Terroir studies usually focus on wine character or style rather than quality, but a good terroir is considered to be one that ensures a slow but complete maturation of grapes with a certain regularity in quality of the product from vintage to vintage (Seguin, 1986).
Why is it important to study natural terroir units
As seen above, the identification of NTUs will form the first step of any terroir study that, in turn, is important for, inter alia, the following reasons:
The identification of viticultural terroirs is considered to be of international importance and the Office International de la Vigne et du Vin (OIV) has passed a resolution (Anon., 1993) requiring terroir identification and characterisation in all wine producing countries.
The concept of terroir stresses that winemaking begins in the vineyard. Although there are many factors affecting the final wine character and style, a stable set of environmental features (the terroir) forms the basis of the viticultural ecosystem.
There is growing interest in the origin of wines and in single vineyard wines. The identification of terroir units with specific application to viticulture is an important step in meeting the consumer challenge and penetrating an important market. Terroir studies are, therefore, of commercial interest.
Demarcation of areas of origin should be based on the terroir concept. Terroir studies are important to provide a scientific basis to such a system in order to maintain its integrity.
Terroir studies enable producers to better understand their vineyards and to improve their product. A map of NTUs is of greater use for planning purposes than a soil map alone, as it combines all the factors having an effect on viticultural production.
How does the identification of natural terroir units aid the wine producer
In order to better understand how a map of NTUs can be of use to the wine producer, it is necessary to look at the interaction of each of its components with wine quality and character.
Topography: The effect of topography on temperature variability (above and below ground) can be considered to be one of the main factors affecting the quality of grapes (Gladstones, 1992). Topographic effects on climate can be indirect, due to drainage, exposure to wind, drainage of cold air, or direct, due to the immediate effects of the change in the incidence of the sun’s rays on the earth’s surface (Crowe, 1971).
Altitude, aspect and inclination of the slope are of the most important landscape attributes affecting the climate of the vineyard (mesoclimate) (Dumas, Lebon & Morlat, 1997). Increasing altitude tends to result in decreasing temperature (1C decrease per 100 m increase for dry air; this value will be lower for water saturated air; Preston-Whyte & Tyson, 1988), but this effect can be alleviated by an increase in radiation, warmer soil surfaces, poor ventilation and movement of cold and warm air (Dumas et al., 1997; Gladstones, 1992).
Slope aspect affects temperature via sunlight interception, as well as exposure to winds and rainfall (Schultz, 1997). Topographic variability results in an open or closed landscape, affecting air movement in an area and sunlight interception (Lebon, 1993). Terrain morphology, due to its constituents of slope inclination and slope shape, affects temperature variability and soil water drainage. Convex landscape forms will generally result in less day-night temperature variation in comparison to concave forms (Branas, Bernon, & Levadoux, 1946), while concave slopes often result in accumulation of soil moisture and nutrients at the foot of the slope (Schultz, 1997).
Climate: A variety of mesoclimates can be identified in a hilly or mountainous region as a result of topographical effects on various climatic elements. Large bodies of water also have a modifying effect on temperature due to their temperature inertia, resulting in the reduction of both the diurnal temperature range and the contrast between minimum and maximum temperatures (Gladstones, 1992). Temperature is probably one of the most important parameters affecting the grapevine as it has an effect on almost every aspect of the vine’s functioning (Coombe, 1987). High temperatures will result in slightly higher sugar contents, up to a temperature threshold. Malate contents will be lower, while tartrate contents will be little affected. Potassium contents will increase, affecting the wine pH. The effect of night temperatures on anthocyanin synthesis appears to be partially dependent on the contrast between day and night temperatures, with day temperature playing the preponderant role (Kliewer & Torres, 1972). Optimum mean temperature for pigment formation is in the region of 20-22C (similar pattern for phenolics) and a mean temperature range of 20-22C in the month of ripening is optimal for physiological ripening in grapes and for the synthesis of colour, flavour and aroma compounds (Gladstones, 1992).
Relative humidity has an effect on the photosynthetic rate when the soil water supply is limited (Champagnol, 1984) and it has been shown that low relative humidity values and high temperatures result in high berry pH values, as well as reducing the growth and yield per unit water transpired (Gladstones, 1992). High relative humidity values can, however, increase disease incidence.
Wind has both positive and negative effects for viticulture. Strong winds in spring and early summer can injure new growth and young bunches, as well as reducing fruit set. Moderate winds of higher than 3-4 m.s-1 can result in closure of stomata in the leaves resulting in inhibition of photosynthesis (Hamilton, 1989). Air circulation, however, prevents high relative humidity and excessively high temperatures from developing in vine canopies. In areas where the soil has a potential for high vigour, strong winds may be conducive to quality by limiting the vegetative growth of the vine.
Geology: There is little literature on the relationship between geology and wine, although a paper on this subject has recently been published in the Wynboer (Wooldridge, 2000). Geology forms an integral part of the eco-geo-pedological sequences used by Morlat (1989) and Lebon (1993) in their identification of basic terroir units in the Loire Valley and Alsace, respectively, and is considered by Dubos (1984) to be one of the most important static components of the terroir complex affecting character and quality of the final product. According to Hancock (1994), geology potentially has both direct and indirect effects on wine character and style and includes the effects of the underlying parent material, the resulting soil physical and chemical characteristics and variations in topography. It contributes to the physical properties of soils, affecting in turn the water supply to the vine. Underlying rocks can also significantly affect water supply to the vine, depending on soil depth and penetration by the vine roots. Soils originating from different parent material often have distinctive chemical compositions. Van Schoor (2001) proposed a possible effect of minerals, e.g. quartz and clay fraction kaolinite. However, the most significant effect of geology appears to be through its contribution to soil physical properties (Seguin, 1986).
Soil: The effects of soil on wine character and style are probably one of the most widely debated topics in viticulture. Soil has a definite effect on the quality of wines under the same climatic conditions but the effect is not consistent over seasons, indicating an inter-relationship between soil and climate (Saayman, 1977; Conradie, 1998). Although soil characteristics such as soil colour, temperature and chemical composition definitely play a role in the effect of soil on the growth pattern of the vine and, consequently, wine character and quality, the most convincing indications are that the main effect of soil type is through its physical properties and, more specifically, through the regulation of the water supply to the grapevine (Seguin, 1986). This must be considered in conjunction with the meso- and seasonal climate. Deep soils without chemical or physical restraints for root development will promote a well-developed root system with a high degree of buffering against climatic extremes (Van Zyl & Van Huyssteen, 1979) and contribute to constancy of the product across vintages, irrespective of the seasonal climate – the hallmark of a good terroir. But, not only does soil temper climatic extremes (such as drought and high temperatures), climate and climate-dependant factors are some of the most important parameters affecting the formation of soil (De Blij, 1983). Soil distribution can, therefore, often be related to landscape positions within a certain geological formation.
From the above discussion it is clear that the NTUs integrate environmental factors affecting wine character and style and that it is impossible to determine the effect of one factor without taking the other components into account. A map of NTUs is, therefore, a vital tool for a producer/ co-operative cellar/viticultural consultant to aid in the understanding of the reaction of the vine to the environment, and the style and character of the resulting wine. By linking existing block data and knowledge based on personal experience to the map, it becomes a powerful predictive tool for planning of new vineyards, as well as for planning agricultural practices. It will also aid a winemaker in his planning of the harvest and in his decision about which grapes to combine for fermentation.
The example of the Bottelaryberg-Simonsberg-Helderberg winegrowing area
A map of natural terroir units has been compiled for an area of approximately 25 000 ha to the west and south west of Stellenbosch (Fig 1).
The relatively complex topography of this area and its proximity to the Atlantic Ocean, with the resulting interplay of sea and land winds, provide many different environments for viticulture (Carey, 2001), which are well represented by the identified NTUs. These NTUs integrate aspect (taking radiation differences and effects of dominant winds into account), altitude, terrain morphology (crest, midslope, footslope, valley bottom), broad soil description and geological parent material. These factors provide a wealth of environmental information and are an important basis for studying the environmental effects on the vine phenology and production and wine character. As mentioned above, NTUs form a platform for further studies on the interaction between the environment, the vine and the resulting wine and a database of viticultural, climatic and wine related data. This goal is pursued on plots situated in close proximity to a weather station and on various landscape positions. Results from these studies will form the topic of future publications.
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* Sections of this paper presented in partial fulfilment of the M.Sc. Agric. (Viticulture) degree at the University of Stellenbosch
About the Authors:
V A Carey1*, E Archer2 & D Saayman3
1. ARC Infruitec-Nietvoorbij, Stellenbosch.
2. Department of Viticulture and Oenology, University of Stellenbosch, Matieland.
3. Distell, Stellenbosch.
*Present address: Department of Viticulture and Oenology, University of Stellenbosch, Matieland. E-mail: firstname.lastname@example.org.