Mechanisms of Current Terrestrial Carbon Sinks and

Future Persistency 

Josep CanadellGCP and GCTE International Office

Canberra, Australia[Email: pep.canadell@csiro.au]

Outline

• Distribution and strength of terrestrial sinks• Candidate sink mechanisms• Where IPCC-2001 left the issue• US terrestrial sink case study• Mechanisms: present and future stability

• Land use change legacy• Fire suppression• Woody encroachment• Climate change• CO2 fertilization• Nitrogen fertilization• Reforestation• Surprises through changes in biodiversity

- 0.8 - 1.7

IPCC 2001Schimel 2001

Achard et al. 2002Malhi& Grace 2000

Terrestrial Carbon Sources and Sinks [1990’s]

Pg C/yr

+ 0.7+ 0.3 + 0.7

Terrestrial Carbon Sinks [1990’s]

Pg C/yr

- 0.7- 0.3 - 0.7

- 0.8 - 1.7

Net Sink

Gross Sink

Terrestrial Biosphere C Sink

Cram

er et al. 2000Why do we need to know the mechanisms?

IPCC 2001

Future atmospheric CO2 concentrations andstabilization scenarios

Late 1990’s:Cropland establishment and abandonment, CO2 and Climate (IPCC 2001).

Mid 1990’s:There was more than CO2. N deposition with unrealistic uptake rates of up to 80%.

Early 2000’s:All due to past land use practices (US-lead), (using forest demography and age structure). CO2 no effect.

Early 1990’s:All due to CO2 fertilization (biogeochemistry models/Physiological)

Sink Mechanisms – The 90’s understanding

• CO2 fertilization• Nitrogen fertilization• Climate change• Regrowth in abandoned croplands• Regrowth in previously disturbed forests

– Logging, fire, wind, insects• Fire suppression (woody encroach., forest thickening)• Decreased deforestation• Improved agriculture• Sediment burial• Future: Carbon Management (e.g., reforestation)`

Candidate Mechanisms of Current Terrestrial Sinks

Direct human induced

Global Sink Attribution by IPCC 2001 [1920-1992]

S1 = CO2

S2 = CO2 + Climate*S3 = CO2 + Climate + Cropland Establishment and Abandonment**

*Climate effect is inferred by S2 - S1** Land-use effect is inferred by S3 - S2

In the 1980s

• North extra-tropics: CO2: -0.2 to -1.6; Climate: +0.4 to –0.2; Land use: 0.0 to -0.4

• Tropics: CO2: -0.6 to -1.4; Climate:+ 0.7 to -0.1; Land use: +0.5 to +1.2

• The analyses included only 3 out of 10 sink mechanisms thought to be important.

1920 1930 1940 1950 1960 1970 1980 1990 2000

S3

Ne

t F

lux

(Pg

C y

r-1

)-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

CO2/O2 Budgets

Net Biota-to-Air

HRBM IBIS LPJ TEM

McGuire et al. (2001)

Forest trees 0.15

Other forest 0.15

Cropland soils 0.04

Woody encroach. 0.13

Wood products 0.07

Reservoirs 0.04

Exports - Imports 0.09

US-Fixed expt.rivers 0.04

Sinks in the Coterminous U.S. [1980-90] PgC yr-1

0.71 PgC yr-1 apparent U.S.

Pacala et al. 2001

Forest trees 0.15

Other forest 0.15

Cropland soils 0.04

Woody encroach. 0.13

Wood products 0.07

Reservoirs 0.04

Exports - Imports 0.09

US-Fixed expt.rivers 0.04

21 % of the total Sink

due to trees

Sinks in the Coterminous U.S. [1980-90] PgC yr-1

35%of the sink is susceptible

CO2 and N depositionfertilization

Forest trees 0.15

Other forest 0.15

Cropland soils 0.04

Woody encroach. 0.13

Wood products 0.07

Reservoirs 0.04

Exports - Imports 0.09

US-Fixed expt.rivers 0.04

Sinks in the Coterminous U.S. [1980-90] PgC yr-1

Forest trees 0.15

Other forest 0.15

Cropland soils 0.04

Woody encroach. 0.13

Wood products 0.07

Reservoirs 0.04

Exports - Imports 0.09

US-Fixed Ex.Rivers 0.04

32%of total Sink due to other

less commonly accounted mechanisms

Sinks in the Coterminous U.S. [1980-90] PgC yr-1

1. Are the sink mechanisms permanent features?

time

Sink

Str e

ngt h

4. Will they disappear?

time

Sink

Str e

ngt h

3. Will they saturate?

time

Sink

Str e

ngt h

2. Will they increase in strength?

time

Sink

Str e

ngt h

Future Dynamics of Carbon Sink Mechanisms

Forest Regrowth in Abandoned Croplands

Eastern United States (5 states)

98% of the C sink attributed to land use change:• Forest regrowth after crop abandonment• Reduced harvesting• Fire suppression

2% remaining attributed to:• Increasing CO2

• Nitrogen Deposition• Climate Change

Caspersen et al. 2000

1980’s-1990’s

ForestInventory 1

ForestInventory 2

Gro

wth

Rate

2%

98%

time

Sink Strength due to Forest Regrowth

Jiquan Chen, Univ of Toledo

0 20 40 60 80 100

0

2

4

-2

Net E

cosy

stem

Pro

duct

ivity

(Mg.

ha-1

)

t3

t4t1

t2

Years

Nemani et al. 2002

Climate as a Driver of C Sinks in the U.S.

2/3 of forest growth rateexplained by increased

precipitation and extensionof growing season due to

warming

1950-1993/Biome-BGC

8% increase in precipt.[1.39 mm yr-1]

No continental T change [increased in west and decreased on East]Decrease annual vapor deficit

Fire exclusion has increased C storage in forests [last 100 yrs]

Carbon Sink: Fire suppressionP h

otos

: M. F

lan n

igan

[Can

ada]

Total Area Burned (US)

Houghton et al. 2000

Annual Flux of C (TgC yr-1)

Eliminating fire completely,US forest could accumulated

2.6 Pg C by 2140

Sinks, for how long and at which cost? Time Bomb

Swetnam et al.

0

2

4

6

8

10

1920 1940 1960 1980 2000

Area

(m

illion

ha)

ClearCut Fire Insects Total

Increase after 1970

Disturbances in Canada’s forests [1920 – 1995]

Kurz & Apps 1999

Variable Temp Constant Temp

-200

-100

0

100

200

300

400

1920 1940 1960 1980 2000

Tg C

/ yr

Source

SinkDecrease after 1970

Net ecosystem C fluxes in Canada [1920 – 1995]

Kurz & Apps 1999

Phot

o: M

artin

197

5, A

rizon

a 19

03 &

194

1

Woody plant encroachment has promoted C sequestration in grassland and savanna ecosystems of N and S America,Australia, Africa, and Southeast Asia over the past century.

Maximum Potential C sequestration in the absence of fire = 2 Pg C yr-1 (upper value) Scholes and Hal 1996

Estimated CO2 sink:

USA: 0.17 PgC/yr for the 1980s (Houghton et al., 1999)

NE Australia: 0.03 PgC/yr (Burrows, 1998)

Woody Encroachment

Jackson et al. 2002Goodale and Davidson 2002

Carbon accumulation due to woody encroachment

• There is a Maximum limit.

• We may be over-Estimating C gain in wet regions.

Biom

ass

Stim

ulat

ion

(%)

GC

TE Sy nthesis. Mooney et al . 1999

Biomass Responses to Elevated CO2

Gro

wth

Enha

ncem

ent

CO2 concentration (ppm)200 400 600 800 1000200 400 600 800 1000

Phot

o: R

. Jac

kson

[Tex

as, U

SA]

Increasing aCO2 Effects on Plant Growth

Canadell et al. (in preparation)

550 ppm476 ppm415 ppm

354 ppm294 ppm250 ppm

0 150 300 450 600 750 9000.0

0.5

1.0

1.5

2.0

2.5

3.0

g S (

mol

m-2 s

-1)

Jackson et al. 2002

Stomatal acclimation - Solanum (C3 forb)

Intercellular [CO2]

600 ppm

CO2

H2O

Saturation of CO2 Increased Water Use Efficiency

Fossil-fuel N Deposition on Land (kg/km2)

1990 Townsend et al. 1996

Net

prim

ary

prod

uctio

n(g

C m

-2 y

–1)1000

100

10

1

10,000

10310210110010-110-2

Nitrogen input(g N m-2 y-1)

NPP Responses to N fertilization

Schlesinger 1997

Nitrogen Deposition

• N deposition explains 100% of current sink 80% 20% 15% (Holland et al. 1995, 97, Nadelhoffer et al. 1999, McGuire (in preparation)).

• The fertilization effect reaches a saturation.

• N deposition will not stimulate C uptake in the tropics (Hall & Matson 1999)

Houghton 2002

Reforestation: Annual Flux of Carbon in China

-150

-100

-50

0

50

100

150

200

250

300

350

1850 1870 1890 1910 1930 1950 1970 1990

An

nu

al f

lux

of

carb

on

(T

g C

yr

-1)

Degradation

Croplands

Industrial harvest

Fuelwood harvest

Plantations

China

[1850-2000]

Historically,450 Pg of C emitted (ff+lucc)(200 Pg from deforestation)

Nothing-to-eat Scenario:700 ppm (by 2100) down to 660 ppm

Maximum potential of C sink with reforestation

Prentice et al. 2001

Ramakutty & Foley 1999

90 ppm(40 ppm from deforestation)

More realistic scenario:Half of the cropland returns to native

20 ppm700 ppm (by 2100) down to 680 ppm

1. Are the sink mechanisms permanent features?

time

Sink

Str e

ngt h

4. Will they disappear?

time

Sink

Str e

ngt h

3. Will they saturate?

time

Sink

Str e

ngt h

2. Will they increase in strength?

time

Sink

Str e

ngt h

Future Dynamics of C Sink Mechanisms

CO 2 fertili

zation

Forest Regrowth

NoneIncreased Precipitation

(depending on timing of warming)

Woody Encroachment

N deposition

Cropland Soils

Surprises

Increasing Dominance of Lianas in Amazonian Forest

Phillips et al. 2002

Phot

o : R

. Hay

s C u

mm

ins

Lianas have increased 1.7-4.6% yr-1 relative to trees (over last two decades).

Tropical sink may decrease sooner than predicted.

Lianas increase mortality and decrease tree growth.

Ambient CO2

550 ppmNativ

e an

nual

sBr

omus

Rela

tive

rati o

(ele

vate

d/am

bien

t CO

2)

1

2

3

4

0

Dens

ity

Seed

rain

Biom

ass

Invasive Bromus takes over at elevated CO2

FACE - Nevada Desert

Smith et al. 2000

1. Major terrestrial biospheric sinks are in mid-latitudes (net sink) and in the tropics (gross sink).

Conclusions (i)

2. Legacy of past land use practices is a major driver of the current Northern hemisphere C sink, and CO2 and N fertilization may play a much smaller role than previously thought.

3. Management practices and disturbances that affect the age structure and demography of ecosystems are critical for understanding current and future C sinks. Both need to be coupled to biogeochemical and ecophysiological models.

4. The causes of the tropical gross sink are less clear but CO2 fertilization may drive part of the sink. Why CO2 should increase NEP in the tropics and not in temperate forests?

5. CO2 fertilization is likely to have a larger effect in the coming decades but not beyond 600 ppm.

Conclusions (ii)

6. Globally, N deposition is responsible for less than 15% of the current sink, much less than previously thought.

7. Timing of precipitation and temperature will determine the net effect of climate change on C sinks.

8. Surprises in sink strength may arise in the future via changes in biodiversity.

9. There are no permanent sink mechanisms that will ensure indefinite terrestrial sinks. Many of the current sinks are likely to decrease or disappear over the next half a century.

Conclusions (iii)

End