
50
0-15 cm15-30 cm30-45 cm45-60 cm
ESI, Moisture PointMany Locations, Discrete Depths, Bi-Weekly, Manual Sampling
day
0 50 100 150 200 250 300 350
Soi
l moi
stur
e (c
m3 c
m-3
)
0.0
0.1
0.2
0.3
0.4
0.5
Daily SamplingWeekly Sampling
Marry the Two Sensor Types
Calibrate Theta-Probe with Moisture PointBetter Spatio-Temporal Resolution
Grassland
weighted by roots (cm3 cm-3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
E/ E
eq
0.00
0.25
0.50
0.75
1.00
1.25
summer rain
Oak Savanna
weighted by roots(cm3 cm-3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
E/ E
eq
0.0
0.2
0.4
0.6
0.8
1.0
ET and Soil Water Deficits:Root-Weighted Soil Moisture
Baldocchi et al., 2004 AgForMet
Grassland
weighted by roots (cm3 cm-3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
E/ E
eq
0.00
0.25
0.50
0.75
1.00
1.25
summer rain
Oak Savanna
weighted by roots(cm3 cm-3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30
E/ E
eq
0.0
0.2
0.4
0.6
0.8
1.0
soil water potential (MPa)
-5 -4 -3 -2 -1 0
E/ E
eq
0.0
0.2
0.4
0.6
0.8
1.0
predawn water potentialsoil water potential
soil water potential (MPa)
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
E/ E
eq
0.00
0.25
0.50
0.75
1.00
1.25
oak savanna
annual grassland
Baldocchi et al., 2004 AgForMet
soil water potential (MPa)
-5 -4 -3 -2 -1 0
E/ E
eq
0.0
0.2
0.4
0.6
0.8
1.0
predawn water potentialsoil water potential
soil water potential (MPa)
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
E/ E
eq
0.00
0.25
0.50
0.75
1.00
1.25
oak savanna
annual grassland
ET and Soil Water Deficits:How do Trees stay Alive with such Low Water Potentials?
Root-Weighted Soil Moisture Matches Pre-Dawn Water Potential
ET of Annual Grass responds to water deficits differently than Trees
Days After Jan. 1, 2002
-50 0 50 100 150 200 250 300 350
volu
me
tric
soi
l moi
stur
e (c
m3 c
m-3
): 0
to
0.6
0 m
0.0
0.1
0.2
0.3
0.4
0.5
oak savannagrassland
Soil Moisture Dynamics at Oak Savanna Differ from Near by Annual Grassland
Smaller Water Reservoir Contributes to Lack of Trees
How Deep is the Soil?
Trenton Franz, EMI
Ione, CA
End of Hydrological Year
2000 2002 2004 2006 2008 2010
Wat
er F
lux
(mm
/y)
200
300
400
500
600
700
800
900
1000
ET, Oak SavannaET, annual grasslandprecipitation
InterAnnual Variation in Water Balance
ppt (mm/y)
200 400 600 800 1000
ET
(m
m/y
)
200
400
600
800
1000
grassland: ET +/- 87 mm/y; ppt +/- 170 mm/yoak savanna: ET +/- 61 mm/y
In Semi-Arid Regions, ET is Conservative:The Most ET lost, scales with Precipitation during the
Driest Years
156 158 160 162 164 166
158
158.02
158.04
158.06
158.08
158.1
158.12
158.14
158.16
158.18
158.2
DOY
gro
un
d w
ate
r e
lev
ati
on
[m
]
groundwater elevation at Tonzi
G. Miller, Y. Rubin, D. Baldocchi unpublished data
Oak Trees Tap Ground Water
Pre-Dawn Water Potential Represents Mix of Dry Soil and Water Table
Miller et al WRR, 2010
During Summer MidDay Water Potential is Less Negative than Shallow Soil Water Potential, Evidence the Trees are tapping Groundwater
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
‘Soil Moisture’ Maps with EMI
Trenton Franz, U Arizona
Oak Savanna, 2001-2011
Day of Year
0 50 100 150 200 250 300 350
Can
opy
Pho
tosy
nthe
sis
(gC
m-2
d-1
)
-6
-4
-2
0
2
Interannual Variation in Net Carbon Exchange
Ione, CA
Hydrological Year
00-01 01-01 02-03 03-04 04-05 05-06 06-07 07-08 08-09 09-10 10-11
NE
E (
gC m
-2 y
-1)
-300
-200
-100
0
100
200
300
400
Oak Savanna Annual Grassland
Carbon Fluxes Scale with Spring Rainfall
Open Grassland
PPT3-6 (mm)
0 50 100 150 200 250 300
Ann
ual F
lux
(gC
m-2
)
-200
0
200
400
600
800
1000
1200
Savanna
PPT3-6 (mm)
0 50 100 150 200 250 300
GPP RecoNEE
Ma et al, 2007 AgForMet
Environmental Controls on Respiration
Soil volumetric water content (m3 m-3)
0.0 0.1 0.2 0.3 0.4
Rec
o/R
ref
0.0
0.5
1.0
1.5
2.0Fast growth period data
Rain pulse
Xu + Baldocchi, AgForMet 2004
Impact of rain pulse on ecosystem respiration: Fast response
Day
150 200 250 300 350
Fc
( m
ol m
-2 s
-1)
0
1
2
3
4
5
6
understoryopen grassland
Baldocchi et al, JGR, Biogeosciences, 2006
274 276 278 280 282 284 286 288
-1
0
1
2
3
4
5
6
7
8
9
DOY
NE
E [
m
ol
m-2
s-1
]
Vaira 2008
Sustained and Elevated Respiration after Fall Rain
Synthesis and Conclusion
a: Photosynthesis >Respiration
c: Seasonal WaterDeficits Occur &
Shorten the GrowingSeason
CO2CO2
d: StomatalClosure Occursand Roots tap
Ground water toEnsure
Evaporation <Precipitation
f: Photosynthetic Capacitymust be Great, for a ShortPeriod when soil water is
ample, to Facilitate High Ratesof Photosynthesis
g: Leaf N and LeafThickness must be
adequate (high) to supportthese demands by the
Photosynthetic Machinery
e: Constraints from WaterBudget force the Ecosystemto produce a Sparse Canopywith Limited Leaf Area and
Reduced ET
20 40 60 80 100 120 140 160 180
20
40
60
80
100
120
140
160
180
200
b:Photosynthesis
scales withWater Use
Day
0 50 100 150 200 250 300 350
Cu
mu
lati
ve
Wa
ter
Flu
x, m
m
0
100
200
300
400
500
600
700
ET ppt
ET (mm d-1)
0 1 2 3 4
GP
P (
gC
m-2
d-1
)
0
2
4
6
8
10
Day0 50 100 150 200 250 300 350Vo
lum
etric
Wat
er C
onte
nt
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.05 m0.50 m
DOY
100 150 200 250 300 350V
cmax
( m
ol m
-2 s
-1)
0
20
40
60
80
100
120
140
Quercus alba (Wilson et al)Quercus douglasii (Xu and Baldocchi)
Broadleaved, Deciduous Trees
Specific Leaf Area (cm2 g-1)
60 80 100 120 140 160 180 200
Am
as
s (
nm
ol g
-1 s
-1)
0
50
100
150
200
250
300
Quercus douglasii
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