WINETECH Technical Yearbook 2020

Impact of local site and inter-seasonal weather variability on grapevine responses (PART 4)

tionships between blocks of variables (con- sidering all environmental factors, plant growth, ripening and wine response) are highlighted below as some of the main fac- tors influencing the grapevine’s response. CLIMATE X GRAPEVINE PHENOLOGICAL EXPRESSION The seasonal variability had an overriding impact (over site differences) on timing of phenological stages and the periods between phenological stages. The time between phenological stages seem to be shifting (increasing/decreasing) from one season to another. We can continue to expect differences in each vintage, emphasising the need for adapted in- season management to ensure consistency in wine quality and style. Considering all the bioclimatic indices used to describe a season, the Winkler- and Huglin indices had the strongest relationship with phenology, and had the best correlation with the absolute dates of flowering and pre-véraison. The higher the indices values (warmer season), the earlier flowering and pre-véraison over the different sites was noted. Higher resolution data highlighted temperatures throughout the growing season affected flowering date and duration, with the exception of August and September. Summer months (December, January and February) has a stronger effect on flowering date for

This article highlights findings from a study that has tested the hypothesis: The grapevine is responding to climate change through altered phenology, growth and ripening responses. The study used multiple factor analysis to evaluate the interaction of environmental factors (over seasons) on grapevine phenology, growth, ripening and wine attributes at various sites and vigour levels. With the aim to identify the possible driving factors influencing the grapevine’s response that can be used in future management. Study area consisted of six heathy Cabernet Sauvignon vineyards monitored in three well-known wine producing regions, namely the Cape South Coast, Coastal and Olifants River Regions (figure 1), representing a range of climates in the Western Cape. Within each Cabernet Sauvignon vineyard, three grapevine vigour plots (namely, high, medium and low) were selected using infrared aerial photography (12-15 vines selected) and monitored over the four growing seasons. The potential impacts of seasonal variabil- ity on plant response were quantified com- paring the standard indices with frequency analyses for the growing seasons analysed at plant level. Details of the methods used for data collection and processing can be requested from tara@sun.ac.za. This article only highlights the major findings from the multiple factor analysis. The strongest rela-

JULY 2020

TARA SOUTHEY: Centre for Geographical Analysis, Department of Geography and Environmental Studies, Stellenbosch University, Stellenbosch KEYWORDS: Climate, terrain, phenology, grapevine

chill units. Air temperature is one of the most important parameters affecting grapevine phenology, growth and has an effect on almost every aspect of the grapevine’s physiological functioning. Globally the timing of phenological events have advanced and periods between phenological events have shortened, resulting in an overall shortening of growing season. Matching the critical developmental phases of grapevines to a suitable climate (best-fit cultivar for a specific climate) is fundamental in the planning of any vineyard development where optimising quality is a priority.

This article links to the previous articles and fits into the overall objective to study the grapevine’s response to climate attributes from a phenology, growth and ripening perspective. The interaction of the grapevine with its immediate environment has long been a research focus in South Africa. The average annual temperature in all Western Cape viticultural regions has increased over the past five decades, consequently grapes ripen earlier. Chilling units have shifted with warmer growing regions resulting in chilling losses and cooler regions gaining

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