Drought, fire and the carbon balance of Africa Bob Scholes CSIR Natural Resources and Environment

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 PgC/y land use change emissions ~10+3 Pg/y NPP and 11+5 R h –Fires ~ 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) 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  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 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 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

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!])

Ecosystem CO 2 fluxes (µmol m -2 s -1 ) Canopy conductance (mmol m -2 s -1 ) The shape of the NEE day 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

Soil temperature at 7 cm (°C) Ecosystem respiration (µmol m -2 s -1 ) Wet soilMedium soilDry 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 34ºC39ºC

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

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 (T air >35ºC, and T soil >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  sat  wp  ad Soil 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/m 2 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 O 3 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 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 km 2. ~ 12.4 TgC/y Many times higher than the total emissions for Namibia!

The miombo woodland case Projected to be transformed into cropland over the next 30 years 6 x 10 6 m 2 x (2.5 (soil)+ 2 (tree) x 10 3 gC/m 2 ) = 27 PgC

The end