Drought, fire and the carbon balance of Africa

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Drought, fire and the carbon balance of Africa

• Bob Scholes
• CSIR Natural Resources and Environment
• bscholes_at_csir.co.za

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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

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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
• 0.390.02PgC/y land use change emissions
• 103 Pg/y NPP and 115 Rh
• Fires 1.10.5 PgC/y contribution to respiration
• High interannual variability
• Southern Africa small net sink, northern Africa
  small net source?

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Rainfall and grass NPP(Noy-Meirs inverse
texture hypothesis)
AGNPP f(rainfall, soil type)
Scholes RJ 2004 J Env Res Economics 26,559
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Consequence interannual variability of grass
production is higher on clays than sands
?clay
?sand
?rain
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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

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Constraints on tree cover
Sankaran et al 2005 Determinants of woody cover
in African savannas Nature 438, 846-9
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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
  gt3ºC warmer and 10 drier

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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)
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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)
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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?
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Soil moisture, temperature and Rsoil(Skukuza
data Musa Mvundla)
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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 (Tairgt35ºC, and Tsoilgt40ºC)
  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?

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The composite picture
Rh
NEE
?sat
?wp
?ad
Soil water content
NPP
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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

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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
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(No Transcript)
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The long-term effects of changes in the fire
regime on system CKruger Park fire trials Otter
(1992)
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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!
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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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The end