Targeting Vineyard Management Practices to Combat Early Onset of Ripening Caused by Climate Change

For winemakers, these types of physiological shifts can have significant consequences on the wine created from the grapes of earlier maturing grape vines.  The alterations in the physiological and enological characteristics of the berries may increase overall alcohol content and the acid:sugar balance.  Typically, sugars accumulate in the berries after veraison in a sigmoidal pattern with specific characteristics, which can change and become off balance with earlier ripening.

The study presented today aimed to address this issue, and specifically the cause of earlier maturity/ripening in grapes.  They addressed whether the change in grapevine maturity was associated with the increase in ambient temperatures over time, a higher rate of sugar accumulation in the berries, or a combination of the two phenomena.  They noted it is important to determine this piece of information in order to properly manage a vineyard in a time of changing climate, particularly if no action would result in a finished wine with undesirable sensory characteristics.

Methods

Using Southeastern Australia as their region of study, the authors collected data from 15 successive vintages and three climatically different regions: Coonawarra was chosen as the cool region; Riverland was chosen as the hot region; and Barossa Valley was chosen as the intermediate region. 

Daily climate data were obtained for these three regions; including maximum, minimum and average temperature, reference evapotranspiration, solar radiation, and the rainfall and vapor pressure deficit at the maximum and minimum temperatures.  Climate data were averaged for a vintage season (from September to April in Coonawarra, from September to March in Riverland and Barossa Valley).

Time trends were calculated using linear regression, which gives a coarse indication of trends in a relatively short time span (15 years).  Averages for each climatic variable were calculated for both the 5 hottest and the 5 coolest vintages in the 15 year span.

Chardonnay, Cabernet Sauvignon, and Shiraz were the varieties studied.  Berry samples were collected once a week during the ripening period, resulting in a minimum of four samples per block before harvest.  The number of blocks with samples collected ranged from 117 in the Coonawarra Chardonnay to 5500 in the Barossa Valley Shiraz, with a total of 2395 and 24,196 observations, respectively.  In order to account for the natural variation of a vineyard, 30 clusters per block from both exposed and shaded sections were randomly sampled.

Total soluble solids (TSS) were measured on site for the Riverland samples.  Grapes were macerated in a plastic bag after collection, and the juice samples were then decanted for sugar analysis using an analogue refractometer or hydrometer.  For the Coonawarra and Barossa Valley samples, juice was collected by using a roller crusher and analyzed using a FOSS WineScan FT120.

Ripening onset and rate of increase of TSS was calculated using derived mathematical equations that had been previously established, but with minor alterations.

Results

Climate Trends

• Maximum temperature across all three regions increased at a rate of 0.1^oC/year, with no detectable trend for the minimum temperature.
• Average temperature across all three regions increased at a rate of 0.06-0.08^oC/year.
• Solar radiation increased in Coonawarra and Riverland, while reference evapotranspiration increased in Riverland.
• Vapor pressure deficit at maximum temperature increased in all three regions, and vapor pressure deficit at minimum temperature increased at Coonawarra and Riverland.
• The difference in maximum temperature between the five hottest and the five coolest vintages was 1.7^oC.
• The difference in minimum temperature between the five hottest and the five coolest vintages ranged from 1^oC in Coonawarra and 1.5^oC in Riverland.

 Ripening Traits and Associations with Climate

• The onset of ripening was closely associated with temperature.

o   Onset of ripening changed at a rate of -8.7 days/^oC.

o   The association of onset of ripening and average temperature was significant for all varieties and the pooled data.

•       The rate of increase of TSS ranged from 0.10^oBé/day for Chardonnay in Riverland to 0.19^oBé/day for Shiraz in Barossa Valley.

o   For the pooled data, this rate was not associated with average temperature.

o   There were some specific cases of increased rate with increased temperature, with a significant association occurring with Cabernet Sauvignon.

•       For the pooled data, the rate of increase of TSS was directly associated with vapor pressure deficit.
•       The threshold time when the grapes reached a maximum concentration of TSS increased at a rate of 10.7 days/^oC.
•       Maximum concentration of TSS ranged from 12.7 to 15.6^oBé, and was not associated with temperature.
•       The duration of linear increase of TSS ranged from 30 days for Riverland Cabernet Sauvignon to 60 days for Coonawarra Shiraz, and was not associated with temperature.
•       Across all varieties, vintage type, and region, grape maturity advanced at a rate of 9.8 days/^oC.

Discussion/Conclusions

The authors of this study showed that resulting from climate change, the rate of change of grapevine maturity was between -8.6 and -10.7 days/^oC, which is within the range of values determined by other studies.  

It is important to note that even though the authors primarily focused on associations of ripening with temperature changes, there are other climatic factors at play in climate change that could potentially alter the ripening rate.  In fact, the results of this study showed that not only was there a negative association between ripening and temperature, but there were also positive associations between reference evapotranspiration, vapor pressure deficit, and radiation.  Even though speaking of results in this case only in terms of temperature was an accurate determination, one must used caution when using temperature only when attempting to determine cause and effect relationships between ripening and climate change.

How much do changes in the onset of ripening really matter?

According to this study, the onset of ripening accounted for 86% of the variation in maturity and the rate for the remaining maturity.  This the onset of ripening accounts for the vast majority of the variation in maturity of the grapes, it is extremely important that we understand the causes and mechanisms behind any changes in ripening rates, from both a biological and a viticultural/enological standpoint.  Therefore, by implementing viticulture practices that target the onset of ripening, vineyards may be able to better adapt and still manage to produce quality wines with desired sensory characteristics, even when facing a changing climate.

It is important to note that these results and conclusions may only be applied to the region and conditions that were used in the study (Southeast Australia) since minute changes in terrior/region/climate have significant consequences for the onset of ripening and other enological characteristics of grapes.  It has been shown that climate change effects different parts of the globe in dramatically different ways, so more studies in all regions should be completed for a more comprehensive understanding of the issue.  More work needs to be done in other climates and regions to determine if similar trends occur, or if viticulture management practices need to be slightly altered in order for different regions to cope with region-specific physiological and enological changes due to climate change. 

I’d love to hear what you all think of this subject.  There is certainly much more to be said that I covered in this single post.  Please feel free to comment below!

Source: Sadras, V.O., and Petrie, P.R. 2011. Climate shifts in south-eastern Australia: early maturity of Chardonnay, Shiraz, and Cabernet Sauvignon is associated with early onset rather than faster ripening. Australian Journal of Grape and Wine Research 17: 199-205.

DOI: 10.1111/j.1755-0238.2011.00138.x
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!