As I sit here on this gorgeous 65-degree day in February (in Virginia), I can’t help but think about global warming. Regardless of how you believe it was caused, be it by anthropogenic sources or a natural cycle of the earth, I think we can all agree that the earths’ climate is changing. How does this affect viticulture and wine quality?
Studies have found that temperature affects the rate of development of the grape, which includes sugar accumulation, acid loss, and the synthesis of color and flavor components. The levels of components such as anthocyanins, phenolics, tannins, and other antioxidants affect color, bitterness, and flavor, and the ratios of which can alter the overall quality of the finished wine. Tannins will bind to pigment anthocyanins, higher levels of which will create more stable color for red wine aging and overall higher quality. In regards to consumer preferences, studies have shown consumers prefer higher levels of tannins, and in regards to anthocyanins, studies have found that there is a positive correlation between anthocyanin concentrations and price.
One major concern about global warming and climate change is that it could shift the timing of the growing season, which would ultimately decrease the quality of the finished wine from earlier grape harvests at high temperatures. Every viticultural area is different, however in warmer climates; higher temperatures may have a negative effect on the quality of the finished wine. Very little research has been done in the field; however some greenhouse studies (controlled environments) have shown that when grapevines are exposed to higher temperatures, they produce significantly lower levels of anthocyanins, while sometimes even degrading the anthocyanins that remain.
One type of grape that may be more sensitive to climate change, due to its’ ideal growing conditions, is Pinot Noir. Pinot Noir is typically a cool-climate grape, requiring approximately 1150 degree-days for maturation. It is these types of grapes; cool-climate varieties; that scientists speculate will be most affected by rising temperatures caused by climate change.
In addition to increased temperatures, it is speculated that light intensity may have an equal, if not greater effect on the development of the grape. Traditionally, light is thought to increase phenolic concentrations (including anthocyanins), however, too much exposure of light may bleach the berries, which may lead to decreased color. It is unknown whether these affects are in fact due to light, or due to temperature, and results in the literature appear to be mixed.
The overall goal of the study presented today was to collect detailed temperature and light measurements, and to analyze their effects on Pinot Noir skin composition, while providing suggestions on possible changes to vineyard management practices.
The study occurred at private, commercial vineyards in Sonoma Valley and Los Carneros American Viticultural Areas. 7 vineyards were studied in 2005, 8 vineyards in 2006, and 6 vineyards in 2007. Pinot Noir vines were at least 5 years old, planted between 1998 and 2001, and were similar clones and rootstocks. In general, rows were oriented northwest-southeast, with sites chosen to represent a large span of temperatures across an area.
There were three sites in Los Carneros AVA: one from the western side of the Sonoma edge, one from the Sonoma/Napa border, and one at the eastern edge of Napa. In the Sonoma Valley AVA, there were five sites which spanned north-south from the southern edge of the AVA to the mid-valley near Eldrige and from the foothills of the Sonoma Mountains in the west to the eastern border of the AVA with Napa. All vineyard sites spanned an east-west range of 16km and a north-south range of 12km.
Grape samples were collected in order to sample a minimum number of berries from a maximum number of vines, and to try to collect 500 berries total per vineyard. Each vineyard was divided up into five blocks, and within each block, six randomly selected vines were chosen. From these randomly selected vines, 3 clusters were randomly selected (per vine) and 5 berries per cluster were randomly selected, for a total of 450 berries per vineyard (15 berries per vine and 30 vines per vineyard). Clusters were not looked at before they were selected, in order to avoid bias. Clusters that were selected were flagged and the same clusters were measured throughout each growing season.
Berry samples were taken at full maturity and also the mid-way point between veraison and harvest. Berries were taken from different positions within the cluster.
Full phenolic extractions were performed on the grape skins and spectrophotometry was employed for analysis. The following components were measured: anthocyanins, tannins, and total iron-reactive phenolic concentrations.
For climate measurements, temperature and humidity were measured every 10 minutes using a datalogger. These readings were averaged to create hourly and daily measurements of min, max, and mean temperatures. The following climatic data were calculated: min, max, and mean temperatures, temperature range, growing degree-days, days below 0oC, days above 30oC, days above 35oC, hours above 22oC, and hours between 16 and 22oC.
Observations of different phenological time periods were created by giving a visual estimate of the percent of all clusters on each vine achieving bloom and/or veraison, and the percent of berries on three randomly selected clusters achieving the same stage. Climate statistics for the following phenological stages were then estimated: the previous fall, winter, budburst to bloom, bloom to veraison, and veraison to harvest. Bloom and veraison were determined when 50% of the vines had completed capfall or color change, respectively.
For light exposure, photosynthetically active radiation (PAR) was measured within 1 week of harvest in 2006 and 2007, and were taken within 90 minutes of solar noon on clear/sunny days. Fruiting zone PAR was measured by taking the PAR reading next to the clusters on the sun-exposed side of the vine. PAR exposure was calculated as the percentage of PAR, relative to the fruitzone PAR.
- Temperature patterns between sites were consistent.
- 2005 had a warm, sunny spring, a more cloudy summer, and a cool ripening period.
- Average dates across all vineyards for phenological stages were: March 19th for budburst, May 12th for bloom, July 27th for veraison, and September 10th for harvest.
- Phenological dates were significantly delayed in 2006 compared to 2005, while 2007 was slightly earlier (not significant).
- 2006 had a cool and cloudy spring, with a hot summer and cooler ripening period.
- 2007 had a warm and sunny spring, with a cooler summer and a warmer ripening period.
- Berries in 2007 were significantly smaller than the other years.
- Total heat accumulation was similar for all three years, though the spring of 2007 was warmer than 2006, and 2006 contained more hot days.
- There was no rootstock or clone effect, so data could be pooled.
- Sites with a previously cool fall had significantly higher anthocyanin and tannin concentrations than sites with a previously warm fall.
- Sites with a previously cool fall had significantly higher total phenolics.
- Statistical analysis indicated that several indicators of moderate heat from veraison to harvest were positively and significantly related to anthocyanin concentrations (and NOT significant for total phenolic and tannin concentrations).
- Heat accumulation in the previous fall and from bloom to veraison were negatively correlated with all measured phenolics, with the effect significantly for anthocyanins and total phenolics for hours greater than 22oC.
- Tannins were significantly increased by warm days between budburst and bloom.
- During the period of budburst to bloom, anthocyanins increased with warm days and cool nights, though it was NOT significant.
- Sites with higher growing degree-days and hot days had higher phenolic concentrations.
- Across all three years, correlations between anthocyanins and tannins were not strong.
- Anthocyanins were slightly more correlated with total phenolics.
- Tannins and total phenolics were highly correlated (as expected).
- Grape samples declined slightly in anthocyanin concentration, and showed large decreases in iron-reactive phenolic and tannin concentrations over time.
o Skin weight increased slightly over this time.
o Berry weight declined by 11% over this time.
o Therefore, the ratio of skin to total berry weight increased by 18%.
- The amount of light was highly significant in explaining the levels of all measured phenolics at harvest, and also the Brix level.
- Increasing light explained 41% and 98% of the variability in the decrease of phenolics measured, and 37% of the variability in the increase in Brix levels.
o In other words, increasing light leads to decreasing phenolics concentrations and increasing Brix.
Overall, the authors of this study found that heat accumulation during the fall/winter (postharvest) prior to the year of maturity and heat from bloom to veraison were both negatively correlated with concentrations of anthocyanins, tannins, and iron-reactive phenolics. They also found that these two time periods were highly correlated with each other. Also, the authors found a positive correlation between anthocyanin concentration and hours from 16oC to 22oC between veraison and ripening, which has been shown to be the temperature interval that is favorable to enzymatic activity related to flavor in the finished wine. Based on high variability from one study to the next, the authors suggested that the mechanism for the previous fall/winter climate to affect the fruit composition the next year is unclear, and that more study is needed for longer time periods.
Another important finding of this study was that light exposure explained more of the variability in total phenolics and tannins than the temperature. The study found that high levels of light resulted in lower concentrations of anthocyanins, tannins, and total phenolics. They speculated that it’s possible that the use of a third training wire and hedging additional shoot growth, which was applied in this vertically-shoot positioned vineyard, removed much of the shading that would have otherwise occurred.
To date, it has been noted recently in California that warmer winter and spring temperatures have been observed. Future estimates of warming indicate that there will be more warming in the summer than in the winter. Models have shown that there is the possibility of a 3.3-6.4oC temperature increase in Northern California, compared to a winter warming of 2.3-3.4oC. According to the authors, it is likely that this scale of warming could have negative effects of the quality of wine in California, which would also be reflected in the price.
Conclusions and Recommendations
This was a complete and comprehensive study in general, though I think that it should include more than just three vintages in order to determine long-term effects of a changing climate on grapevines. Also, many more vineyard sites under different climatic conditions are needed for study.
Regarding vineyard management implications, the study found that high levels of light and temperatures during the postharvest season and between bloom and veraison resulted in decreases in the phenolic components of the grape. It may be recommended that vineyard managers adopt an approach that increased the shading to the grape clusters on the vines, by increasing the leaves in the canopy to protect the clusters against intense light and temperature exposure. This may mean changing the trellis system in the vineyard, and perhaps also irrigation practices, in order to create more shady and cooler conditions for the Pinot Noir grape in the Sonoma/Napa region of California to grow and develop with ideal phenolic levels as climatic conditions continue to warm.
I’d love to hear what you all think! Please feel free to comment below by clicking on the “comment” link at the end of this post.
Source: Nicholas, K.A., Matthews, M.A., Lobell, D.B., Willits, N.H., and Field, C.B. 2011. Effect of vineyard-scale climate variability on Pinot noir phenolic composition. Agricultural and Forest Meteorology 151: 1556-1567.
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!