Doctoral Degrees (Botany and Zoology)

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Browsing Doctoral Degrees (Botany and Zoology) by browse.metadata.advisor "Brummer, Christian"

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    Exchange of carbon dioxide, water and energy in South African semi-arid ecosystems
    (Stellenbosch : Stellenbosch University, 2024-03) Maluleke, Amukelani; Midgley, Guy F. ; Feig, Timothy Gregor; Brummer, Christian; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH 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.