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Drought, fire and the carbon balance of Africa. Bob Scholes CSIR Natural Resources and Environment [email protected] Outline. Effects of drought on Net Ecosystem Exchange Rainfall-NPP relationships Soil moisture-respiration relationships The effect of very high temperatures

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drought fire and the carbon balance of africa

Drought, fire and the carbon balance of Africa

Bob Scholes

CSIR Natural Resources and Environment

[email protected]

outline
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
slide3
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
Rainfall and grass NPP(Noy-Meir’s inverse texture hypothesis)

AGNPP = f(rainfall, soil type)

Scholes RJ 2004 J Env Res Economics 26,559

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

clay

sand

rain

rainfall and tree npp charlie shackleton dataset
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 cover
Constraints on tree cover

Sankaran et al 2005 Determinants of woody cover in African savannas Nature 438, 846-9

ecosystem scale npp in relation to water and temperature
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
slide9
Hot air is dry air

Dry air reduces canopy conductance

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

Canopy conductance

(mmol m-2 s-1)

Water vapour pressure deficit of the air

(bar)

slide10
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

Ecosystem CO2 fluxes

(µmol m-2 s-1)

Canopy conductance

(mmol m-2 s-1)

slide11
Night time fluxes

Skukuza site

Wet soil

Medium soil

Dry soil

Ecosystem respiration

(µmol m-2 s-1)

Soil temperature at 7 cm (°C)

34ºC

39ºC

31ºC

Does the optimum shift to higher temperatures in dry soil, or is this just an artifact

of sampling – there are no hot wet days?

the effects of very high future temperatures
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
The composite picture

Rh

NEE

sat

wp

ad

Soil water content

NPP

drought effects on albedo
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
slide16
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
The long-term effects of changes in the fire regime on system CKruger Park fire trials Otter (1992)
slide19
The Namibia caseTree biomass increased following cattle ranching.Thought to be due to reduced intensity and frequencyof fire

Approximate estimate of C

uptake through bush

encroachment: 620 TgC

over 50 years, on 494 000

km2.

~ 12.4 TgC/y

Many times higher than the

total emissions for Namibia!

the miombo woodland case
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

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