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Published on September 3, 2007

Author: Naples

Source: authorstream.com

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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:  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):  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) 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:  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 andgt;3ºC warmer and ~10% drier Slide9:  Canopy conductance (mmol m-2 s-1) Water vapour pressure deficit of the air (bar) Hot air is dry air Dry air reduces canopy conductance (data courtesy of Werner Kutsch [and Ian McHugh!]) Slide10:  Ecosystem CO2 fluxes (µmol 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 Slide11:  Soil temperature at 7 cm (°C) Ecosystem respiration (µmol m-2 s-1) Wet soil Medium soil Dry soil Night time fluxes Skukuza site Does the optimum shift to higher temperatures in dry soil, or is this just an artifact of sampling – there are no hot wet days? 31ºC 34ºC 39ºC Soil moisture, temperature and Rsoil(Skukuza data: Musa Mvundla):  Soil moisture, temperature and Rsoil (Skukuza data: Musa Mvundla) 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 (Tairandgt;35ºC, and Tsoilandgt;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 sat wp ad Soil water content NEE Rh 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 In Southern African savannas, fire emissions go down in the dry season after a low-rainfall growing season:  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] Slide17:  The long-term effects of changes in the fire regime on system CKruger Park fire trials Otter (1992) :  The long-term effects of changes in the fire regime on system C Kruger Park fire trials Otter (1992) The Namibia caseTree biomass increased following cattle ranching.Thought to be due to reduced intensity and frequencyof fire:  The Namibia case Tree biomass increased following cattle ranching. Thought to be due to reduced intensity and frequency of 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 The end:  The end

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