The IPNW has emerged as a premium European wine grape growing region with Washington State as the dominant producer. Washington is second only to California in wine grape production in the United States [U.S. Department of Agriculture (USDA), 2011a]. In 2011, nearly 44,000 acres of wine grapes existed in Washington, a 395% increase over the last 18 years (USDA, 2011b). The region hosts 13 American Viticultural Areas (AVAs) acknowledged by the U.S. Alcohol and Tobacco Trade Bureau on the basis of national or local name recognition, usage, and distinguishing features (U.S. Alcohol and Tobacco Tax Trade Bureau, 2013). Several larger AVAs contain open land currently not planted to wine grape (Fig. 1).
Climate is the determinant limiting factor in wine grape production. Growing-degree day (GDD) accumulation is one common method of reporting climate and allows comparison between different locations under similar macroclimate. GDD accumulation for wine grapes is calculated as the summation of average temperatures [i.e., (maximum temperature + minimum temperature)/2] less a threshold of 10 °C between 1 Apr. and 31 Oct. In a major U.S. wine region, five grape type categories were developed based on this index of heat accumulation (Amerine and Winkler, 1944).
In temperate climates where heat accumulation is adequate to ripen wine grapes, winter cold damage may be the limiting factor for vineyard survival. Phenology, cultivar, and temperatures preceding potentially damaging low temperatures all influence risk of cold damage (Ferguson et al., 2011). Sites with lower extreme minimum temperatures will generally be at greater risk for cold damage, which can range from loss of fruitful buds to outright death of the entire vine. The typical minimum temperature threshold at peak dormancy for most wine grape cultivars is around −23 °C (Ferguson et al., 2011). Frost-free days (FFDs), the period between the last spring and first autumn frosts (0 °C), is frequently examined in determining the suitability of an area for wine grape production (Jackson and Cherry, 1988; Wolf and Boyer, 2003). FFDs indicate growing season length and serves as a proxy of the period over which wine grapes can develop and ripen.
Topography also plays a role in site suitability. Topographic suitability relates to the physical ability to manage a vineyard (i.e., ability for machinery to safely operate on a site) and influence over mesoclimatic (subregional to vineyard scale) conditions. Slope and aspect are both readily quantified topographic characteristics. In the Northern Hemisphere, slopes with a southern aspect have higher levels of insolation, and consequently heat accumulation, and are typically considered ideal; however, wine grapes can be successfully grown on aspects that are often considered “undesirable” (Wolfe, 1999). Because of this, the degree of slope is generally given greater consideration. Moderate slopes (5% to 15%) are considered the best sites for wine grape production as they allow air drainage without hindering equipment operation (Jones et al., 2004). Sloped sites can reduce cold air pooling as they promote air drainage to alternate locations. Sites located above potential cold air pools may also benefit from additional elevation through lower daytime temperatures, which can promote fruit quality in hot regions (Gladstones, 1992). Unfortunately, slope alone cannot predict mesoclimate conditions and sites must be considered within the greater context of surrounding topography, obstructions to air flow and prevailing winds (Jackson and Schuster, 2001).
Wine grapes tolerate a range of soil conditions. Waterlogged soils retard vine growth, hinder mechanical operations in the vineyard, and favor the development of several root diseases and chlorosis in calcareous soils (Davenport and Stevens, 2006). Free-draining soils maintain oxygen concentrations near roots and facilitate moderate water stress with proper irrigation management (Foss et al., 2010). Unrestricted soil drainage to a depth of at least 2 to 3 m is recommended for vineyards in most situations (Gladstones, 1992; Jackson, 2008). Failla et al. (2004) found grapevine (Vitis sp.) roots at depths of over 3 m in soil surveys in northern Italy. Vines may grow roots to depths of 30 m or more if no impenetrable barriers are present (Keller, 2010). Only under severe water stress will wine grapes access substantial water from greater than 2 m. Shallow soils above parent material or other impenetrable barriers where root penetration is problematic are considered unsuitable for grape production and increase the likelihood of waterlogging (Foss et al., 2010; Jackson, 2008). Well-drained soils, along with greater soil depth, encourages the growth of robust, perennial root structures. While the AWC of soils in the IPNW is relatively low to moderate, directed applications of irrigation allow for consistently high-quality grape production.
Soil pH is also important in wine grape production in Washington State, as wine grape is grown on its own roots. Absorption of many nutrients for wine grape is optimal at soil pH of 6.6 to 7.2 (Meinert and Curtin, 2005). Overly alkaline soils lead to deficiencies of phosphorus, iron, manganese, boron, and zinc (Gladstones, 1992). Overly acidic soils can generate toxic levels of aluminum, copper, and manganese; induce phosphorus deficiency; restrict root growth; and lead to grapevine nutrient and soil microbial imbalances (Bargmann, 2003; Foss et al., 2010; Gladstones, 1992).
The expansion of the IPNW wine grape industry has resulted in the inability of viticulture consultants and university Extension to travel to every potential new vineyard location. Efficient remote assessment of a site is necessary to facilitate this expansion and avoid potential pitfalls of a site that need to be addressed before vine establishment. This project was designed to establish a decision support system (DSS) for wine grape production in the IPNW to help facilitate these remote assessments.
The specific objectives of this project were to: 1) establish a DSS for wine grape that includes information on common site characteristics, such as topographic, edaphic, and climatic parameters; and 2) begin preliminary evaluation of the effectiveness of the DSS to elucidate potential problematic components in wine grape production by mapping existing vineyards and obtaining qualitative perceptions of vineyard performance from experienced viticulturists.
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