The age of C respired from tropical forest

Susan Trumbore, Jim Randerson (UC Irvine)

Jeff Chambers (Tulane)

Simone Vieira, Plínio Camargo, Everaldo Telles,

Luiz Martinelli

(Centro de Energía Nuclear na Agricultura, USP, Brazil)

Members of CD08 team

CD08 Team

The age of C respired from forest

Photosynthesis

leaf

stem

root

Allocation

storage

Microbial community

Stabilized SOM

Soil respiration

Litter and SOMdecomposition

Ecosystem respiration

RootRespiration

Fire

Loss by leaching, erosion

-1.5-1

-0.50

0.51

1.5

0 100 200 300 400 500

time

Production

Ecosystem Respiration

Heterotrophic decomposition

Why this might be interesting

• Measure of the capacity for storage of C and interannual variability in C balance

• Potential for direct comparison of data with models

Time

C stored C lost C stored

Dev

iati

on f

rom

mea

n

Radiocarbon of soil-respired CO2 provides a direct measure of isodisequilibrium “mean age” of several years up to a decade

14 C

14 C

50

150

250

350

450

550

650

1960 1970 1980 1990 2000

50

100

150

200

1998 2000 2002 2004

Harvard Forest MAatmosphere

Howland Forest, METapajos Forest, BrazilBonanza Creek, Alaska

Manitoba, Canada

Atmosphere- from Levin

and Hessheimer 2000

50

100

150

1999 2001 2003 2005

background air

control surface

dry surface

root respiration

Soil respired CO2 is a mix of heterotrophic and autotrophic sources

Data from Seca Floresta Plots

14C = 11-24Mean age ~2-4 yrs

14C

Heterotrophic Respiration can be measured by putting litter and 0-5 cm soil cores in sealed jars, then measuring the rate of CO2 evolution and the isotopic signature of evolved CO2.

50

100

150

1999 2001 2003 2005

background air

control surface

dry surface

root respiration

Soil respired CO2 is a mix of heterotrophic and autotrophic sources

Data from Seca Floresta Plots

14C = 11-24Mean age ~2-4 yrs

14C From incubations

Why is heterotrophic respiration so old?

• Leaves are 2-3 years old on average before they fall to the forest floor (Telles et al. 2003); branches and woody debris will be older

• Fine roots – those that do not die and decompose rapidly live for several years to decades (Trumbore et al. 2005)

• Soil organic matter – ‘Fast’ turnover pools have turnover times of several years to a decade (Telles et al. 2003)

0

25

50

75

100

125

150

0 10 20 30 40 50

Manaus

Santarem

Rio Branco

Determine age of respired CO2 using pulse-response function for CASA

Years since pulse

CO

2 r

espi

red

Thompson,and Randerson, Global ChangeBiol., 1999.

Year 1 productivity is still being respired 50 years later

0

5

10

15

20

25

0 0.25 0.5 0.75 1

Age

of

resp

ired

C (

yr)

Fraction of total respiration

Mean age of heterotrophically respired C from CASA is 22 yearsbut 50% is <4 years old

0

25

50

75

100

125

0 5 10 15 20 25-200

0

200

400

600

800

1000

1950197019902010

1

4C

(p

er m

il)

Fraction of total respiration

14

C(d

iffer

ence

fro

m

atm

opsh

ere

valu

e)

14C from CASAX

0

50

100

0 0.25 0.5 0.75 1

Range of values in CO2

respired in incubations ~5 years

PredictedBy CASA~22 years

Fraction of total respiration

14

C(d

iffer

ence

fro

m

atm

opsh

ere

valu

e)Model and measurements agree only for the fastest ~60% of respired CO2 - the difference is in that long tail distribution

0

50

100

0 0.25 0.5 0.75 1

Range of values in CO2

respired in incubations ~5 years

PredictedBy CASA~22 years

Problems with incubations

• Overemphasis of ‘young’ part of the respiration distribution

– Exclusion of woody debris from soil sampling will bias against the longer ‘tail’

- Artifacts with incubations in general

- Inclusion of roots in incubations emphasizes ‘young’ pools

Potential issues with CASA

Too long of a ‘tail’

- treatment of the wood pool – wrong turnover time (certainly true in Manaus, maybe not for Santarem) Vieira et al. in revision PNAS)

- model may allocate too much NPP to stem growth in tropical forests (stem allocation <1/3 of NPP)

2.0 dead wood (50-100) yr

Total heterotrophic respiration ~10

(11-24 yr)

Total autotrophic respiration ~20

(0.01-1 yr)

Total ecosystem respiration ~30(3 – 8 yr)

3.3 leaf litter

(2-3 yr)

4.7 Root/SOM

(5-10 yr)

Photosynthesis(~30)

RootRootrespirationrespiration

Fluxes from Chambers et al 2004. Units are

MgC ha-1 yr-1

Dead wood excludes trees <10cm DBH

ObservationsManaus ZF2Terra firme

Export <1

0 dead wood (50-100) yr

Total heterotrophic respiration ~10

(4-7 yr)

Total autotrophic respiration ~20

(0.01-1 yr)

Total ecosystem respiration ~30(0.9-1.6 yr)

3.3 leaf litter

(2-3 yr)

6.7 Root/SOM +wood

(5-10 yr)

Photosynthesis(~30)

RootRootrespirationrespiration

Fluxes from Chambers et al.

Units are MgC ha-1 yr-1

Dead wood excludes trees <10cm DBH

Sampling bias removes

wood component

3.3 dead wood (50-100) yr

Total heterotrophic respiration ~10

(20-37 yr)

Total autotrophic respiration ~20

(0.1-1 yr)

Total ecosystem respiration ~30(7-24 yr)

3.3 leaf litter

(2-3 yr)

3.3 Root/SOM

(5-10 yr)

Photosynthesis(~30)

RootRootrespirationrespiration

Fluxes from Chambers et al.

Units are MgC ha-1 yr-1

Dead wood excludes trees <10cm DBH

CASAPrediction too long

Interannual variability in C fluxes

• Wood increment ~ 0.7 MgC ha-1 yr-1 (15 - 20% ) (Vieira et al 2004)• Litterfall ~ 0.7 MgC ha-1 yr-1 (10 - 16% )• Mortality can vary 3-fold from one year to the next

(at least equal to wood increment variations)• If coherent over large areas of the basin, these

are globally significant • Are we likely to see a forest at steady state?

Time lags of ~5-20 years between production and decomposition mean that periodic changes in GPP will lead to periodic changes in NEP

The longer term components (SOM, wood) make up 25-30% of the respiration on an annual basis but are critical for understanding time lags - hence C storage or interannual variability

Conclusion