A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy, 2017 / Plant phenology, the study of the timing of biological events such as leaf flush, has been identified as a key tool in monitoring and understanding the impacts that changing climates may have on the world. Unfortunately there is a noticeable lack of phenological research in Africa’s savannas in comparison to other global ecosystems, such as temperate forests. Savannas are known for their complexity in structure and composition and the phenology of their flora is no exception to this. Owing to the highly seasonal climates in this system, plants have had to evolve a range of phenological strategies to cope with the limited window of suitable growth conditions during the periodic wet season in the austral summer. One of these phenological strategies is that of early-greening; where deciduous trees will produce new flush prior to the onset of seasonal rainfall when the environmental conditions are at their driest. There is likely no other ecosystem in the world where the early-greening phenomenon occurs, however, it has been recorded in savannas from Africa, Australia, Asia and South America. The global prevalence of this strategy suggests that early-greening trees must gain some benefit from utilising this seemingly risky leaf flushing strategy. The main aim of this study was to investigate two of the hypothesised benefits of an earlygreening strategy for semi-arid savanna trees. The first hypothesis suggested that earlygreeners are extending their growing season beyond that of grasses and late-greeners – whose leaf flush is limited by the availability of water through the onset of seasonal rainfall. The second hypothesis is that early-greeners produce new leaves ahead of the emergence of invertebrate herbivores with the onset of the first rains, therefore decreasing the risk of damage to the vulnerable tissues and allowing the defence mechanisms within/upon the leaf to develop. This study took place in the Nylsvley Nature Reserve (NNR) over three consecutive austral growing seasons (August 2012 to April 2015) and was conducted on the two dominant deciduous broad-leaved trees: Burkea africana and Terminalia sericea. Burkea africana is known to flush prior to the onset of seasonal rainfall and was considered the earlygreening species in this study. Terminalia sericea has been recorded as flushing leaves prior to the onset of rainfall, but is more often a facultative-greener – rapidly producing new leaves with the onset of seasonal rainfall. This study experienced high seasonal variability between years with early-greening only occurring during the final (2014-2015) season. Nevertheless, I was able to assess the weekly green-up phenology of both species during the first three months of green-up and found that the green-up of the trees was largely disconnected from the main system driver – water – while grass green-up was closely linked to the onset of rainfall. This complements the theory of temporal niche separation; however, when assessing the functionality of the new leaves produced, carbon gain only occurred after the first two weeks post bud-break. In a system such as the NNR where rainfall onset variability is high, trees will only gain the advantage of an extended growing season if the onset of rainfall is more than two weeks after the start of flush. Using historical rainfall records (1980-2014), I estimated that 46% of years could have potentially experienced early-greening with rainfall commencing after the 15th October – the earliest date of green-up prior to rainfall onset in the NNR during this study. One of the benefits tested relating to early-greening in this study was that early-greeners avoid invertebrate herbivore damage on vulnerable new leaves. This study provided evidence for the use of a phenological defence strategy to cope with invertebrate herbivory pressure. Leaves which emerged before the rains had lower rates of herbivore damage than those which emerged after. Moreover, it was demonstrated that the constant turnover of leaves with high photosynthetic rates (T. sericea) is a reasonable mechanism for dealing with high leaf herbivory, and can result in equivalent end of season leaf area (and carbon gain) to species which invest in defence and have slower turnover rates (B. africana). Using the ground-based phenological measures in conjunction with remotely sensed NDVI imagery, the frequency of early-greening across seven comparable broad-leaved woodland sites in southern Africa from 2002 to 2014 was quantified. Of the environmental variables considered, the predictability of early-season rain (rather than total rainfall amount) was best correlated with early-greening. In savannas where rainfall onset and annual amounts were highly variable (such as the NNR), early-greening was less frequent (20% of the years) while in savannas closer to the equator where rainfall amounts were consistently >900 mm per annum and the onset dates began within a two week window each season, early-greening occurred in 80-90% of all years. The decrease in the proportion of early-greening events in the NNR from the 1980s to the past decade could be driven by the changing rainfall regimes over South Africa – with a predicted decrease in the number of precipitation events, but an increase in the storm intensity and rainfall amounts in each of these events. Fewer precipitation events may increase the risks associated with the early-greening strategy and this may be driving the NNR trees to use this strategy less frequently. This study has highlighted the need for a long-term phenological monitoring network within southern Africa’s savannas and has illustrated how early-greening species can benefit over other flora when environmental conditions are suitable for them to commence early leaf flush. This thesis has shown that early-greening broad-leaved savannas trees in South African savannas are more likely to avoid invertebrate herbivory than extend their growing seasons. / XL2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/23555 |
| Date | January 2017 |
| Creators | Whitecross, Melissa Andrea |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (232 pages), application/pdf, application/pdf |
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