The Future of German Riesling

(We’re happy to share this visionary article written by Professor Hans Reiner Schultz from the University of Geisenheim, Germany for TONG 9 – GERMAN RIESLING. Published back in 2011, it still clearly defines and describes the challenges but also misconceptions about Riesling in its main wine regions.
You will find more articles and information about TONG 9 – GERMAN RIESLING on www.tongmagazinedigital.com.)

“Riesling and its beautifully balanced linear wines have always been associated with the cool climates of Germany. This is where the interplay between acidity and residual sweetness is at its best, producing wines with great tension. But, as everywhere, Germany’s climates are warming up and the question now is whether Riesling growers will have to adapt their viticultural practices.

Although Riesling is traditionally considered Germany’s quality grape variety, it is grown in many of the world’s wine regions. Of the 34,000 hectares of vineyards planted with Riesling worldwide, 22,400 hectares are in Germany. Alsace in France has 3,500, Austria 1,640, Australia 4,500, the US 1,700 and New Zealand around 900. Considering that there are more than 7 million hectares of vineyards around the world, this doesn’t amount to much!

Obviously, these regions don’t all have cool climate conditions, although Riesling is considered a cool climate grape variety. The New World regions are usually the warmest, like the Okanagan Valley in Canada, the Yakima Valley in Washington State, US, or the Adelaide Hills, Australia; Blenheim in New Zealand is on the cooler side of the “Riesling wine regions” with a reputation. How much does climate affect Riesling’s distinct character? And can we look at the warmer regions outside Germany as an illustration?

Riesling is thought to be a cross between two very old grape varieties that may have been around before the Middle Ages: Traminer and Heunisch. Traminer was apparently spread across various wine regions in Europe – it is genetically linked to grape varieties like Muscat, Gewürztraminer and Sauvignon Blanc –, while Heunisch is thought to have been a lower-quality grape from Central Europe, mainly Germany. The roots of “Heunisch” are in the Roman “vinum hunicum”, meaning “wine of poor quality”, as opposed to “vinum francicum”, or “high quality wine”. The opposition between good and bad wines was particular Middle Ages, when people were yet not interested in varietal differences. The name “Traminer” first appeared in Europe in 1349, while the earliest record of “Riesling” dates back in 1435. Riesling is definitely an old grape variety.

Does Riesling have a favourite climate?
In general, Riesling needs cool to intermediate climates for its crop to ripen properly. Cool regions have average growing season temperatures of 13 to 15°C. They are traditionally suited to grape varieties like Müller-Thurgau, Pinot Gris, Gewürztraminer, Riesling, Pinot Noir, and to a lesser extend Chardonnay and Sauvignon Blanc. We don’t have specific information about the upper temperature suitability thresholds for these varieties, but we know that Riesling shares, along with the other varieties mentioned, an adaptability to intermediate climates, with average growing season temperatures of 15 to 17°C.

Other cultivars suited to intermediate climates include Semillon, Cabernet Franc, Tempranillo, Dolcetto, Merlot, to some extend Malbec, Syrah, and Viognier, and on the lower margin Cabernet Sauvignon. Riesling is suited to regions across the world famous for other (traditional) grape varieties. Yakima Valley in Washington State in the US and Clare Valley in Australia are just two examples.

Of course, it’s not as simple as that. Many climatic factors contribute to the formation of a grape’s composition and, ultimately, a wine’s quality. Yakima Valley, for example, has the warmest summer, while New Zealand has the coolest average temperature, cooler even than Germany. The warmest nights in summer are in Austria and Alsace, while Washington State, the Okanagan Valley and the Adelaide Hills have the coolest summer nights, and thus the highest diurnal temperature differences, up to 20°C in Yakima. In Germany, diurnal temperature differences are much less, between 10 and 12°C during the growing season and between 10 and 8°C in September and October nearer harvest time. Yakima has the most sunshine hours, Geisenheim in Germany and Colmar in Alsace have the least.

The differences between Riesling areas can be considerable, but the question remains: are Riesling wine styles in these different countries and regions equally distinct? Some would say yes, although despite these differences, they still have a Riesling character, which indicates that for the coolest regions, like Germany, the variety has a significant potential to adapt to climate change.

Climatic variations are smaller within most Riesling-growing regions. But at the cooler end of climate requirements, so in the coolest climates, these small variations can have substantial consequences on the wine styles produced and different sites may lead to slightly different wines (warmer, fruitier etc.).

As a rule, Riesling appears to be a grape variety that responds well to moderate climatic differences and retains its varietal character and quality within a broad range of cool and intermediate climates.

Riesling soils
Along with Riesling’s climatic preferences, we also need to look at its suitability to different soils. It seems logical to assume that if this grape variety grows in different regions and climates without loosing its varietal character, it can also be grown on different soils and retain its distinctiveness. In Germany, Riesling is grown on at least six different soil types: rhyolite (red shale), loam, limestone, quartzite, sand, slate and red clay. These basic types imply more subtle differences – slate, for  instance, red, blue, grey or yellow.

Different soil types have different compositions and nutrient availability. Nutrient levels are lower in stone soils like slate, for instance, which is less fertile than alluvial soils. Vines on stone soils will produce wines with a more restrained and steely character than vines on alluvial soils, which is why many wine lovers say that stone soils produce wines with more minerality. There is, however, no direct proven link between soil composition and what a wine’s taste. The soil’s content may not be the decisive factor per se if water availability is limited, which makes nutrients travel less easily from soil to vine.

But things are more complicated than that. The dark colour of some stony soils, like grey slate in the Mosel region, may compensate for the lack of nutrients in the soil and may lead to more aromatically powerful grapes. The heat captured by the stones during the day is reradiated into the grapevine canopy in the early hours of the night, thus lowering diurnal temperature differences and prolonging enzymatic processes within the vine and the fruit, like the accumulation of sugars and the degeneration of acids. In other words, the colour of the soil may be almost as important as its composition.

At the Research Centre in Geisenheim, Rheingau, we have conducted research on the interaction between Riesling vine physiology, fruit composition and soil colour. For this, we used an unusual set up. In a vineyard with the same soil, the same pruning and trellising system, the same row and vine distance and with vines of the same age, we changed the colour of the soil by adding substrates of varying colours to its surface. We took four soil types and colours as parameters: white for chalk and limestone, red for red clay, black for black coarse slate and brown for a loamy soil, which was our control soil.

Soil colour affects the reflectance of sunlight radiation into the canopy both in quantity and quality. The ratio of red to far-red light (660nm-730nm) has an effect on a pigment complex (phytochrome) that determines the activity of certain enzymes in the grapes, like invertase (important for sugar accumulation), PAL (for colour formation) and nitrate reductase (for amino acid supply).

We found that red to far-red light was reflected more powerfully into the canopy on white soils, followed by red and brown soils. Black soils have a markedly lower reflected radiation. The differences between the different soils were the most marked in July and decreased near the end of the growing season in September.

Other aspects of this experiment involved the influence of soil temperature and reflection on fruit and canopy temperature. The results here were inversely linked to those of our soil colour study, with the highest soil temperatures in black slate, followed by red clay, limestone and loam. Differences were less marked, but led to differences in berry temperature and thus presumably in enzymatic activity in the grapes (although we did not measure this).

The link between berry temperatures and grape composition is interesting; since for most fruit components there is an optimum-type temperature relationship, it indicates the complexity of these interrelationships. The increase of phenolic compounds in the grapes, for instance, was higher on white soils, followed by brown, red and black soils. But again, the differences were not considerable, leading us to conclude that Riesling can preserve at least some components of its unique varietal character over a broad range of soils, as far as soil composition and colour are concerned. Does that mean that terroir or viticulture in general are not important? No, it doesn’t.

Riesling aromas
As in all other vineyards of the world, grape composition and especially sugar and potential alcohol levels have changed over the last 40 years. The main reason, at least in the most northern grape-growing regions like Germany (Geisenheim is situated at the 50th degree latitude north), is global warming, reinforced by increasing knowledge about viticultural techniques and canopy management. But long-term data clearly shows that climate evolution is  paramount. Within the same Guyot pruning systems developed for cool climates to optimise sunlight on the canopy, and characterised by elongated and thin canopies, the average potential alcohol levels have risen by 26 percent from 1970-1986 and 1994-2010. Total acidity levels have decreased from 15.5 grams per litre to 9 grams per litre (expressed as tartaric equivalent). Grapes tend to be riper, but then the concept of ripeness is not objective, and perfect ripeness has not yet been defined.

What does that mean for the distinctive aromas of Riesling? Have they changed and, more importantly, will they change in the future?

Different chemical compounds are responsible for Riesling’s unique aromatic profile. Citronellol and ?-terpineol (both terpenes) are responsible for citrus aromas; ?-ionon and ?-damascenon (both norisoprenoids) for aromas of tropical fruit, apricot and peach; 3-mercaptohexylacetate (a thiol) produces passion fruit aromas, while 3-mercapto-hexanol and 2-phenylethanol are responsible for grapefruit and rosy, flowery aromas respectively. Linalool, geraniol and nerol (terpenes) lead to flowery and orange-like aromas. But one of the most controversial aroma compounds in Riesling, especially in aged wines, is TDN (1,1,6-trimethyl-1,2-dihydronaphtaline), the norisoprenoid responsible for kerosene and petrol aromas. In general, passion fruit and grapefruit play a big part in Riesling’s aromatic profile, while TDN, despite not being unique to Riesling, can add to complexity at low concentrations and can be overwhelming at high concentrations, when the wines age and their aromatic backbone diminishes.

Science has not yet fully explained the cause of TDN and its resulting kerosene aromas. In general, TDN levels in Riesling grapes, which is present in the grape flesh, rise when the grapevine experiences drought, heat and nitrogen deficiency. This appears to indicate that TDN is connected to stress. Its content is linked to sugar increase, and so its level rises with grape ripening, especially in hot climates. But different clones and different yeast strains during fermentation can enhance the level of TDN in grapes and must.

One of the most easily measurable parameters is temperature and sun exposure. When directly exposed to sunlight, and when the leaves have been removed in the fruit zone, Riesling grapes  have much higher levels of TDN than shaded grapes. Other research has found that TDN levels decrease when the pH of the grape juice rises. In general, this is linked to soil fertility and nutrient availability, especially  nitrogen and potassium. The higher the nutrient availability, the lower TDN levels appear to be. TDN is clearly linked to viticultural management, and winegrowers can substantially influence its levels in grapes.

The future
Average temperatures during grapevine growing season have been increasing in the northern and southern hemispheres and average minimum temperatures have increased substantially.

Although Riesling has a broad climatic scope, and can produce high-quality wines in cool to moderate climates, German winegrowers are concerned their wines will lose their classical character and will have higher alcohol levels and a warmer style. Precipitation distribution during the season is another concern, since summer rainfall is expected to decrease and winter precipitation to increase with a continuous increase in the evaporative demand of the atmosphere.

Winegrowers can adapt their vineyards to rising average temperatures and to control water consumption. When establishing a new vineyard, if it is not on steep slopes, the winegrower can change row direction. In Geisenheim, we are carrying out an experiment with different row directions; we have found that planting in an east-west direction lowers berry temperature, mainly because of less (shorter) exposure of the western part of the canopy in the afternoon and more shaded canopies. This goes directly against the traditional belief that in cool climates vineyards are best planted in a north-south direction and preferably on south-facing slopes.

Another element is the adaptation of rootstock, with a preference for drought-tolerant species with Vitis rupestris genes, such as Richter 110 instead of the more common Vitis riparia and/or Vitis berlandieri types in Germany (such as for example SO4).

The tendency towards higher natural alcohol contents due to climate warming may require novel strategies to counteract this trend in the vineyard. One of our more surrealistic experiments is defoliation of the upper canopy, instead of the fruit zone. Grapevine leaves need all the carbohydrates they produce until they reach about 33 to 70% of their maximum size. Only then do they start to contribute to the accumulation of sugars in the plant. Once they reach their maximum size after bloom, their photosynthetic capacity supplies most carbohydrates directly to the fruit. The old, fully-grown leaves in the fruit zone start losing photosynthetic activity after three to four months.

Traditional defoliation of the fruit zone removes these leaves, facilitating ripening thanks to direct sun exposure and reducing the risk of rot. It also makes harvesting easier, especially by hand.

By defoliating the upper canopy, plucking away some of the younger leaves above the fruit zone – but not the shoot tips! – growers can decrease photosynthetic rates which means slower sugar accumulation, while physiological ripening can continue. Leaving less leaves on the vines may also lead to less potassium in the grapes, resulting in a slightly lower pH, which means more acidity and freshness. But the ultimate goal are wines with a fuller flavour potential, but lower alcohol levels. This is a big problem in California, where there appears to be competition for the longest hang time, resulting high and refined phenolics but also in disturbingly high alcohol levels of 15 to 16% and more in red wines.

With all this in mind, we can conclude that global warming does not pose a direct threat to German Riesling and the refined linear wine styles it produces in Germany. High-quality Riesling is not only grown on a much larger climatic scope than commonly believed, it also maintains its unique character on a large variety of soils. Furthermore, viticultural techniques can substantially alter the aromatic profile of the variety, creating many possibilities for the winegrower (and winemaker) both to create new wine styles and “protect” existing styles by adapting viticultural management to changing climatic conditions.”

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