what about low yields?

The relationship between wine quality and grape yield is the stuff of which wine legend is made. It is derived from the fact that all the great wines of the world are made from low or lowish yielding vines. It is simply impossible to make concentrated rich Napa Cabernet or those equally intense and beautifully structured Grand Cru Burgundy if the vines are cropped too heavily. However, it is not true that simply having low yields will ensure wine quality. What is of equal importance is the circumstances under which those low yields were obtained. It’s true that the notions of “low yields” are congruent with the best wines and that most of the best wines ive ever tasted ironically come from vineyards that have very specific canopy management regimes and dormant season planning of activities and objectives.
Let's put things in perspective first. Yields can range from 3 to over 40 tons per hectare. (1ha=2.47 acres) What is an appropriate yield depends on a number of factors, but the most important are the style of wine that the winemaker wants to make and the grape variety itself. If a winemaker wants to make a medium bodied wine with moderate levels of concentration for early consumption, then it makes no commercial sense to crop at the lower end of the scale. The reason why consumers have such a great choice of good (but not great) wines at below $15 is because most wine producers crop their vineyards at the middle or 'middle plus' end of the yield spectrum. Some grape varieties are also very yield sensitive, producing very ordinary wines even when only slightly overcropped. Examples include Pinot Noir, and the classic Italian varieties Sangiovese and Nebbiolo.
But why do higher cropping vines produce wines of less concentration and character? The answer lies not in the fruit itself, but in the leaves of the vine. As the grape ripens, it draws its reserves from the leaves. The leaves collect sunlight, and use its energy to produce carbohydrates. These are translocated into the grape during ripening, and result in an accumulation of sugars, flavor compounds and tannins. The less fruit the vine has per unit amount of leaf area, the more flavor, color and tannin it will gather during it ripening. An analogy is to think of the vine canopy as a solar panel, and each grape as a small light bulb that is connected to the panel. If you have two vines with the same amount of leaf area (i.e. same area of solar panel), but one vine has three times the number of grape berries (i.e. light bulbs), you would expect that the light coming from each of these light bulbs would be much dimmer.
Let’s emphasize the word “functioning” because some leaves may actually produce very few carbohydrates. Leaves that are shaded by other leaves (as occurs in dense canopies) or leaves that are diseased are not effective in producing the stuff that drives character in grapes. Viticulturists have two options as to how to maximize the amount of effective leaf area per unit weight of grapes. The first is to decrease the weight of grapes. This can be achieved by adjusting how hard the vines are pruned in the previous year, or by simply cutting off the bunches when they have just formed. 'Bunch thinning', is a radical approach as much of the vignerons potential yield simply ends up on the vineyard deck. However it is almost a “must do” approach when making good wines from yield sensitive varieties such as Pinot Noir. Other approaches involve manipulating the position of the vines' shoots to allow better interception of sunlight by the leaves. This is done by trellising the canopy in such a way that it is split it into various layers, or more commonly by lifting the shoots up off the natural drooping position, into an erect upright position using wires attached to the vine trellis. This almost Viagra-like approach is called 'vertical shoot positioning', (VSP) and is perhaps the most common method used to improve light interception by leaves. The underlying philosophy here is not having less fruit per leaf area, but more leaf area per amount of fruit. The commercial implications are obvious. If you do things right, you can have higher yields with no reduction in quality. However, this only works to a point as there is only so much that can be done to improve leaf efficiency by a vine.
So next time you hear that a difficult vintage will produce great wine because of lower yields, think again. If the conditions that caused the lower yield also caused a loss of working capacity on part of the leaves, which is very common, then it probably bologna. But then again, every vintage is better than the last right?