July 31: With the changing climate, heat and hydric stress alter the grape composition and affect the quality and yield depending upon complex interactions between temperature, water availability, plant material and viticultural techniques, so we must start making adjustments now, writes John Salvi, Master of Wine who focuses on Bordeaux vineyards and makes various suggestions that can be generally implemented elsewhere too
We know that great heat and hydric stress alter the composition of the grapes. Technically it alters the colloidal composition of the must (grape juice). Colloidal means a mixture or solution having particles in suspension. Climate change will impose increasingly hot and dry conditions on vineyards. Both quality and yield are strongly influenced by climatic conditions and depend upon complex interactions between temperatures, water availability, plant material and viticultural techniques. To meet changing climatic conditions, we must make adjustments and start making them NOW. We need to modify plant material and viticultural techniques to delay both phenology and ripening and to increase drought tolerance.
Dryer conditions lead to yield reduction and excessive water stress jeopardises quality. What can we do to maintain quality and still vintage perfect grapes at the climatically optimum time?
The best vintages are the dry vintages (1945). Within limits a certain amount of drought reduces yield but favours quality, avoiding fat, lazy grapes. More so for red wine than for white wine. It results in smaller berries, more phenolics in the skins and enhances aromas. The vine is highly resistant, but beyond certain limits it cannot cope. We need to plant drought resistant material, plant on soils that have maximum water retention and adapt the pruning system to require less water. Eventually the French Standards Body, Institut national de l'origine et de la qualité (INAO) must be persuaded to allow limited irrigation.
HANDLING HIGHER TEMPERATURES
Temperature is the main driver of phenolics. Grapes ripen earlier resulting in higher sugar and alcohol, lower acidity, less anthocyanins and eventually less aroma. More evaporation from the vine and the soil add to the hydric stress. Both will increase drought conditions in Bordeaux. We will have to adjust plant material (see below) and adjust viticultural techniques (below).
LATER RIPENING VARIETIES
Using these varieties is a good proposition, but in the very long term, the solution is for INAO to modify the present regulations. For Bordeaux and Bordeaux Supérieur 7 new varieties have been permitted within low limits, out of which one can be more optimistic about Touriga Nacional. We also need new clonal material. Clones of the same grape variety can vary by up to 17 grams sugar/litre (1° alcohol). We need to concentrate on the less productive ones.
DROUGHT RESISTANT VARIETIES
With greater heat and drought, we put both Merlot and Sauvignon under pressure. Merlot does not like great heat and will have to be dramatically reduced. Sauvignon cannot support drought and will have to be reduced and even eventually phased out. Cabernet Sauvignon and Franc must increase as must Petit Verdot. The change is clear. Cabernet Sauvignon used to be picked green and Petit Verdot rarely ripened fully. Today they both ripen to perfection. Replanting more Malbec is being studied and the introduction of new later ripening varieties was mentioned above.
LATER RIPENING ROOTSTOCKS
We have insufficient knowledge here, but we know that the rootstock does influence the phenology of the grafted vine. Rootstock resistance to drought varies considerably. More research is required.
DROUGHT RESISTANT ROOTSTOCKS
We need to plant drought resistant rootstocks to sustain yields and avoid loss of quality due to stress. This would be a powerful and environmentally friendly way to handle increased drought and avoid increase in production costs. More research is needed.
INCREASE OF TRUNK HEIGHT
This is very important. The closer to the soil the hotter the temperature. Traditionally Cabernet Sauvignon was grown on gravel soil and pruned with a low trunk. Increase the height of the trunk and you will decrease the effect of increased temperatures.
Reduce leaf area to Fruit Weight Ratio. This is very important indeed, but also complex. Fiercer sun is already burning exposed grapes after excessive de-leafing but on the other hand too many leaves increase alcohol levels through photosynthesis and encourage moisture and subsequently fungal diseases and rot. Against this, too few leaves delay colour change and affect total acidity. A balance must be sought for each viticultural region and its climatic conditions each year.
Bush vines (head pruning) are highly resistant, but impractical in Bordeaux as they do not allow mechanisation and only support low yields. Our present systems are both efficient and adaptable, but with greater heat and drought we need to consider systems that are higher off the ground, and which can be trained to have optimum foliage. Various forms of trellis pruning should be considered.
Late pruning delays budbreak and seems (uncertain) to improve the sensorial quality of the wine. Research is underway regarding a second pruning after budbreak, which will considerably delay maturity.
Not possible in Bordeaux
For red wines closer spacing (10,000 vines/hA) optimises sunlight interception and allows high quality with relatively high yields. BUT, wider spacing means more water availability and may eventually become essential.
WATER RETENTIVE SOILS
Already mentioned above, such soils should be used to the maximum wherever available. Jean-Philippe Delmas of Château Haut Brion says that more vigorous rootstocks will develop roots that will go deeper to seek water, which is down there if they can get to it. This, he says, is more vital than less vigorous rootstocks to produce less alcohol. Water retentive soil can be encouraged by “deep ripping”.
As mentioned above, irrigation may become essential eventually, but would need permission from the INAO. Must be carefully considered before use because it has major drawbacks linked to the environment, other potential water users and the natural water table.
The vine is of Mediterranean origin. Highly adaptable and highly resistant. Higher temperatures alter phenology and shift the ripening period and vintage to less favourable periods for quality. Dry conditions reduce yields, but limited stress improves the quality of red wine. We have adequate resources to handle both heat and drought.
Some of the above steps are easy, like later pruning while others are very long term perspectives like replanting. Overall, depending upon the rate of climate change, these adjustments should be sufficiently effective to ensure high quality for decades to come. Climate change will NOT dramatically decrease viticultural suitability in main wine producing areas by 2050. The vast genetic diversity in vines is a precious resource to continue to produce high quality wine with sustainable yields in a changing climate.
Most of the information in this article was supplied by Kees van Leeuwen and Jean Philippe Delm of Château Haut Brion. Though the information is specific to Bordeaux, it may be extrapolated to other vineyard regions too.
John Salvi, Master of Wine
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