Exchange of carbon dioxide, water and energy in South African semi-arid ecosystems

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dc.contributor.advisorMidgley, Guy F. en_ZA
dc.contributor.advisorFeig, Timothy Gregoren_ZA
dc.contributor.advisorBrummer, Christianen_ZA
dc.contributor.authorMaluleke, Amukelanien_ZA
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Botany and Zoology.en_ZA
dc.date.accessioned2024-02-28T15:06:42Z
dc.date.accessioned2024-04-26T19:15:15Z
dc.date.available2024-02-28T15:06:42Z
dc.date.available2024-04-26T19:15:15Z
dc.date.issued2024-03
dc.descriptionThesis (PhD)--Stellenbosch University, 2024. en_ZA
dc.description.abstractENGLISH ABSTRACT: Recent research indicates that semi-arid regions contribute more than previously understood to global atmospheric carbon dioxide growth rates. Human activities and rising atmospheric carbon dioxide can alter ecosystem and biogeochemical processes in African drylands, potentially affecting land-atmosphere feedbacks. In terms of primary productivity and water-use efficiency, semi-arid ecosystems are expected to respond to these drivers more strongly. A paired-site eddy covariance (EC) based technique was used in the study to assess the fluxes of carbon, water, and energy across two semi-arid vegetation types that represent different semi-arid biomes in southern Africa: the Nama-Karoo and the Savanna. The Nama Karoo Site recorded -160 g C m⁻² over 33 months, while the Savanna Site recorded -567 g C m⁻². The mean net ecosystem exchange (NEE) was -189 g C m⁻² y⁻¹ for the Savanna site and -53 g C m⁻² y⁻¹ for the Nama Karoo Site. NEE was driven by incoming radiation, vapour pressure deficit (VPD), and air temperature, with soil moisture being a critical explanatory variable for productivity. The Nama-Karoo site had better ecosystem water-use efficiency (WUE) than the Savanna site while a greater closure rate of 96% was observed at the Savanna site compared to the Nama-Karoo site's 80%, with Energy Balance Ratios of 0.93 and 0.88 for each site, respectively. The responses of NEE, gross primary productivity (GPP), ecosystem respiration (ER) and evapotranspiration (ET) to wetting events showed that NEE generally peaked after five days, corresponding with GPP and ER peaks during the wet season while no significant peaks were observed during the dry season with both ecosystems remaining neutral. After wetting, ET rates peaks immediately and decreases to pre-wetting levels after around eight days during the wet season. At the Savanna site, rainfall thresholds that initiated sustained photosynthesis (point at which GPP surpasses ER and NEE denotes net uptake for a duration longer than 15 days) varied from 131 mm to 172 mm, whereas at the Nama-Karoo site, they varied from 98 mm to 165 mm. The second period had higher rainfall thresholds, possibly due to fire delaying the growing season. Although rainfall is a significant productivity driver, semi-arid ecosystems are likely to translate wetting events into productivity due to a combination of factors including temperature, vegetation status, soil properties, and preceding soil wetness conditions. Lastly, the study assessed the use of Sentinel-2 vegetation indices to estimate GPP from three semi-arid sites. The indices (the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and land surface water index (LSWI)) were evaluated independently and in conjunction with meteorological factors. Results showed that adding meteorological factors to single vegetation index estimations could improve GPP estimation accuracy. The phase of productivity in semi-arid ecosystems is better represented by standard MODIS products than its amplitude, accounting for 68-83% of GPP variability. The study contributes to the understanding of the role of semi-arid ecosystems in climate change and in the global carbon cycle. It addresses uncertainties in these ecosystems, which make up half of Africa. This expertise is crucial for informed resource allocation and policy formulation for ecosystem restoration, mitigation, and resilience, essential for the continent's growing population.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Onlangse navorsing dui daarop dat half-droë streke meer bydra as voorheen verstaan is tot die globale atmosferiese koolstofdioksied-groeikoerse. Menslike aktiwiteite en stygende atmosferiese koolstofdioksied kan ekosisteem- en biogeochemiese prosesse in Afrika droë streke verander en moontlik land-atmosfeer terugvoer beïnvloed. Daar word verwag dat half-droë ekosisteme sterker op hierdie drywers sal reageer in terme van primêre produktiwiteit en watergebruikseffektiwiteit. Die studie het koolstof-, water- en energievloei tussen twee half-droë plantegroeitype vergelyk wat verskillende suider-Afrika half-droë biome verteenwoordig, naamlik die Nama-Karoo en die Savanna, deur gebruik te maak van 'n gepaarde-site eddy covariance (EC) gebaseerde benadering. Die Nama Karoo-site het -160 g C m⁻² oor 33 maande aangeteken, terwyl die Savanna-site -567 g C m⁻²aangeteken het. Die gemiddelde netto- ekosisteem-uitruiling (NEE) was -189 g C m⁻² y⁻¹ vir die Savanna-site en -53 g C m⁻² y⁻¹ vir die Nama Karoo-site. NEE is aangedryf deur inkomende straling, dampdruktekort (VPD) en lugtemperatuur, met grondvog wat 'n kritiese veranderlike was vir produktiwiteit. Die Nama-Karoo-site het beter ekosisteem-watergebruikseffektiwiteit (WUE) gehad as die Savanna-site, terwyl 'n groter sluitingstempo van 96% waargeneem is op die Savanna-site in vergelyking met die Nama-Karoo- site se 80%, met energiebalansverhoudings van 0.93 en 0.88 vir elke site, onderskeidelik. Die reaksies van NEE, bruto primêre produktiwiteit (GPP), ekosisteemrespirasie (ER) en evapotranspirasie (ET) op bevochtiging gebeure het getoon dat NEE oor die algemeen na vyf dae 'n hoogtepunt bereik het, wat ooreenstem met GPP- en ER-piektye gedurende die nat seisoen, terwyl geen beduidende pieke waargeneem is tydens die droë seisoen met beide ekosisteme wat neutraal gebly het. Na bevochtiging het ET-koerse onmiddellik 'n hoogtepunt bereik en het dit na sowat agt dae gedurende die nat seisoen na voor-bevochtiging-vlakke afgeneem. By die Savanna- site het reënvaldrempels wat volgehoue fotosintese geïnisieer het (punt waarop GPP ER oorskry en NEE netto opname aandui vir 'n tydsduur langer as 15 dae) gewissel van 131 mm tot 172 mm, terwyl dit by die Nama-Karoo-site gewissel het van 98 mm tot 165 mm. Die tweede tydperk het hoër reënvaldrempels gehad, moontlik as gevolg van vuur wat die groeiseisoen vertraag het. Alhoewel reënval 'n beduidende drywer van produktiwiteit is, is dit waarskynlik dat half-droë ekosisteme bevochtiging gebeure na produktiwiteit omsit as gevolg van 'n kombinasie van faktore, insluitend temperatuur, plantestatus, grondeienskappe en voorafgaande grondwater toestande. Laastens het die studie die gebruik van Sentinel-2 plantindekse geëvalueer om GPP vanaf drie semi-droë areas te skat. Die indekse (die genormaliseerde verskil plantindeks (NDVI), versterkte plantindeks (EVI) en landoppervlakwaterindeks (LSWI)) is onafhanklik en in samehang met meteorologiese faktore beoordeel. Resultate het getoon dat die byvoeging van meteorologiese faktore tot enkele plantindeks raminge die akkuraatheid van GPP raming kon verbeter. Die fase van produktiwiteit in semi-droë ekosisteme word beter verteenwoordig deur standaard MODIS- produkte as sy amplitude, met 'n rekening vir 68-83% van GPP variasie. Die studie dra by tot die begrip van die rol van semi-droë ekosisteme in klimaatverandering en in die globale koolstof siklus. Dit spreek onsekerhede in hierdie ekosisteme aan, wat helfte van Afrika uitmaak. Hierdie kundigheid is noodsaaklik vir ingeligte hulpbron toekenning en beleidsformulering vir ekosisteemherstel, mitigasie en veerkrag, wat essensieel is vir die kontinent se groeiende bevolking.af_ZA
dc.description.versionDoctorateen_ZA
dc.format.extentxvii, 153 pages : illustrations (some color), mapsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/130483
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshAtmospheric carbon dioxide -- Environmental aspects -- South Africaen_ZA
dc.subject.lcshBiogeochemical cycles -- Effect of human beings onen_ZA
dc.subject.lcshNet ecosystem exchange -- Measurementen_ZA
dc.subject.lcshCarbon dioxide -- Climatic factorsen_ZA
dc.subject.lcshClimatic changes -- Effect of atmospheric carbon dioxide onen_ZA
dc.subject.lcshGlobal warmingen_ZA
dc.subject.lcshSemiarid ecosystems -- Nama-Karooen_ZA
dc.subject.lcshSavannas -- Africa, Southernen_ZA
dc.subject.lcshEddy Covarianceen_ZA
dc.subject.nameUCTDen_ZA
dc.titleExchange of carbon dioxide, water and energy in South African semi-arid ecosystemsen_ZA
dc.typeThesisen_ZA
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