A thesis submitted to the Faculty of Science in fulfilment of the requirements for the degree of Doctor of Philosophy. 30 August 2016 in Johannesburg, South Africa. / The presence of Portulacaria afra affects the flow of ecosystem services from the thicket landscapes of the Eastern Cape. Degraded and intact thicket ecosystems deliver different bundles of ecosystem services, and within each bundle the services are linked non-linearly, so that land-use and management decisions which seek to increase a particular service affect the delivery of other services. One of the challenges of restoring the historically degraded thicket areas is understanding the trade-offs between various objectives, including re-establishing biodiversity and critical services such as carbon sequestration, climate regulation, water provision and forage supply. This thesis uses a variety of techniques to assess some of these trade-offs at several spatial and temporal scales. The amount, rate and nature of carbon assimilation by P. afra at a variety of spatial and temporal scales are also explored. Methods At a landscape scale stream flow data from transformed and intact catchments, near Jansenville and Bucklands respectively, are used to compare the freshwater ecosystem services provided by the presence of P. afra. Similarly, at a landscape scale, the new high resolution Multi-angle Imaging SpectroRadiometer (MISR-HR) products are used to explore the differences in the surface energy budget of intact thicket and degraded ecosystems. Two simple radiative transfer models are used to assess the radiative forcing (RF) trade-off between carbon uptake-related global cooling and albedo-related global warming. From samples and measurements taken from a field site 15 km south-west of the town of Kirkwood, radiocarbon and stable carbon isotope analysis, growth ring analysis, dendrometry, infrared thermometry, water use efficiency and eddy covariance data are used to quantify the rate of carbon assimilation by P. afra, and unpack the contribution of C3- and CAM-derived carbon to total carbon sequestration. Results Intact thicket cover results in a higher total yield of water as a fraction of rainfall, fewer extreme high flow events, and a higher volume of low-flow in comparison to landscapes thought to represent transformed former thickets.
Intact thickets have a lower albedo than degraded thickets. Restoring to an intact state will result in a mean albedo decrease of 5% (absolute) which constitutes a direct radiative forcing of +0.019 pW
m-2 at a global scale. This warming effect overwhelms the climate benefit derived from carbon sequestration related to P. afra growth by a factor of 1.6. The growth data suggests that P. afra produces two growth rings annually on average, with the average growth increment of 0.8mm in diameter. The multi-year and multi-source aboveground relative growth rate mean is 0.006 (±0.005 SD) g DM per g DM per year, which is mean average increment about 1.1% per year. The eddy covariance data highlighted the temporal variability in the system, measuring intact P. afra thicket vegetation as a net source of carbon to the atmosphere 0.018 gC m-2 hr-1, over the three short-duration campaigns. CAM is now understood to come in several variants. The sampled stems, averaged across all rings, had a δ13C signature of -15.8±1.0 ‰ indicative of “strong CAM” behaviour. P. afra plants in the Kirkwood locality over the past decades have derived 61% of their carbon through the CAM pathway and 39% through the C3 pathway, with substantial inter-stem variation. The soil isotope data suggests 51 - 63% C3 derived carbon, with values increasing with sampling depth. There are occasional large excursions, always in a single growth ring, towards more negative δ13C values (-19.0‰); in other words towards the C3 range. Although thought to be related to water stress, it was hard to determine a single trigger for the switching between C3 and CAM modes. This is consistent with findings for other strongly CAM plants. From the dendrometers and infrared thermometry, P. afra shows flexible stomatal behaviour on a range of timescales. For the majority of the study period the stomata were open both day and night. There is variation in stomatal closure over the year, which appears strongly seasonal, and suggests a link to the bimodal rainfall pattern experienced in the region. Using decision tree to classify the photosynthetic mode, this data suggest that the individual plants being measured are spending only 4% of the time in classical CAM mode, and the majority of the time in CAM-cycling. Conclusions Restoration of transformed thicket will improve freshwater ecosystem services associated with useful water yield, sustained supply, and flood control. It is important to include land-atmosphere feedbacks other than carbon assimilation when assessing the climate service delivered by restoration. The negative and positive radiative forcing will operate over different timelines.
CAM metabolism plays an important but not exclusive role in carbon gain. Contrasting patterns between plant and soil C3 fraction may relate to allocation issues and the contribution by plants other than P. afra. The growth rates of thickets are not substantially greater than similar water-limited ecosystems. / LG2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21695 |
| Date | January 2016 |
| Creators | Smart, Kathleen Grace |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (152 leaves), application/pdf |
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