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

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 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 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.

FIGURE 1. Study sites selected (areas encircled with red) over a climate band from the cooler Elgin area to the warmer Vredendal area.

The potential impacts of seasonal variability on plant response were quantified comparing 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 form tara@sun.ac.za. This article only highlights the major findings from the multiple factor analysis. The strongest relationships between blocks of variables (considering all environmental factors, plant growth, ripening and wine response) is highlighted below as some of the main factors 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.

FIGURE 2. Hourly maps transitioned into an index map for hours observed between 30 – 35°C from November to February. The map highlights darker areas where earlier flowering and harvest can be expected compared to lighter areas.

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 the coming season. Frequency analysis highlighted more observed hours between 30 – 35°C and 35 – 40°C in summer months, resulted in earlier flowering (calculated as days after 1 September) in the following season.

Flowering: Flowering tended to “set the pace” for phenology in the seasons. Flowering was the most sensitive phenological stage to climatic conditions and had a strong correlation with harvest date, with an excellent correlation in more “normal” seasons. Flowering date as days after budburst could potentially be used to predict harvest date for Cabernet Sauvignon over sites within an accuracy of a few days. Similar seasonal temperature variations were observed in a case study over a diverse climatic band for seven seasons, this included commercial sites and a variety of cultivars. Confirming the results from this four-year study, the phenological variability is more likely to be dictated by between-season variability rather than variability between localities. Figure 2 is a spatial image of the observed hours at 30 – 35°C for the 2017/18 summer ripening months (November to March), darker areas highlighting a warmer summer, therefore the areas where earlier flowering can be expected in the next season, 2018/19. The lighter areas highlight later flowering. These maps are based on observed hours at specific temperature ranges to aid management decisions. Series of hourly maps and seasonal summary maps are available at www.terraclim.co.za.

Climate X grapevine growth, ripening and wine expression

Growth: Warmer temperatures had a positive correlation with growth early in the season and negative correlations later in the season. Shoot growth tempo was significantly slower for grapevines at the cooler site compared to the warmer sites. Vigour also had an effect on the tempo of growth and final shoot length, with the low and medium vigour having had a faster tempo of growth, but shorter shoots compared to the high vigour plots. The final shoot length attained seemed to be driven by temperature and water constraints, more balanced vines were attained with moderate water constraints and moderate to warm climatic conditions.

Ripening: Moderate to high water constraints ensure the canopy fills out to have sufficient balance of vegetative and reproductive growth to allow for a good tempo of sugar and anthocyanin accumulation. The balance in the grapevine therefore could drive sugar accumulation or lack thereof and indirectly affect anthocyanin accumulation due to the co-regulation nature of the compounds. Unbalanced grapevines will either not complete sugar loading before harvest is attained, or the opposite, where the sugar accumulation could stop (“gets stuck”), hampering the phenolic development of the berry. Warmer temperatures can be buffered with low to moderate water constraints.

Wine: Climate was positively correlated with wine colour, as the total colour pigments and phenolics increased over the three growing seasons in relation to the increase in seasonal growing degree days. The results suggested that anthocyanin biosynthesis was more sensitive to atmospheric conditions than to water constraints under the given study conditions over sites. Season was the overriding factor affecting the anthocyanin profile, with the warmer seasons and sites being more closely related to the coumaroyl derivatives; while cooler sites, seasons and higher vigour levels (shaded fruits) showed more glucosides and acetylglucoside derivatives.

Season and water constraints was isolated as the primary driving factors affecting the sensory attributes of Cabernet Sauvignon. The cooler and lower water constraint sites, such as the high vigour areas, were strongly associated with herbaceous and vegetable attributes. The warmer sites and areas of medium to low vigour more associated with black fruit and prune attributes, as well as tasting sweeter.

Methoxypyrazine (ibMP) expression was driven by temperature and water constraints over sites and seasons. The summer rainfall in one season (2013/14) hampered the degradation of ibMP due to increased vegetative growth. Seasons with warmer pre-véraison temperatures (like in 2014/15), showed to be favourable for ibMP synthesis. Shorter seasons did not allow for the effective degradation. Overall the expression seemed to be more prominent in the warmer climates, as the cooler climates had cooler pre-véraison temperatures resulting in less synthesis of ibMP. The kinetics of ibMP degradation also seemed to be slower at the sites with higher percentage of course sand and silt content. This study gave some insights into the management and planting distribution of Cabernet Sauvignon in the context of ibMP expressions, especially as the pre-véraison temperatures seem to be increasing in warmer areas where Cabernet Sauvignon plantings are prolific.

The study showed the medium vigour sites ensured sufficient light in the canopy, allowing respectable tons per hectare and more favourable sensory attributes. Moderate climates with moderate water constraints seemed to produce more complex wines than extreme climates, but it can be emphasised that effective in-season management of canopy and irrigation will ensure consistency and complexity in wines.

Conclusion

The study confirmed the hypothesis that grapevine will respond to climate change and continue to do so in the expression of phenology, growth and ripening, as the grapevine’s performance are affected by the constant environmental parameters despite the differences on vineyard and site level. Season variability was prominent in driving grapevine response, the variability compelled by extreme out of the ordinary climate events. Seasonal variability may be counteracted by viticultural practices, such as supplementary irrigation to induce or allow moderate water constraints, ensuring a more balanced grapevine in the selection of trellis system, pruning and canopy management. In the context of climate change, the aim is to match the cultivar growth and ripening response to the climate, for ripening to match the cooler part of the season.

Globally, the varietal spectrum would change substantially in future since the suitability for the cultivation of a given cultivar is largely temperature driven. This study proves that the climate is warming/cooling, dependent of area and months, which emphasise the need for more semi-real time climate data that can be used within season for decision making. Frequency analysis would greatly and effectively guide decision making in this regard. Geostatistical modelling of phenology and grapevine responses using isolated driving factors as inputs, can aid as an in-season monitoring and management tool for better climatic adaptions for the future within farms and regions (www.teraclim.co.za).

Abstract

Seasonal variability is increasing in the context of climate change, it is imperative to understand the grapevine’s growing environment at all climatic scales to ensure effective adaption to an ever changing environment. Climatic features that affect key grapevine physiological processes need to be assessed on a finer scale in order to optimise distribution of a grapevine cultivar in a specific environment, based on grapevine physiology thresholds in the context of climate change. Matching cultivar and terroir requires cultivar-specific studies, together with knowledge of grapevine reaction in terms of growth and ripening when confronted with different climatic/extreme weather conditions. The plant’s reaction to seasonality of environments can be seen in phenology, a sensitive indicator of climate change. The study tests the hypothesis that the grapevine is responding to climate change through altered phenology, growth and ripening responses, considering that the grapevine’s performance is affected by global environmental parameters despite differences on vineyard and site level.

– For more information, contact Tara Southey at tara@sun.ac.za.