August 2017 Issue of Wines & Vines

Understanding Terroir Expression

How to optimize vineyard-management practices

by By Cornelis van Leeuwen, Jean-Philippe Roby and Laure de Rességuier

Terroir is a cultivated ecosystem in which grapevines interact with soil and climate. The main climatic parameters include temperature, rainfall and reference evapotranspiration. Vine phenology and grape ripening is mainly driven by air temperature and soil temperature. The soil provides water and minerals to the vine, particularly nitrogen.

In the past several decades, tools have been developed to quantify terroir parameters. Small-scale weather stations can yield temperature data at high resolution, which can be used to provide refined maps of temperature summations. Models have been developed to predict phenology in relation to temperature. Vine water status can be assessed with a pressure chamber, or by means of carbon isotope discrimination measuring grape sugar (so-called δ13C).12 Vine nitrogen status can be assessed with measurement of yeast assimilable nitrogen (YAN). In this way, terroir parameters can not only be measured but also mapped.

This approach allows precise vineyard management to optimize terroir expression through plot selection, choice of appropriate plant material in relation to soil and climate, vineyard floor management, fertilization and training system.

Defining terroir
Wine quality depends on the environmental characteristics of where the grapes are grown. Many factors are involved such as climatic conditions, soil composition (geology, soil type and soil depth) and topography. All these factors act simultaneously, and they interact. If each terroir factor is studied separately, studies remain highly descriptive and fail to explain why wine shows such extraordinary sensory diversity.

Moreover, terroir factors like soil and climate are complex and need to be broken down to enable studying their impact on vine development, grape ripening and wine composition. This multidisciplinary approach is the basis of Gérard Seguin’s definition: “Terroir is a (cultivated) ecosystem, in a given place, in which the vine interacts with the natural environment and, in particular, the soil and the climate.”8

Human factors also play an important role in terroir expression.11 In this text, major factors involved in terroir expression are addressed at different scales. They can be managed by the appropriate choice of plant material and viticultural practices in order to optimize terroir expression in winegrowing areas with a wide range of climatic conditions and soil characteristics.

Major environmental factors involved in terroir expression
Among the environmental factors acting on vine development, phenology, grape ripening and wine composition, three are of major importance: air and soil temperature, vine water status and vine nitrogen status. Each can be measured at different scales. They can also be mapped, which is the ultimate tool to implement fine-tuned management at the parcel scale, or even at the intra-parcel scale.

Air and soil temperature
Vine development and phenology are closely related to air temperature. High temperatures trigger early phenology. Timing of the ripening period is critical in the production of terroir wines. If ripeness occurs too late (after Oct. 15 in the Northern Hemisphere), grapes may struggle to reach full ripeness, particularly in regions where the temperatures drop quickly during October. In those situations, wines made from grapes harvested after Oct. 15 may be acidic and green. If ripening happens too early (in July or August), the timing of technological ripeness (sugar/acid balance), aromatic ripeness and phenolic ripeness in grapes are uncoupled. Wines produced under these conditions are unbalanced while they generally lack freshness and aromatic complexity. Hence, the ideal window to reach full ripeness is approximately Sept. 10-Oct. 15 in the Northern Hemisphere.11

To a lesser extent (compared to air temperature), the timing of phenology is also influenced by soil temperature. A warm soil can speed up phenology by approximately one week. Warm soil can be an asset in a cool climate, while it can have an undesirable effect on early ripeness in a warm climate.

Vine water status
Vine physiology is heavily impacted by vine water status. Vine water deficit provokes shoot growth cessation and limits berry size. As long as it remains moderate, it also enhances the accumulation of grape skin phenolic compounds. These conditions favor the production of high-quality red table wines. When water stress is excessive, it can impair photosynthesis and provoke stuck-ripening.

Vine water status depends on many factors: soil water-holding capacity, rainfall, reference evapotranspiration (ET0), grapevine variety, rootstock and training system (in particular leaf area). Hence, it is under the combined influence of climate, soil, plant material and training system.

Vine nitrogen status
Vines absorb nutrients from the soil. Lack or excess in nutrients can impair vine physiology. However, it has been shown that most nutrients and soil minerals do not play a major role in terroir expression, with the exception of nitrogen.10

Vine nitrogen status impacts yield, vigor, shoot growth, berry size, grape acidity, grape sugar content, skin phenolics and grape aroma compounds. Moderate to low vine nitrogen status favors the production of grapes with high quality potential for red winemaking (small berries and high grape skin phenolics). For high-quality white wine production, vine nitrogen status should be at least moderate, because low nitrogen status can impair the production of aroma compounds, in particular those from the volatile thiol family.5

Measurements of terroir parameters
In the past several decades, many tools have been developed to measure major terroir parameters. Some of these are easy to implement and can be used at high resolution. They create the opportunity to produce maps, which are very useful in precise vineyard management to optimize terroir expression.

Soil mapping
Soil maps are produced with auger sampling and soil pit studies. Their precision can be significantly improved when soil electric tomography is measured beforehand.9

Assessment of climatic parameters
Climatic parameters are measured with weather stations. Most winegrowing regions dispose of long-term climate data series, which allows studying the vintage effect on grape composition and wine quality and to assess possible long-term trends in climate evolutions.1 However, few studies address climatic variability within winegrowing regions, which can potentially have a great impact on terroir expression.

Recent miniaturization of temperature sensors, shelters and data loggers allow measuring temperature variability at a very fine scale.6 The impact of temperature on vine phenology can be assessed with the Grapevine Flowering Véraison model2 for a wide range of grape varieties.3 Coupling fine-scale temperature maps to these phenology models can help growers to optimize the choice of plant material in relation to local temperature variability.

Assessment of vine water status
Many tools have been developed in recent decades to measure vine water status. Two are of particular interest because they are precise and easy to implement at reasonable cost: measurement of stem water potential in the field or assessment of carbon isotope discrimination on grape juice at ripeness (so-called δ13C).12

Stem water potential is measured with a pressure chamber on leaves, which are bagged with an opaque plastic bag close to solar noon (around 2 p.m.). Repeated measurements provide a very precise assessment of the variations of vine water status in the growing season in relation to climatic (rainfall and evapotranspiration) and soil-related (soil water-holding capacity) parameters. The drawback of this tool is that the number of parcels that can be monitored is limited, which makes it not suitable for spatial representation of vine water status.

The δ13C can be measured from grape juice in labs specialized in stable isotope analyzes for between $30 and $50 per sample. It represents the average water deficit of the vine during the first weeks after véraison, when grapes accumulate sugar. Although it does not give precise information on the timing of water deficits, it allows evaluation of vine water deficit intensity in many locations, opening the possibility of creating very precise maps showing variability in vine water status at the intra-block scale. See “Mapping Vine Water Status.”

Assessment of vine nitrogen status
Plant-based measurements are, by far, more powerful compared to soil-based measurements to assess vine nitrogen status. Among these, yeast assimilable nitrogen measured on grape juice at ripeness is an easily accessible indicator at low cost. With high-density sampling (10 samples per hectare), very precise maps can be obtained showing vine nitrogen status variability at the intra-block level. See “Vine Nitrogen Status Map.”

Managing terroir parameters
Site selection:
Site selection is a major tool in terroir management. The best vineyard sites for production of terroir wines are located in moderate to cool climates. In warm climates, cooler locations can be found in north-exposed slopes (in the Northern Hemisphere) or at high altitudes. In cool climates, south-exposed slopes might be the best locations to obtain full ripeness.

Warm (dry, stony and shallow) soils can, to a certain extent, compensate for a cool climate.4 Moderately dry climates are best suited for production of high-quality wines. Low soil water-holding capacity (SWHC) can compensate in a wet climate, while at least moderately high SWHC is preferable in dry climates.

Choice of plant material: The choice of plant material is a major tool to optimize terroir expression. Early ripening grape varieties should be planted in cool climates and late-ripening varieties in warm climates in order to obtain ripeness during the ideal window (Sept. 10-
Oct. 15 in the Northern Hemisphere).
Timing of ripeness can also, in a more limited way, be influenced by rootstock selection. In dry conditions, only drought-resistant grape varieties should be planted (Grenache, Carignan and Cabernet Sauvignon), and drought-sensitive grape varieties should be avoided (Merlot and Sauvignon Blanc). Planting with drought-resistant rootstocks such as 110R is a very cost-effective and environmentally friendly way to adapt vine growing to dry climates.

Vineyard floor management and fertilization: When nitrogen availability of the soil is not well adapted to the type of production, vine nitrogen status can be managed by either cover crop (reduction in vine nitrogen status) or fertilization (to obtain a higher nitrogen status).

Training system: In cool and wet climates, leaf area per hectare should be high to optimize both light interception and vine transpiration. High-density planting or divided canopies are accurate ways to obtain high leaf areas. In warm and dry climates, leaf area should be limited on a per-hectare basis to limit transpiration. Mediterranean bush vines (gobelet) are perfectly adapted to those conditions. Because leaf area to fruit weight ratio is an important quality factor, those vines cannot be high yielding. High yields can only be obtained in dry climates through irrigation. However, irrigation may raise issues around water resource management and salt stress in vines.

Many environmental factors are involved in terroir expression. The effects of climate and soil are complex, and they should be divided into measurable parameters such as temperature and water status. To understand their impact on grape composition and wine quality, these must be ranked in order of importance.

Climate primarily acts through temperature, rainfall, Evapotranspiration and sunlight, while soil acts through its impact on water status and nitrogen availability. Temperature, vine water status and vine nitrogen status can be accurately measured at high resolution. This opens up the possibility for fine-tuned vineyard management in order to optimize terroir expression.

Among available tools, the choice of plant material adapted to local conditions (grapevine variety and rootstock), vineyard floor management, fertilization and training systems allow growers to produce high-quality wines expressing distinctive terroir characteristics in a wide range of environmental conditions.

This paper was originally presented at the XIth International Terroir Congress coordinated by Greg Jones and held in Oregon’s Willamette Valley.

Cornelis (Kees) van Leeuwen is professor of viticulture and head of the viticulture and enology department at Bordeaux Sciences Agro. His papers are available at Van Leeuwen is also editor of the scientific journal OENO One and consultant for Château Cheval Blanc in Saint-Emilion, France.

Jean-Philippe Roby is senior lecturer and researcher at Bordeaux Sciences Agro. His specialization is vineyard management and plant material. He is publishing director of the OENO One scientific journal and consultant for RdV Vineyards in Virginia.

Laure de Rességuier is lecturer and researcher at Bordeaux Sciences Agro researching fine-scale climate variations in vineyards and their impact on vine development and grape ripening. She specializes in geographic information systems and remote sensing.


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