Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by Author "Carey, Victoria Anne"

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    The use of viticultural terroir units for demarcation of geographical indications for wine production in Stellenbosch and surrounds
    (Stellenbosch : Stellenbosch University, 2005-04) Carey, Victoria Anne; Archer, E.; Barbeau, G.; Saayman, D.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology. Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: Due to increased consumer demand for products labelled by origin, and the requirement that these labels are a guarantee of both quality and product character, there is an increasing global focus on delimitation of denominations of origin. The integrity of denominations of origin and their defensibility can be ensured through the use of terroirs as a basis for delimitation. The aims of this study were to establish the dominant environmental criteria that affect the viticultural behaviour and wine character of two important cultivars (Cabernet Sauvignon and Sauvignon blanc) in the Stellenbosch Wine of Origin District, to use an appropriate methodology to identify viticultural terroirs in this district based on these criteria and with the use of a geographic information system, and finally to use these viticultural terroirs to identify denominations of origin within the same area. A terroir can be defined as a grouping of homogenous environmental units, or natural terroir units, based on the typicality of the products obtained. Identification and characterisation of terroirs depends on knowledge of environmental parameters, the functioning of the grapevine and the characteristics of the final product. Field studies, resulting in point data, are necessary to investigate the functioning of the grapevine but in order for this information to be of use within zoning studies it must be placed in a spatial context. As a first phase in data acquisition, the Stellenbosch Wine of Origin District was characterised and natural terroir units were identified using existing digital data and a geographic information system. A natural terroir unit (NTU) can be defined as a unit of land that is characterised by relatively homogenous topography, climate, geological substrate and soil. A total of 1389 NTUs were identified in the Stellenbosch Wine of Origin District (84 537 ha). The identified NTUs were homogenous with respect to terrain morphological unit, altitude, aspect and soil type. Each of the identified units was further described with respect to the extent of the expected sea breeze effect and, for certain of the soil types, the associated parent material. As a second phase of data acquisition, a network of plots of Sauvignon blanc and Cabernet Sauvignon were delimited in commercial vineyards in proximity to weather stations and their viticultural and oenological response monitored for a period of seven years. Regression tree analyses were performed on the complete data set and the relative importance of the environmental and management related variables determined for each dependent variable. Excepting for scion clone, which had a high relative importance for bunch mass of Sauvignon blanc and yield to pruning mass index of Cabernet Sauvignon, no other non-environmental variable included in the analyses appeared to have a strong effect on grapevine performance and wine character. The performance of Cabernet Sauvignon was affected by the potassium content of the subsoil and the climate of the season. The performance of Sauvignon blanc appeared to be related to soil texture, wind exposure and temperature, both during the green berry growth stage and the month prior to ripening. From the results presented, it appears that environmental parameters have an overriding effect on the performance of both Cabernet Sauvignon and Sauvignon blanc, but that these two cultivars react differently to environmental stimuli. A knowledge-driven model used the rules generated in the regression tree analyses to directly classify natural terroir units with respect to expected response of Cabernet Sauvignon and Sauvignon blanc in the Stellenbosch Wine of Origin District. The natural terroir units were thus grouped into terroir units that were homogenous with respect to predicted viticultural and oenological response for each cultivar. The use of representative sites to determine the response of the grapevine to its environment is time consuming and costly and limits terroir studies to research related investigations. Vineyard managers were therefore surveyed with respect to the functioning of established Sauvignon blanc vineyards in the Stellenbosch Wine of Origin District in an attempt to obtain the necessary data. Comparison of the data generated with these questionnaires to measured data in commercial vineyards suggested that the vineyard managers were able to characterise the performance of vineyards with respect to vigour, signs of drought stress and yield. Each vineyard was mapped and the responses were linked to modelled environmental variables. Classification and regression trees were used to construct decision trees, which could be applied to environmental data in a geographic information system to determine viticultural terroirs for production of Sauvignon blanc. These terroirs, although fewer, were comparable to those generated using field data. Data gathered during terroir studies, and the identified viticultural terroirs for Cabernet Sauvignon and Sauvignon blanc, were used to revisit the boundaries of the Stellenbosch Wine of Origin District and the Simonsberg-Stellenbosch ward. Modifications were proposed based on expected wine characteristics. Boundaries for two new wards in the Helderberg basin were proposed. It was also possible to identify vineyards within a ward for the production of terroir specific wines.