GEOMORPHIC CONTROLS ON AIR TEMPERATURE VARIATION IN THE NORTHERN WILLAMETTE VALLEY AMERICAN VITICULTURAL AREA

The Willamette Valley American Viticultural Area (AVA) lies in northwestern Oregon between the Coast Range on the west and the Cascade Mountains to the east. It is a cool-climate grape-growing region, considered ideal for the cultivation of Pinot Noir. Air temperatures in vineyards in the northern part of the Willamette Valley AVA are significantly affected by geomorphology and forest shading. Air temperature variations strongly influence the rate of physiological development of grapevines and can be used to determine the specific varieties and clones that are most suited to a particular site. Twenty-four HOBO temperature data loggers were installed within 12 premiere vineyards of the northern Willamette Valley AVA in order to better understand the influence of geomorphology and shading on air temperature variations. Sites were selected to provide a broad range of elevations, slopes, aspects, and positions relative to surrounding forests. The lowest vineyard site in the study was located at 67 m elevation and the highest at 235 m. The degree to which a site was lower or higher than the surrounding topography was determined via "relative lowness coefficient” (RLC), which was computed by comparing the elevation of the site to the lowest and highest elevations within a 2.5 km radius. During the period of study the sites accumulated 193 growing degree-days (10˚ C GDD) on average for each 1° C increase in average air temperature. An average increase of 44 GDDs was associated with each 100 m of elevation gain. A maximum difference of 6° C of diurnal temperature variation was observed between the lowest and highest vineyard sites in the study area. Higher average temperatures correlated weakly with lower slope angles. There was no correlation between air temperature and aspect. Areas of vineyards proximal to forests typically experienced shading during part of the day, which lowered the average high temperature of that particular site. Proximity to forests also prevented radiational cooling, contributing to higher average lows in some areas and suppressing effects related to cold air pooling in topographic lows. Vineyards at lower elevations also experienced earlier and more intense fall frost events. Sites with RLC of less than 0.4 accumulated 81% of the total frost hours accumulated by all study sites.