Seasonal patterns of evapotranspiration, soil moisture, rainfall, and normalized difference vegetation index in a Nama-Karoo ecosystem

Thesis (MSc)--Stellenbosch University, 2023.

ENGLISH SUMMARY: This study mainly focused on the South African semi-arid region, the Eastern Karoo, which is located on an ecotone between the Nama Karoo and the Grassland Biomes. The Eastern Karoo is classified as a summer rainfall area, and is known for its highly variable rainfall patterns, which vary in time, space, amount and duration. Seasonal changes in rainfall patterns can affect the hydrological cycle, environmental processes, vegetation, and other biological processes of this region. Dryland ecosystems such as in the Eastern Karoo seem to be one of the most responsive ecosystems to climate change and rising atmospheric CO2, for example, leading to globally widespread desert “greening”. Despite these important changes underway globally and locally, and the value of semi-arid systems in southern Africa, much remains to be learned about their functioning and resilience in response to biophysical drivers. The main aim of this study was to determine if the soil moisture regime, and the related vegetation evapotranspiration and production response to rainfall and other climatic drivers in the Eastern Karoo are predictably linked, and if the ecosystem shows predominantly “pulsed” (eventdriven) or seasonal functioning. The inter-biome ecotonal location was specifically selected to enhance the likelihood of gathering data representing a range of climatic conditions over a succession of years. Results suggest that while soil moisture in this system is somewhat more strongly event-driven rather than seasonally driven, particularly for shallow soil depths, system evapotranspiration appears to be under strong plant physiological control, especially via a reliance of evapotranspiration on deeper soil moisture. These findings do not support the notion that the Nama-Karoo is strongly event-driven with respect to vegetation physiological activity. They also do not support the inference that ongoing climate change will necessarily increase evapotranspiration and cause system aridification. Results for vegetation productivity showed that average normalized difference vegetation index (NDVI) is driven by evapotranspiration and air temperature, not soil moisture, while short term shifts in NDVI (DNDVI) are driven by rainfall and soil moisture conditions. Finally, in-field daily average NDVI measurements on a dominant grass species strongly indicated that shallow soil moisture is a significant driver of the C4 grass productivity response. Taken together, these results show the value of using a range of approaches as lenses in answering questions of how climate conditions affect vegetation water use and production as a whole, and that in-field sampling adds value to the mechanistic understanding gained by studying the individual components of biodiversity. Future research could usefully build on this platform to develop a fuller picture of the interplay between these components of plant biodiversity, and how they contribute to the resilience of overall productivity under climatic variability and ongoing change.

AFRIKAANSE OPSOMMING: Hierdie studie het hoofsaaklik gefokus op die Suid-Afrikaanse semi-droe streek, die Oos-Karoo, wat op 'n ekotoon tussen die Nama Karoo en die Grasveldbiome gelee is. Die Oos-Karoo word as 'n somerreenvalgebied geklassifiseer, en is bekend vir sy hoogs veranderlike reenvalpatrone, wat wissel in tyd, ruimte, hoeveelheid en duur. Seisoenale veranderinge in reenvalpatrone kan die hidrologiese siklus, omgewingsprosesse, plantegroei en ander biologiese prosesse van hierdie streek beinvloed. Droeland-ekosisteme soos in die Oos-Karoo blyk een van die ekosisteme wat die meeste reageer op klimaatsverandering en stygende atmosferiese CO2 te wees, byvoorbeeld, wat lei tot wereldwyd wydverspreide woestyn "vergroening". Ten spyte van hierdie belangrike veranderinge wat wereldwyd en plaaslik aan die gang is, en die waarde van semi-ariede sisteme in Suider-Afrika, moet nog baie geleer word oor hul funksionering en veerkragtigheid in reaksie op biofisiese drywers. Die hoofdoel vanv hierdie studie was om te bepaal of die grondvog regime, en die verwante plantegroei evapotranspirasie en produksie reaksie op reenval en ander klimaat drywers in die Oos-Karoo voorspelbaar gekoppel is, en of die ekosisteem oorwegend "gepols" toon (gebeurtenisgedrewe) of seisoenale funksionering. Die tussen-bioom ekotonale ligging is spesifiek gekies om die waarskynlikheid te verhoog om data te versamel wat 'n reeks klimaatstoestande oor 'n opeenvolging van jare verteenwoordig. Resultate dui daarop dat hoewel grondvog in hierdie stelsel ietwat sterker gebeurtenis-gedrewe eerder as seisoenaal gedrewe is, veral vir vlak gronddieptes, blyk dit dat sisteem-evapotranspirasie onder sterk plantfisiologiese beheer is, veral as gevolg van 'n afhanklikheid van evapotranspirasie op dieper grondvog. Hierdie bevindings verwerp die idee dat die Nama-Karoo sterk gebeurtenisgedrewe is met betrekking tot plantegroeifisiologiese aktiwiteit. Hulle ondersteun ook nie die afleiding dat voortdurende klimaatsverandering noodwendig evapotranspirasie sal verhoog en verdroging sal veroorsaak nie. Resultate vir plantegroeiproduktiwiteit het getoon dat die “normalized difference vegetation index” (NDVI) gedryf word deur evapotranspirasie en lugtemperatuur, nie grondvog nie, terwyl korttermynverskuiwings in NDVI (DNDVI) deur reenval en grondvogtoestande gedryf word. Laastens, daaglikse gemiddelde NDVI-metings in die veld op 'n dominante grasspesie het sterk aangedui dat vlak grondvog 'n beduidende drywer van die C4-grasproduktiwiteitsreaksie is. Gesamentlik toon hierdie resultate die waarde van die gebruik van 'n reeks metodes as lense in die beantwoording van vrae oor hoe klimaatstoestande plantegroeiwatergebruik en -produksie as geheel beinvloed, en dat veld-proewe toevoeg tot die meganistiese begrip wat verkry word deur die individu te bestudeer. Toekomstige navorsing kan nuttig op hierdie platform voortbou om 'n vollediger prentjie te ontwikkel van die wisselwerking tussen hierdie komponente van plantbiodiversiteit, en hoe hulle bydra tot die weerstaanbaarheid van algehele produktiwiteit onder klimaatsveranderlikheid en voortdurende verandering.

Masters