Ecological Controls on Net Ecological Controls on Net Ecosystem Productivity of a Ecosystem Productivity of a

Seasonally Dry Annual Seasonally Dry Annual Grassland under Current and Grassland under Current and

Future Climates: Modelling with Future Climates: Modelling with EcosysEcosys

Grant, R.F.Grant, R.F.11, Baldocchi, D.D., Baldocchi, D.D.22 and Ma, and Ma, S.S.22

11 Department of Renewable Resources, University of Alberta, Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2E3Edmonton, AB, Canada T6G 2E322 Department of Environmental Science, Policy and Management & Berkeley Department of Environmental Science, Policy and Management & Berkeley Atmospheric Science Center, University of California, Berkeley, CAAtmospheric Science Center, University of California, Berkeley, CA

Ecological controls on NEP of Ecological controls on NEP of annual grasslands in annual grasslands in

Mediterranean climate zones Mediterranean climate zones Net C uptake Net C uptake

determined by the duration and intensity of determined by the duration and intensity of precipitation in the rainy season during which precipitation in the rainy season during which soils are wet enough to sustain net C uptake by soils are wet enough to sustain net C uptake by plants. plants.

Net C emission Net C emission slow during the dry season when grasslands slow during the dry season when grasslands

have senescedhave senesced rapid, rainfall-induced pulses at the start of the rapid, rainfall-induced pulses at the start of the

next rainy season before grasslands regrow, next rainy season before grasslands regrow, rapid at the end of the rainy season if grassland rapid at the end of the rainy season if grassland

growth terminates before the start of the growth terminates before the start of the following dry season. following dry season.

How can we model these How can we model these controls?controls?

(1) Climatic and phenological signals that induce (1) Climatic and phenological signals that induce germination during soil wetting at the start of the germination during soil wetting at the start of the rainy season, and senescence during soil drying rainy season, and senescence during soil drying at the end of the rainy season at the end of the rainy season

(2) Soil-root-canopy-atmosphere hydraulic (2) Soil-root-canopy-atmosphere hydraulic scheme by which soil and atmospheric water scheme by which soil and atmospheric water status determine plant water status, and hence status determine plant water status, and hence GPP during soil wetting and drying between GPP during soil wetting and drying between germination and senescencegermination and senescence

(3) Stimulation or suppression of (3) Stimulation or suppression of RRhh during during wetting or drying of surface residues and soil wetting or drying of surface residues and soil evident in precipitation-driven pulses that evident in precipitation-driven pulses that characterize C emissions in seasonally dry characterize C emissions in seasonally dry ecosystems ecosystems

(1) Climatic and phenological (1) Climatic and phenological signalssignals

requirements for time accumulated at requirements for time accumulated at cc above or below set thresholds to be above or below set thresholds to be met during earlier and later plant met during earlier and later plant growth stages respectivelygrowth stages respectively 480 h above -0.2 MPa for germination, 480 h above -0.2 MPa for germination,

and 240 h below -2.0 MPa for senescence. and 240 h below -2.0 MPa for senescence. avoid premature germination or avoid premature germination or

senescence and hence wastage of senescence and hence wastage of resources during false starts or ends to resources during false starts or ends to the rainy season the rainy season

bo

un

dar

y

canopy layer 1

atmosphere

atm. vapor density

canopy vapor density

ccanopy layer n

r,n

r,2

r,1

s,n

s,2

s,1

soiln

soil2

soil1

radialn

radial2

radial1axia

l 3 axia

l 1

axia

l 2

cap

acit

ance

stomatalshaded,n

stomatalshaded,1

stomatalsunlit,n

stomatalsunlit,1

soil layer n

soil layer 2

soil layer 1

Rn LE H(2) Soil-root-(2) Soil-root-canopy-canopy-atmosphere atmosphere hydraulic scheme:hydraulic scheme: derive c at which root water uptake+ capacitance= transpiration

soil hydraulic resistance, rootlength density

root surface area

roo

t ax

is l

eng

th

CO2 fixation,turgor, Ca - Ci

residue layer

(3) Stimulation or suppression (3) Stimulation or suppression of of RRhh during wetting or drying during wetting or drying

Scheme water and vapor transfer Scheme water and vapor transfer through soil surface and surface through soil surface and surface residue to solve for residue to solve for rr and and rr..

Specific microbial respiration in soil Specific microbial respiration in soil and residue constrained by aqueous and residue constrained by aqueous concentration [M/concentration [M/] and ] and ..

Vaira Ranch (38.418N, 120.958W)MAT 16.3 oCPrecipitation 559 mm

Year a†

b R2 RMSD

RMSE

n

(a) Measured CO2 Fluxes mol m-2 s-1 mol m-2 s-1 mol m-2 s-1

2001 0.0 0.95 0.75 2.4 1.2 4610

2002 0.0 0.98 0.84 1.9 1.1 4780

2003 0.3 1.08 0.84 2.1 1.2 4477

2004 0.2 1.11 0.77 1.9 1.0 3742

2005 0.2 1.06 0.88 2.4 1.4 4095

2006 -0.1 0.93 0.72 2.2 1.0 4430

2007

0.4 1.10 0.81 2.1 1.1 4152

2008 0.0 1.21 0.51 1.5 0.8 5932

(b) LE Fluxes W m-2 W m-2 W m-2

2001 2.2 1.00 0.73 28 25 5933

2002 2.5 1.08 0.84 19 28 6893

2003 -0.2 1.06 0.80 22 29 6554

2004 -1.5 1.01 0.70 26 31 5271

2005 3.2 1.17 0.90 20 33 5856

2006 -0.3 0.98 0.81 23 30 5931

2007

0.3 1.16 0.86 20 29 5135

2008 -0.3 1.00 0.67 23 27 6041

Intercepts (Intercepts (aa), slopes (), slopes (bb), correlations (R), correlations (R22), root mean ), root mean square of differences between modelled and measured square of differences between modelled and measured fluxes (RMSD), root mean square of error in measured fluxes (RMSD), root mean square of error in measured

fluxes (RMSE)fluxes (RMSE)

Large interannual variation in NEP Large interannual variation in NEP caused by precipitationcaused by precipitation

0 30 60 90 120150180210240270300330360-6-4-20246

0.0

0.1

0.2

0.3

NEP (g C

m-2 d

-1)

(m

3 m-3)

Pre

cip. (m

m h-1)

Day of Year 2004

0

5

10

15

20

10 cm 20 cm

0 30 60 90 120150180210240270300330360

Day of Year 2005

Rainy seasonsshorter, less intense vs. longer more intense

Soil dryingearlier vs. later

End of net C uptakesooner vs. later

Net C emission at start of rainy season

Later start to rainy season lowers Later start to rainy season lowers net C uptake in late winter of 2004 net C uptake in late winter of 2004

vs. 2005vs. 2005

99 100 101 102 103 104 105 106

-10

0

10

20

0

200

400

600

800

1000

Radia

tion (W

m-2)

5

10

15

20

25

30

-400

-200

0

200

400

600

Energ

y Flu

x (W

m-2)

CO

2 Flu

x (

mol m

-2 s

-1)

Day of Year 2004

100 101 102 103 104 105 106 107-15-10-505

10152025

0

200

400

600

800

1000

0

5

10

15

20

25

Tem

perature (oC

)

Day of Year 2005

-400

-200

0

200

400

600

Earlier end to rainy season lowers Earlier end to rainy season lowers net C uptake in spring 2004 vs. net C uptake in spring 2004 vs.

20052005

99 100 101 102 103 104 105 106

-10

0

10

20

0

200

400

600

800

1000

Radi

atio

n (W

m-2)

5

10

15

20

25

30

-400

-200

0

200

400

600

Ene

rgy

Flu

x (W

m-2)

CO

2 Flu

x (

mol

m-2 s

-1)

Day of Year 2004

100 101 102 103 104 105 106 107-15-10-505

10152025

0

200

400

600

800

1000

0

5

10

15

20

25

Tem

perature (oC

)

Day of Year 2005

-400

-200

0

200

400

600

Wetting of dry surface soil and Wetting of dry surface soil and residue at the start of the rainy residue at the start of the rainy

season drives rapid pulses of COseason drives rapid pulses of CO22 emissionemission

304 305 306 307 308 309 310 311 312 313 314-12.5

-10.0

-7.5

-5.0

-2.5

0.0

0

2

4

6

8

10

Prec

ipita

tion

(mm

h-1)

0

5

10

15

20

25

CO

2 Flu

x (m

ol m

-2 s

-1)

Day of Year 2003312 313 314 315 316 317 318 319 320 321 322

-12.5

-10.0

-7.5

-5.0

-2.5

0.0

0

2

4

6

8

10

Prec

ipita

tion

(mm

h-1)

0

5

10

15

20

25

Temperature (

oC)

CO

2 Flu

x (m

ol m

-2 s

-1)

Day of Year 2007

Annual NEP is closely Annual NEP is closely associated with the duration of associated with the duration of

net C uptakenet C uptake

2000 2002 2004 2006 2008

-200

-150

-100

-50

0

50

100

150

200

NE

P (

g C

m-2 y

-1)

Year

60

80

100

120

140

160D

ura

tio

n o

f n

et

C U

pta

ke

(d

) EC model

Ta Precip. Ca Ta Precip. Ca

oC y-1 y-1 y-1 oC y-1 y-1 y-1

Scenario A1fi B1

Dec. – Feb. 0.0364 0.9962 1.00866 0.0209 0.9968 1.00324

Mar. - May 0.0496 0.9974 1.00866 0.0286 0.9987 1.00324

Jun. – Aug. 0.0755 0.9995 1.00866 0.0418 0.9974 1.00324

Sep. – Nov. 0.0496 0.9974 1.00866 0.0286 0.9987 1.00324

Annual changes in seasonal air temperatures (Ta) and precipitation predicted from 1961 – 1990 to 2090 – 2099 by the UKMO Hadley Centre Climate Model V.3 (HadCM3), and in atmospheric CO2 concentrations (Ca) for the SRES A1fi and B1 emission scenarios in California

NEP modelled from 2003 to 2006, and under 2003 – 2006 weather altered hourly over 54 years (2057 – 2060) and 99 years (2102 – 2105) under climate change simulated by the UKMO Hadley Centre Climate Model V.3 (HadCM3) for the SRES (a) A1fi and (b) B1 emission scenarios in CA

-6-4-202468

10

2002 2003 2004 2005 2006-6-4-202468

NEP (g C

m-2 d

-1)

2003/2006 2057/2060 2102/2105

Year

(b) B1

(a) A1fi

2056 2057 2058 2059 20602101 2102 2103 2104 2105

earlier maturity in longer growing seasons

leaves wetter, respiring soil during dry season

increased productivity with rising Ta, Ca, but …

Annual and 9-year moving average NEP and soil + litter C modelled under current climate (2000 – 2008), and under climate change scenarios from the UKMO Hadley Centre Climate Model V.3 (HadCM3) under SRES A1fi and B1 from 2009 to 2143.

NEP mean and IAV are stable under current climate

2020 2040 2060 2080 2100 2120 214010

11

12

13

14 current A1fi B1

(d)

SO

C (

kg

C m

-2

)

Year

-300

-200

-100

0

100

(c) B1

-300

-200

-100

0

100

(b) A1fi

NE

P (

g C

m-2

y-1

)

-300

-200

-100

0

100

200

(a) current

pattern of IAV changes after 35 to 70 years, avg. NEP declines

SOC stops rising, declines gradually after IAV changes