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 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

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

Annual Grassland

Day

0 50 100 150 200 250 300 350

LE (M

J 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 (M

J m

-2 d

-1)

0

2

4

6

8

10

12

14

A Decade of Evaporation Measurements

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 BalancePpt: 350 to 900 mmET savannna: 380 to 520 mmET grass: 280 to 520 mm

ppt (mm/y)

200 400 600 800 1000

ET

(mm

/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

Drier Years

‘Mediterranean Ecosystems Don’t evaporate more than 500 mm per year’, Serge Rambal

Day

0 50 100 150 200 250 300 350

LE, M

J m

-2 d

-1

0

2

4

6

8

10

grasslandSavanna

Evaporation from an Oak Savanna > Annual GrasslandHow and Why?

Days After Jan. 1, 2002

-50 0 50 100 150 200 250 300 350

volu

met

ric s

oil m

oist

ure

(cm

3 cm

-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

Eco-hydrology:ET, Functional Type, Physiological Capacity and Drought

?

?

?

E/

Eeq

Rcanopy (s m-1)10 100 1000 10000

E/

E eq

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)

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

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

How Deep is the Soil?

Trenton Franz, EMI

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

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

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

156 158 160 162 164 166158

158.02

158.04

158.06

158.08

158.1

158.12

158.14

158.16

158.18

158.2

DOY

grou

nd w

ater

ele

vatio

n [m

]groundwater elevation at Tonzi

G. Miller, X Chen, Y. Rubin, D. Baldocchi WRR 2010

But, Oak Trees Seem to 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

‘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

Water Fluxes are Coupled with Carbon Fluxes

Interannual Variation in Net Carbon Exchange:Grassland is Carbon Neutral; Savanna is a C Sink

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 (g

C 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

Soil Moisture Controls on Respiration

Soil volumetric water content (m3 m-3)

0.0 0.1 0.2 0.3 0.4

Rec

o/Rre

f

0.0

0.5

1.0

1.5

2.0Fast growth period data

Rain pulse

Xu + Baldocchi, AgForMet 2004

274 276 278 280 282 284 286 288

-1

0

1

2

3

4

5

6

7

8

9

DOY

NEE

[m

ol m

-2 s

-1]

Vaira 2008

Sustained and Elevated Respiration after Fall Rain

Impact of Rain Pulse, Timing of Rain and Photodegradation on Ecosystem Respiration

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

Synthesis and Conclusion

a: Photosynthesis >Respiration

c: Seasonal WaterDeficits Occur &

Shorten the GrowingSeason

CO2 CO2

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

Day0 50 100 150 200 250 300 350

Cum

ulat

ive

Wat

er F

lux,

mm

0

100

200

300

400

500

600

700

ET ppt

ET (mm d-1)0 1 2 3 4

GPP

(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

ass (

nmol

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