Browsing by Author "Twala, Thando Caroline"

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    The impact of natural and simulated herbivory on compensatory leaf production under different light conditions
    (2019) Twala, Thando Caroline
    The ability of plants to compensate for disturbances such as herbivory and fire is driven by the type and extent of the disturbance, as well as the availability of resources. In savannas, plants utilise a variety of mechanisms to tolerate disturbances and resource limitations. Compensatory growth, resource reallocation, and photosynthetic and phenological changes have been recognized as mechanisms used by plants to tolerate environmental stress and resource limitation. Herbivory has been extensively researched as one of the main drivers of savanna biomes, but only a few studies have considered the influence of light availability (shading) as a resource affecting plant establishment and recruitment, in particular during the early stages of tree development. There is a gap in our understanding of how juvenile savanna trees (seedlings and saplings) tolerate shade, with many studies considering light as a limiting resource in forests and not in savannas, where light is considered to be a limiting resource to grasses but not woody species. The research that has been conducted on the influence of shading on juvenile trees has neglected how shading affects plant leaf turnover, phenology and leaf traits as well as plant morphology and physiology. Over the past decade only a few seed biology studies have been conducted on Terminalia sericea. These studies were aimed at understanding the seed nutrient content and factors influencing seed germination, but there is still limited information about the main drivers of seed germination, predation and physical properties. The main aims of this study were to: (1) investigate the effect of light availability, as well as natural and/or simulated herbivory on the compensatory growth capabilities of Terminalia sericea, a deciduous broad-leaved savanna tree species of considerable ecological and economical importance, and (2) determine the effect of seed predation and location on samarae (seed) physical properities, as well as the effect of artificial and natural cues on seed germination. The study took place on the roof top of the Oppenheimer Life Sciences Building at the University of the Witwatersrand in Johannesburg, South Africa, over one growing season (September 2016 to May 2017). To investigate the effects of simulated herbivory and light availability (shading) on leaf turnover, longevity and phenology, two pairs of adjacent leaf clusters were selected from the canopies of seedlings and saplings in the sun and 80% shade. On each pair, one cluster was exposed to simulated herbivory (removing 50% of the leaf area along the mid-rib vein) while the adjacent cluster was left untouched. Leaf turnover and longevity were measured at three week intervals and leaf phenology (i.e. timing of leaf age/development classes, e.g. new, fully expanded, mature and senescent) was measured monthly. The herbivory treatments did not affect leaf turnover or phenology. However, simulated herbivory had an effect on leaf longevity in seedlings although not in saplings, with leaf longevity being higher in control leaves compared to herbivory treatment leaves. Shading resulted in lower leaf production and loss, and longer leaf longevity. In addition, there was a higher proportion of new and fully expanded leaf phenophases in full sunlight compared to the shade, with the shade not having an effect on the proportion of mature and senescent leaf phenophases. Although, leaf production was lower in seedlings compared to saplings, the leaves of seedlings quickly matured but delayed leaf senescence, being shed only after the sapling canopy was bare, this is termed phenological avoidance which is an adaptative strategy used to maximise light interception and gain carbon. The influence of shading on Terminalia sericea seedling and sapling architecture, storage reserves (dry mass and allocation patterns), and leaf traits was also tested. This study provided evidence that saplings were larger (taller, higher stem diameter, canopy area and volume as well as biomass), thus indicating that storage reserves increase with an increase in plant stage. Plants grown in the shade allocated more resources to their leaves than sun plants which invested more resources in their roots and shoots. This strategy is used to maximise light capture in the shade through the presence of more leaves (greater leaf size (i.e. length, width, area and thickness)), whereas the high allocation of resources to roots and shoots is an indication that plants were adapted to obtain more water, which is a more important resource in full sunlight, because sunlit plants have higher leaf temperatures and tend to lose more water through transpiration than those in the shade. On saplings in the sun (only) two pairs of leaf clusters were selected on each plant and chlorophyll content (using the SPAD 502-Plus and CCM-300) and stomatal conductance were measured. Phalera imitata, Druce (Notodontidae) larvae were placed on one cluster in each pair with the adjacent cluster left untouched and both clusters were bagged. Phalera imitata larvae were removed after 24 hours and chlorophyll content and stomatal conductance were measured 3 days after herbivory. On the same leaves exposed to simulated herbivory (above) leaf chlorophyll content (measured using the SPAD 502-Plus and CCM-300), stomatal conductance and maximum efficiency of photosystem II (Fv/Fm) were measured at 3 week intervals. Phalera imitata larvae herbivory did not have an effect on chlorophyll content and stomatal conductance on herbivory treatment and control leaves. Simulated herbivory did not have an effect on stomatal conductance and Fv/Fm, however, there were variable chlorophyll content results measured with the different devices, where chlorophyll was not influenced by simulated herbivory when using the CCM-300, but control leaves had higher chlorophyll content than herbivory treatment leaves in seedlings only. Chlorophyll content and Fv/Fm were higher in shaded compared to sun leaves, however, stomatal conductance was higher in sun leaves in seedlings. Shading, however, did not have an effect on stomatal conductance in saplings. Overall, seedlings performed better in the shade than in sunlight and saplings were able to grow well in the sun and shade. I also investigated the influence of seed (samara) predation on seed dimensions from seeds collected from Nylsvley Nature Reserve (Nylsvley) and the Skukuza region of Kruger National Park (Skukuza), as well as the seed germination responses of T. sericea under different natural (temperature and photoperiod) and artificial (soaking and scarification) environmental cues. Seed predation was very site specific, with seed predation having an influence on seed dimensions in Nylsvley and not Skukuza. This suggested that seed predation likely occurred during seed development at Nylsvley, whereas it was post-dispersal at Skukuza. From the germination experimental trial, it was observed that soaking seeds in water at ambient temperature (control), nicking the seeds and germinating them at 12/12h photoperiod resulted in higher levels of seed germination, however, overall seed germination was very low due to overall low seed viability. This dissertation has shown that light should no longer be ignored as a limiting resource in savannas, because it affects many plant responses, particularly during juvenile plant stages. This study has also shown that in order to propagate T. sericea from seed, photoperiod (12/12h), seed soaking (control) and nicking are techniques that promote seed germination. My study has also again highlighted the low viability of T. sericea seeds.
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    South African Podocarpaceae distribution interpreted from a physiological and population genetics perspective
    (University of the Witwatersrand, Johannesburg, 2023-09) Twala, Thando Caroline; Fisher, Jolene T.; Glennon, Kelsey L.
    Podocarpaceae (podocarp) are the most diverse conifer family with a Southern Hemisphere distribution. Podocarps occur in Afrotemperate and Afromontane forests at high elevations that are cool and humid. Podocarps once dominated the forest canopy but due to their slower growth rate and photosynthetic rates they have are in competition withangiosperms which have faster growing and higher photosynthetic rate. Due to the competition between podocarps and angiosperms, Bond (1989) proposed that podocarps were excluded to nutrient poor and unfavourable environments due to their limited competitive ability. However, podocarps persist under the forest canopy until conditions become favourable. This power dynamic shifts with climate oscillations where podocarps distributions expand and dominate when conditions become cooler. Owing to podocarps being the most diverse and widespread conifers they make for a good study system. This thesis focuses on understanding the climatic variables driving the current and future distribution of podocarps, how their seedling physiology may influence their ability to recruit and establish under climate change, and how this can influence their ability to disperse in their South African distribution. Ensemble species distribution modelling was used to characterise the current and future distribution of podocarps and identify the climatic variables that influence their distribution. The current and future environmental niche was quantified using environmental niche modelling. I found that variables predicting rainfall seasonality were the most important at determining the distribution of podocarps in South Africa. Afrocarpus falcatus and P. latifolius were predicted to have the largest geographic distribution, with P. henkelii and P. elongatus having restricted distributions. Both A. falcatus and P. latifolius were predicted to occur in the Limpopo, Mpumalanga, KwaZulu-Natal, Eastern Cape and Western Cape provinces of South Africa. Podocarpus henkelii was predicted to occur in the KwaZulu-Natal and Eastern Cape provinces. Podocarpus elongatus is endemic to the Western Cape Province. All four podocarps were predicted to expand to higher altitudes (up the escarpment) under climate change and contract in its coastal distribution. Although P. elongatus was predicted to occupy the smallest geographic distribution it was predicted to have the widest environmental niche than the other species, which was predicted to contract under climate change. The environmental niche of P. latifolius and P. henkelii was predicted to remain stable. Afrocarpus falcatus, P. latifolius, and P. henkelii showed niche conservatism, however, P. elongatus under RCP 4.5 → current and the RCP 8.5↔ current niche comparisons showed niche divergence. Podocarpus elongatus was predicted to expand to an environment it currently does not occupy. Ecophysiological and morphological experiments were conducted to understand how podocarp seedlings respond to drought and elevated temperatures. The experiments indicated that P. henkelii seedlings were more drought and heat tolerant than A. falcatus seedlings. Conditions are predicted to become hotter and drier in some parts of South Africa, and this study has shown that P. henkelii seedlings will be able to tolerate these conditions better than A. falcatus seedlings. Furthermore, this suggests that the distribution of P. henkelii is not constrained by its physiology but rather by other mechanisms such as competition, reproductive biology, and/or shade tolerance. Microsatellites were used to inform us about possible podocarp dispersal patterns in A. falcatus, P. latifolius and P. henkelii in South Africa. The results suggested that podocarp populations in South Africa were shown to have higher genetic diversity than other podocarps globally, however these results may be due to the limited number of microsatellites used in this study, smaller population sizes in comparison to other studies and methods used to measure population structure and diversity. As expected, the geographically widespread species (A. falcatus and P. latifolius) are more diverse than the geographically restricted P. henkelii. Geographically distant A. falcatus and P. henkelii populations showed higher differentiation than geographically proximal populations. In P. latifolius South African populations, there was strong isolation by distance. Although the distribution of podocarps is disjunct, there is dispersal between populations. Podocarps are resilient to climate change as was demonstrated by the work in this thesis, and by their paleodistribution expanding and contracting with climate oscillations. In this thesis I considered climate, ecophysiology and genetics as determinants of podocarps distribution. Under climate change, podocarps are predicted to expand to higher elevations to track favourable climatic conditions. Seasonal drought is the most important climatic determinant of podocarp distribution. The ability of these species to tolerate drought and heat stress suggests that the seedlings might be able to tolerate short periods of drought and heat stress, however prolonged exposure may lead to seedling mortality, but populations will then be maintained by adults. Populations show evidence of gene flow, indicating they will be able to persist through changing climates, as they have done in the past. This thesis has highlighted that the factors constraining podocarp distributions might be demographic, and future works should investigate the role of fire in podocarp seedling establishment and longevity, as well as their interactions with angiosperm competitors.