Sunday, March 30, 2014

A generalized, grapevine-needs-driven, viticulture architecture

Orly Ashenfelter and Karl Storchman have recently completed a Working Paper titled Wine and Climate Change, portions of which I intend to highlight on this blog in the future. My goal going forward is to evaluate their findings in relation to a viticulture baseline but no baseline was presented as part of the study. I address that deficit in this post and will build the baseline around the grapevine needs presented in the table below.

Source: Compiled from arcserver2.iagt.org

In addition to the needs listed above, the vine plant also requires a healthy environment in order to successfully complete its reproductive and vegetative cycles.

Climate

The climatic requirements for successful viticulture 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. Climate, according to Dr. Tony Wolff (Lecturer and Viticulturist, Virginia Tech) and John D. Boyer, is the average course of weather in a region over an extended period as measured by temperature, precipitation, and wind speed, among other variables (Vineyard Site Selection, Virginia Cooperative Extension).  Weather is itself defined as the state of the atmosphere at a specific point in time using the same variables as referenced in the climate definition above.  The climate of a grape-growing region will determine, to a large extent -- and all things being equal -- both the grape varieties that can be grown and the styles of wine that can be produced.

As it relates to the wine regions of the world, the ideal climates for vitis vinifera are Mediterranean and marine west-coast climates, both of which are characterized by mild, wet winters and warm, dry summers.  The mild winters promote long-term survivability of the vines (and increased quality of the juice as the vines age) and the wetness provides a reservoir of water that the vine roots can tap into during the grape maturation cycle.  The warm, dry summers provide the heat and light that are the engines of vegetative and crop growth while keeping at bay the threat of rot and flavor dilution that would accompany summer/fall rains.

Source:www.buywineonline.co.uk

Continental climates are modified by large land masses and are characterized by hot summers and cold winters.  Maritime climates, on the other hand, are modified by proximate large bodies of water which heat up and cool down at a slower rate than does the adjoining land mass.  This scientific fact results in the warming of winter winds as they blow over a warmer body of water and the warming of landside vineyards as the winds make landfall.  This warming could act to extend the growing season and minimize the potential vine impact of winter low-temperature events. On the other side of the coin, warm spring air blowing in over the still-cold water will be cooled down and will retard the development of landside vineyards, minimizing their potential for damage from spring frosts.

Two key grapevine needs are adequate sunlight and heat to allow both the fruit and the vegetative aspects of the plant to mature.  Vitis vinifera requires a minimum of 1250 hours of sunshine to provide ripe fruit. The progression of the grape through its various stages of maturity is influenced by the ambient temperature with research indicating that growth of the grapevine begins when temperature exceeds 10℃.  A measure -- growing degree days (GDD) -- has been developed to measure the accumulation of heat (as measured by temperature) in excess of 10℃ over a growing season.  Extensive research has yielded the following GDD parameters which can be used as input in vineyard site selection.

Source: Compiled from oregonviticulture.net

A map of the distribution of grape varieties by growing season temperature is shown in the figure below.

Source: Susnik, Kajfez-Bogataj, and Kurnik, GIS
assessment of Climate Warming Impact onWine
Growing Regions, Workshop on climatic analysis
 and mapping for agriculture, 2005, Bologna, Italy
The canopy is comprised of the stems, leaves and fruit cluster (s) that comprise the most visible portion of the vineyard. Viticulturists manage this canopy as a means of ensuring a uniform vine structure and promoting the flow of air and sunshine within. The thinning and positionaing of leaves, stems, and fruit clusters during the summer ensures that all canopy elements have equal access to the available sunlight and airflow. A well-managed canopy should have one grape cluster per shoot --assuming an average size of 5 to 8 ounces per cluster -- and 10 - 15 leaves per shoot in order to ensure proper ripening.

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.

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.

Mineral Nutrients

Adequate amounts of the appropriate nutrients are required to support proper growth of the grape vine, fruit development, and fruit maturity and those nutrients are obtained from the soil.  The table below shows the mineral requirements of the vine plant, the role of each mineral, acceptable ranges of each mineral in the soil, and the impact of mineral deficiency on the vine.

Source: Compiled from LGRGP.org and others

Soil composition affects the availability of nutrients for soil uptake.  Soil pH is a measure of the acidity (3.5 - 6.5) or alkalinity (7.4 - 9.0) of soil which, through its influence on nutrient solubility and micro-organism activity, affects the number and types of nutrients in the soil. Soil pH between 6 and 7 is considered optimal for vine plant growth as most of the needed nutrients and micro-organisms are available in that range.  The optimal soil type also has a moderate content of low cation exchange capability (CEC) clay (Clay minerals act as harbors for nutrients because the positive ions of the nutrients are trapped by the negative charge of the clay minerals.  The abundance and types of minerals determine whether the clay is classed as low- or high-CEC.).

Water

Water is a key player in the development and growth of the grape vine (soil component, key raw material in photosynthesis, nutrient carrier (both in the soil and in the plant), in-vine transportation vehicle, structural element of the vine). In cooler regions a vine needs approximately 500 mm water/year while the need increases to 750 mm/year in hotter climates. Key determinants as to how much water is actually delivered to the vine include vine density and soil water-holding capacity. Water can be delivered to the vine roots either as a result of precipitation or, in the areas where it is allowed, irrigation.

Air and Water Drainage

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 allows 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. Slope effects can be ameliorated by terracing, an expensive proposition both in terms of establishment and maintenance.

Soils

According to Wolf and Boyer (Vineyard Site Selection, Virginia Cooperative Extension), the best vineyard soils "permit deep and spreading root growth" and provide a moderate supply of water year-round.  Mark Chien (Soil and Site Selection Considerations for Wine Grape Vineyards, Pennsylvania State University) posits that wine grapes do best in moderately fertile soils that are unsupportive of vigorous vine growth.  What are the soil characteristics that will permit "deep and spreading root growth" and year-round access to water?  Those characteristics are presented in descending order of importance in the table below.


The most important requirements, according to the table, are internal water drainage and water-holding capacity.  Geologic permeability (the capability of a porous rock or sediment to permit the flow of fluids through its pore spaces -- Dictionary.com) is seen by Wolf and Boyer as perhaps the most important consideration in a candidate vineyard's soil.  Chien sees well-drained soils as a  common denominator among all great vineyard sites.  These soils "strike a balance between adequate depth and drainage and water-holding capacity" and vines deployed therein will have adequate water access during the summer and can rapidly drain water from the soils in the event of rainfall during the grape-ripening period.  Vineyards sited on convex land patterns are preferable to those on concave landforms in that the former shed surface water while the latter import water as well as soils which erode from higher ground.

Vineyard soil fertility is one of those cases where more is not necessarily better. Adequate amounts of the appropriate nutrients are required to support proper growth of the vine, fruit development, and fruit maturity.  Fertile soils are generally rich in organic material and moisture.  In that grapevines are naturally vigorous, vines grown in highly fertile vineyards will produce abundant canopies and fruit but the fruit will be mediocre because of limited access to the sun and the vine having spread its resources too thinly. Most high-quality vineyards are sited on low-/moderate-fertility soils.

The next soil feature mentioned in the table is effective rooting depth.  The roots of the vine plant: i) anchor the vine; ii) absorb water and nutrients; iii) store nutrients that nourish the plant during dormancy; and iv) produce hormones that control plant functions. The vine deploys a three-part root structure to meet these varied needs.  First, quick-growing, short-lived roots deployed close to the service are tasked with moisture collection. Second, subterranean roots provide the anchoring function.  The principal roots are tasked with nutrient delivery and storage.  According to UCDavis, about 60% of the root structure of a vine plant can be found in the first two feet of the surface but individual roots can grow as deep as 20 feet depending on soil permeability, the level of the water table, and the rootstock variety.

Soil texture refers to the nature, size, shape, orientation, and arrangement of particles.  In the soil-type page I show that sand, silt, and clay have standalone properties which are transformed when the soils are combined.  Clay forms flexible elastic bridges between soil particles to maintain soil structure and preserve porosity.  Pebbles and rocks in clay-rich soils break up the soil, providing pathways for water and root penetration.  Deep, rich soils will provide high-vigor growth and large, watery grapes.

Soil pH is a measure of the acidity (3.5 to 6.5) or alkalinity (7.4 to 9.0) of soil which, through its influence on nutrient solubility and micro-organism activity, affects the number and types of nutrients in the soil.  Soil pH between 6.0 and 6.8 is considered optimal for vine plant growth as most of the needed nutrients and micro-organisms are available in that range.  Alkaline or acidic soils can be treated to bring them closer to optimal.



Click here to see my page on soil types of interest to viticulturists.

Pest- and Disease-Free Environment

The vineyard, left to its own devices, can become a haven for a wide range of pests and diseases with the potential for degradation of the quantity and quality of fruit produced. According to grapegrowingguide.com, the most effective means of combating diseases are (i) a good canopy management program and (ii) a rigorous preventative fungicide treatment program. A good canopy structure allows air circulation between the canopy components and rapid drying after rain or dew, or as a result of low humidity. Such an environment is less favorable for the development of fungal disease.

Establishment and Management of the Vineyard

A variety of factors will be considered in selecting the varieties to be planted in a new vineyard: (i) experience of other vineyards in the area; (ii) the quality of the variety; (iii) the sped at which the variety completes its annual reproductive and vegetative cycles and how do those cycles match to the climate of the region; (iv) the yield potential; (v) adaptation to climate; (vi) adaptation to soil conditions; (vii) its resistance to disease; and (viii) the varieties that are allowed by existing legislation (in an area where that is a concern). The common wisdom holds that varieties produce at their best when grown at the coolest margins of viable ripening.

Grapevines are selected either through mass or clonal selection and cuttings are grafted on to rootstocks based on the attributes required.

It is important that there be a balance between the vine root system and its canopy. In that regards, vines should be planted with higher density in poorer soils and less-densely in fertile soils. Many of the high-quality European vineyards are planted at between 5,000 and 10,000 vines/ha.

Vine pruning and training allows the grape grower to overcome the natural tendencies of the vine and force it to produce fruit that is more suitable to the production of quality wine. Canopy management techniques provide the berry with more or less access to the sun or protection from the elements as required.  For example, a significant challenge to Santorini viticulturists is the stiff wind that buffets the island during the growing season and could damage the berries if they were exposed to the elements. The solution that has been employed for eons is to (i) eschew vine density and (ii) train the vines such that they can afford protection to the otherwise vulnerable berries. Vine canes are intertwined and trained into a circle and the berries grow within this protective cordon. The circular structure can be positioned above ground or in a below-ground hollow where the top of the vine is parallel to the surface. A viticulturist has a wide variety of trellising and vine-training techniques available for deployment based on requirements.

©Wine -- Mise en abyme

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