Terroir Studies in Washington and Wisconsin American Viticultural Areas

The concept of terroir has been evaluated since the 12th century, when Cistercian monks from Burgundy realized that the physical environment in which grapes are grown has a major influence on the character and quality of the resulting wine. These environmental conditions affecting grape and wine quality are known as terroir in viticulture and have become increasingly important in the grape growing and wine industry. In this dissertation, three studies investigating the terroir of vineyards located in Washington and Wisconsin American Viticultural Areas (AVAs), which are defined by the Alcohol and Tobacco Tax and Trade Bureau (TTB) as delimited grape-growing regions having distinguishing features and defined boundaries, are presented. The research objective was to understand and evaluate the interplay of the environmental factors that influence the character and quality of grapes and wines produced in vineyards from Washington and Wisconsin AVAs. These two AVAs have drastically different climates, with the Washington site hosting a xeric to aridic soil moisture regime, and the Wisconsin site having an udic soil moisture regime, allowing for the assessment of climate influence on the interrelated properties of the soil and vine. In Chapter 2, the terroir of historic Wollersheim Winery, the only winery within the confines of the Lake Wisconsin AVA, is examined to understand the interplay of environmental factors influencing the character and quality as well as the variability of Wollersheim wines. Soil texture, chemistry, and mineralogy in conjunction with precision viticulture tools such as electromagnetic induction and electrical resistivity tomography surveys, are utilized in the Wollersheim Winery terroir characterization and observation of spatially variable terroir at the vineyard scale. Establishing and comparing areas of variability at the plot level for two specific vineyard plots (Domaine Reserve and Lot 19) at Wollersheim Winery provides insight into the effects of soil properties and land characteristics on grape and wine production using precision viticulture tools. In Chapter 3, the source of water acquired by grapevines during the critical phenological stages of the 2015 growing season is evaluated using stable isotopes oxygen and hydrogen in water at Wollersheim Winery’s vineyard plot Lot 19. The stable isotope analyses of source and vine water provide insight into seasonal water use, vine water uptake processes, and active rooting zones and the role of water in the vine ecological and physiological processes during the growing season to support development of efficient irrigation strategies while managing grape and wine quality. The seasonal vine water use trend supports a variable functional rooting depth for water acquisition by the vines during the different phenological stages, with rain water as a preferential source during the spring months and a root system developed to reach for water progressively deeper as the growing season advances as greater storage of water is found in deeper soil in the autumn months. Chapter 4 presents a regional scale reconnaissance survey that examines the relationships between soil characteristics and the phenolic compound concentrations of Syrah grapes collected during the 2014 harvest from 11 vineyards planted across four different terroirs in the Walla Walla Valley AVA in an assessment of the useful application of the terroir construct at this scale. Soil properties, including drainage, depth, available water-holding capacity (AWC), texture, bulk and plant available chemistry, and mineralogy in conjunction with concentrations of grape phenolic compounds, including tannin, polymeric and total anthocyanins, quercetin glycosides, and catechin are assessed to explore the link between vineyard soil and grape chemistry. The relationships between soil characteristics and phenolic compounds of Syrah grapes from the Walla Walla Valley AVA vineyards, generally indicate that although the four terroirs have distinctly different soil properties, the grape phenolic concentrations reveal only subtle variations; overall, these minor differences show higher concentrations of phenolics may be associated with vineyards that feature soils exhibiting the influence of basalt from weathered basalt bedrock or basalt-derived alluvium. The study demonstrates that large scale characterizations at the AVA level can be limited by the variability of soil properties at the vineyard level