Drought, fire and the carbon balance of Africa

Bob Scholes

CSIR Natural Resources and Environment

bscholes@csir.co.za

Outline

• Effects of drought on Net Ecosystem Exchange– Rainfall-NPP relationships– Soil moisture-respiration relationships– The effect of very high temperatures

• Drought, fire extent and fire emissions– Effects on burned fraction– Effects on net emissions

Overview of the African carbon balance(Williams et al, Africa and the global carbon cycle submitted to Science)

• 0.2 PgC/y fossil fuel emissions• 0.39+0.02PgC/y land use change

emissions• ~10+3 Pg/y NPP and 11+5 Rh

– Fires ~1.1+0.5 PgC/y contribution to respiration

– High interannual variability– Southern Africa small net sink, northern Africa

small net source?

Rainfall and grass NPP(Noy-Meir’s inverse texture hypothesis)

0

100

200

300

400

500

0 200 400 600 800 1000

Annual rainfall

Gra

ss A

G N

PP (g

/m2/

y)

clay soil

sand soil 0

2

4

6

8

10

12

50 60 70 80 90 100

Sand %

Rai

n u

se e

ffic

ien

cy (

kg/h

a/m

m)

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

0 5 10 15

Rain use efficiency (kg/ha/mm)

Inte

rcep

t o

f A

GG

NP

P v

s R

ain

(k

g/h

a/y)

Scholes RJ 2004 J Env Res Economics 26,559

AGNPP = f(rainfall, soil type)

Consequence: interannual variability of grass production is higher on clays than sands

0

100

200

300

400

500

0 200 400 600 800 1000

Annual rainfall

Gra

ss A

G N

PP

(g

/m2/

y)

clay soil

sand soil

clay

sand

rain

Rainfall and tree NPP(Charlie Shackleton dataset)

• Tree increment is not a function of rainfall or soil type!– But prolonged drought leads to increased tree

mortality

• Is a function of inter-tree competition and tree stem diameter

Constraints on tree coverSankaran et al 2005 Determinants of woody cover in African

savannas Nature 438, 846-9

Ecosystem-scale NPP in relation to water and temperature

• What happens when things get really hot?• Especially if they get drier:

– southern Africa on west side projected to get >3ºC warmer and ~10% drier

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

Airtemperature > 30°Airtemperature < 30°

Can

opy

cond

ucta

nce

(m

mol

m-2 s

-1)

Water vapour pressure deficit of the air (bar)

Hot air is dry airDry air reduces canopy conductance

(data courtesy of Werner Kutsch [and Ian McHugh!])

-12

-10

-8

-6

-4

-2

0

0 50 100 150 200 250 300 350 400

Airtemperature > 30°Airtemperature < 30°

Eco

syst

em C

O2

fluxe

s

(µm

ol m

-2 s

-1)

Canopy conductance (mmol m-2 s-1)

The shape of the NEEday vs VPD curve does not change with temperature but on hot days you are more likely to be at the dry end

Therefore, hot dry weather reduces NPP

y = 0.3079e0.0472x

R2 = 0.1252

0

1

2

3

4

5

6

7

8

9

10

10 20 30 40 50

y = 0.8822e0.0382x

R2 = 0.3109

0

1

2

3

4

5

6

7

8

9

10

10 20 30 40 50

y = 0.4529e0.0769x

R2 = 0.4019

0

1

2

3

4

5

6

7

8

9

10

10 20 30 40 50

Soil temperature at 7 cm (°C)

Eco

syst

em r

espi

ratio

n

(µm

ol m

-2 s

-1)

Wet soil Medium soil Dry soil

Night time fluxesSkukuza site

Does the optimum shift to higher temperatures in dry soil, or is this just an artifactof sampling – there are no hot wet days?

31ºC 34ºC 39ºC

Soil moisture, temperature and Rsoil(Skukuza data: Musa Mvundla)

0

1

2

3

0 10 20 30 40 50Soil temperature @ 5 cm (C)

So

il r

esp

irat

ion

rat

e (g

/m2 /h

r)

Envelope DRY WET MOIST

The effects of very high future temperatures

• Soil and air temperatures reach their maximum when there is insufficient water to cool the system and buffer it through heat capacity

• These temperatures (Tair>35ºC, and Tsoil>40ºC) are above the postulated optima for both carbon assimilation and respiration, and can approach the lethal maxima.

• How adaptable are these optima and maxima to a global rise of a further 2-5ºC?

The composite picture

satwpadSoil water content

NEE

Rh

NPP

Drought effects on albedo

• On the light-coloured soils that predominate in Africa, drought leads to an increase in albedo equivalent to several 10s of W/m2

• If drought is accompanied by high livestock numbers, this raised albedo is persistent

• There may be a regional-scale precipitation feedback

• This effect may be as significant for global warming as the C emissions

In Southern African savannas, fire emissions go down in the dry season after a low-

rainfall growing season

• Data from Modis burned area product (in prep)• Evidence from CO measurements at Cape Point

• Reason is that – Fire extent is a function of fuel load– Number of ignitions also apparently goes down– Emissions also a function of fuel load

Brunke, E-G. and Scheel, H.E. (1997). On the contribution from biomass burning to the concentrations of CO and O3 at Cape Point. Conf. Proceedings of the fifth international conference on Southern Hemisphere Meteorology and Oceanography (American Meteorological Society), Pretoria, South Africa, 7-11 April 1997, P3.33. [poster presentation]

The long-term effects of changes in the fire regime on system C

Kruger Park fire trials Otter (1992)

0

5

10

15

20

25P

rote

cted

Tri

enni

al

Bie

nnia

l

Ann

ual

50-year fire regime

So

il O

C (

30 c

m)

mg

/g

Kambeni sandy

Shabeni sandy

Satara clay

Nwanetsi clay

The Namibia caseTree biomass increased following cattle ranching.

Thought to be due to reduced intensity and frequencyof fire

Time (years)

0 50

Car

bon

dens

ity (g

C/m

2 )

Bush encroachment begins

Still encroached, but further carbon uptake now zero

Total amount of carbon taken up

Time (years)

0 50

Car

bon

dens

ity (g

C/m

2 )

Bush encroachment begins

Still encroached, but further carbon uptake now zero

Total amount of carbon taken up

Approximate estimate of C uptake through bushencroachment: 620 TgC over 50 years, on 494 000 km2.

~ 12.4 TgC/y

Many times higher than thetotal emissions for Namibia!

The miombo woodland case

• Projected to be transformed into cropland over the next 30 years

• 6 x 106 m2 x (2.5 (soil)+ 2 (tree) x 103 gC/m2)

= 27 PgC

The end