The science of viticulture: Topography and site selection

Selecting the "right" site for a new vineyard is a key requirement for the eventual delivery of high-quality wine grapes to the winemaker.  The climatic requirements for this vineyard (discussed previously) include: a growing season long enough to mature both the fruit and vegetative aspects of the plant; production of sufficient carbohydrates to ripen the fruit as well as to maintain future productive potential; and an adequate supply of water.
The physical characteristics of a potential site can serve to enhance/retard climate effects while also serving, in their own right, to meet specific needs of the grapevine.  These physical characteristics -- often referred to as topography -- are elevation, slope, and aspect.  Each will be discussed in turn in this post.

Elevation

Elevation can be discussed either within the context of a specific location -- high point versus low point -- or in absolute terms -- feet/meters above sea level.  Regardless of the reference point, however, elevation can have a significant impact on vineyard temperatures; especially if the vineyard is located in a hilly or mountainous area.

Planting at or near the highest feasible points in the vineyard would allow the viticulturist to meet the grapevine's need for good air and water drainage.  Cold air is heavier than warm air and will flow downhill to replace the warm air as it rises.  This air movement will cause the cold air to pool in areas of low elevation and can result in the formation of frost pockets.  In addition to shedding cold air, high elevations afford cooler daytime temperature during the summer and fall.  There is a point beyond which elevation becomes detrimental to the survival of the vine plant and planting at or above those levels are not recommended.  The optimal elevation range for grape vines to survive and thrive is called the thermal belt.

As in the case of air, water will flow from areas of high elevation to areas of lower elevation both on the surface and below.  This condition meets the vine's need for internal soil drainage.  Standard sub-surface water will limit the amount of oxygen available to the root system and can also destroy the small fibrous roots which are involved in the absorption of water and nutrients from the soil.

Slope

Slope is the degree of inclination of the land from the horizontal and a slight to moderate incline is desirable for air and water drainage.  Slopes in excess of 15 degrees will require (expensive) hand-harvesting of ripe fruit due to the danger of equipment rollover.   The costs of managing a high-slope vineyard need to be balanced against the style/type of wine the winemaker is after.  As has been shown in high-slope vineyards like Bremmer Calmont (Mosel) and Rüdesheimer Berg Schlossberg (Rheingau), the paucity of soil in these environments forces the vine roots deep in search of moisture and nutrients and this results in a desirable intensity of aroma, flavor, and terroir characteristics coupled with freshness.

As shown below, the effects of slope can be ameliorated by terracing, an expensive proposition both in terms of establishment and maintenance.

Aspect

Aspect refers to the prevailing compass direction in which the vineyard slope faces.  Aspect is important in that it affects the angle at which sunlight hits the vineyard and, as a result, its total heat balance.  For example, in areas with cool summers and a relatively low number of degree growing days, north-facing slopes will be facing away from the sun as it "moves" across the sky.  South-facing slopes, on the other hand, will have more direct access to the sun's rays over the course of the day.  In cool climates slopes with southern aspects (S, SE, SW) allow vines to accumulate the maximum amount of sunshine as they pursue growth and fruit maturity.

In continental climes, on the other hand, eastern, northern, and northeastern exposures are preferred.  At Quinto do Vesuvio in the Douro, for example, the slopes are primarily north-facing, allowing the grapes, according to company President Paul Symington, to benefit from the luminosity of the sun without having to deal with the long, direct sunlight (in Douro heat) associated with south-facing slopes.

Further, southern slopes warm earlier in the spring and this can result in early bud break and the potential for spring-frost damage.  On sunny winter days, the vines on south-facing slopes can warm up resulting in decreasd cold resistance and the potential for cold injury.

These, then, are the topographical issues that the viticulturist has to consider when selecting a site for a new vineyard.  We will address the soil issues in site selection in the next post.

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