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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 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

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Drought, fire and the carbon balance of Africa

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  1. Drought, fire and the carbon balance of Africa Bob Scholes CSIR Natural Resources and Environment bscholes@csir.co.za

  2. 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

  3. 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?

  4. Rainfall and grass NPP(Noy-Meir’s inverse texture hypothesis) AGNPP = f(rainfall, soil type) Scholes RJ 2004 J Env Res Economics 26,559

  5. Consequence: interannual variability of grass production is higher on clays than sands clay sand rain

  6. 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

  7. Constraints on tree cover Sankaran et al 2005 Determinants of woody cover in African savannas Nature 438, 846-9

  8. 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

  9. 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)

  10. 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)

  11. 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?

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

  13. 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?

  14. The composite picture Rh NEE sat wp ad Soil water content NPP

  15. 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

  16. 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]

  17. The long-term effects of changes in the fire regime on system CKruger Park fire trials Otter (1992)

  18. 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!

  19. 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

  20. The end

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