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Universität Bayreuth (2021)

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

Mukwashi, Kanisios

Titre : 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

Auteur : Mukwashi, Kanisios :

Université de soutenance : Universität Bayreuth

Grade : Doktors der Naturwissenschaften (Dr. rer. nat.) 2021

Résumé partiel
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.

Mots clés  : Carbon dioxide fluxes ; land-atmosphere carbon dioxide exchange ; carbon budget ; livestock grazing intensity and carbon fluxes ; rain pulses ; hot moments of carbon dioxide efflux ; savanna ecosystems in South Africa ; vegetative functional season ; maximum photosynthetic capacity ; ecosystem water use efficiency ; energy balance closure ; land use disturbance

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Page publiée le 1er novembre 2022