title: Explaining temporal variability of and quantifying the impact of livestock grazing intensity on carbon and energy exchange in semi-arid near-natural and managed savanna ecosystems in South Africa
Creators: name: Mukwashi, Kanisios
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Carbon dioxide fluxes
subject: land-atmosphere carbon dioxide exchange
subject: carbon budget
subject: livestock grazing intensity and carbon fluxes
subject: rain pulses
subject: hot moments of carbon dioxide efflux
subject: savanna ecosystems in South Africa
subject: vegetative functional season
subject: maximum photosynthetic capacity
subject: ecosystem water use efficiency
subject: energy balance closure
subject: land use disturbance
subject: 550

description: This dissertation focuses on the responses of carbon and turbulent energy fluxes to land use disturbance and micro-climatic controls in semi-arid areas of South Africa using the eddy covariance method. Flux tower sites examined were Skukuza, a near-natural savanna ecosystem, and two Karoo sites (Karoo 1 and Karoo 2), managed under different livestock grazing intensities. Flux measurements at the Karoo sites were conducted over two years from November 2015 to October 2017 while six hydro-ecological years were assessed at Skukuza, from among the existing flux datasets from 2000 to 2014. The main aims, investigated at both the Skukuza and Karoo sites, were: to better understand the rain-pulse driven 'hot moments' of ecosystem respiration (Reco) efflux, as well as the connectivity of precipitation and net ecosystem carbon dioxide (CO₂) exchange (NEE) to explain the onset and end of vegetative functional seasons, and to study the temporal variation of ecosystem water use efficiency (EWUE). Additionally, site-specific issues were studied separately on individual sites in order to complement previous work and to derive new findings. The phenomenon of ‘hot moments’ of CO₂ efflux or Reco spikes, as well as the amount of precipitation required to trigger them, has not been explored before for these types of ecosystems. Ecosystem respiration spikes at Karoo sites responded to rain pulses that ranged in size from about 1 mm to less than 15 mm and falling within 1 day to a maximum of 5 days. The minimum precipitation thresholds that triggered Reco spikes at Karoo 1 and Karoo 2 were 3 mm and 1 mm, respectively. The dissimilarity was most likely due to higher organic matter substrate for microbial activity at the Karoo 2 site, rested from livestock grazing, compared to the gently grazed Karoo 1 site. There were no distinct ‘hot moments’ of CO₂ efflux at Skukuza due to increased size of rain pulses, frequency of precipitation occurrence and amount of precipitation. A novel approach to determine the onset and end of vegetative functional seasons at the study sites was introduced using connectivity between precipitation and NEE. The minimum precipitation thresholds to reach the onset of vegetative functional seasons at Karoo 1, Karoo 2 and Skukuza were 33.9 mm, 40.8 mm and 52.8 mm, respectively. However, it was noted that the end of vegetative functional seasons at Skukuza for successive hydro-ecological years were generally late as the deep-rooted plants continued to show a photosynthesis response a couple of days or weeks after the cessation of rainfall. This indicates the need to consider plant functional types in determining the end of the vegetative functional seasons. EWUE ranged from around 1 g C kg⁻¹ H₂O at Karoo sites to about 2 g C kg⁻¹ H₂O at Skukuza showing a rise with increasing rainfall and productivity between Karoo sites and Skukuza. Livestock grazing intensity (gentle grazing vs long-term resting) did not cause a significant difference in EWUE between Karoo 1 and Karoo 2 for two consecutive growing seasons. In regard to the impact of livestock grazing intensity on annual carbon budgets, the annual NEE of Karoo 1 and Karoo 2 were similar. However, the long rest of 8 years at Karoo 2 relatively increased carbon sequestration compared to the gently grazed Karoo 1 site suggesting that land use management can potentially modify vegetation structure and function, which may cause variation in the magnitude of gross primary production. For instance, during the second year of measurement, the annual carbon budgets for Karoo 1 and Karoo 2 were 41 ± 74 g C m⁻² yr⁻¹ and -57 ± 83 g C m⁻² yr⁻¹, respectively. Vegetation greenness and micro-meteorological conditions were similar between Karoo 1 and Karoo 2, allowing a rational comparison on the effect of livestock grazing intensity on carbon exchange components between the sites. Using the thermal diffusion equation, soil heat storage above 10 cm depth was estimated and it improved energy balance closure (EBC) at Karoo 1 by 4 % and 3 % for year 1 and year 2, respectively. At Karoo 2, accounting for soil heat storage improved EBC slightly by 1 % for both year 1 and year 2. A comparison of ground heat flux between Karoo 1 and Karoo 2 showed no significant difference between the two sites. At Skukuza, soil moisture was the main control of the variability of maximum photosynthetic capacity (GPPopt) and net ecosystem CO₂ exchange offset (NEEoffset). GPPopt and NEEoffset increased with increasing soil moisture, supporting the notion that photosynthesis and Reco are higher during wet periods compared to dry periods. Air temperature and vapor pressure deficit did not show a clear relationship with GPPopt at Skukuza. From this study, it is concluded that land management and climate related controls are influential in the temporal dynamics of carbon and energy exchange components at these semi-arid areas of South Africa.