The quality of the vintage and the estimated yield of grapes are often closely linked by the pundits. Avid readers of the wine press would have heard it before. “Yes, we had a kind vintage, with perfect ripening conditions. So expect a lot of stunning wines this year.” Fair enough. Then there are statements such as: “It was a tough vintage, cold during grape set, but incredibly hot during the latter stages of ripening. But the yields were way down, so quality will be up.” Mmmm… surely a case of having your cake and eating it too, you would think. And it is.

The relationship between wine quality and grape yield is the stuff of which wine legend is made. It derives 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 Barossa shiraz wines or those equally intense and beautifully structured Margaret River cabernets or great 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 in which those low yields were obtained.

Let's put things in perspective first. Yields can range from three to over 40 tonnes per hectare. What is an appropriate yield depends on a number of factors, but the most important are the style of wine that is being aimed at and the grape variety itself. If a wine producer wishes 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 brilliant) 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, flavour compounds and tannins. The less fruit the vine has per unit amount of leaf area, the more flavour, colour and tannin it will gather during its ripening. An analogy is to think of the vine canopy as a solar panel, and each grape as a small light bulb connected to the panel. If you have two vines with the same amount of leaf area (the same area of solar panel), but one vine has three times the number of grape berries (light bulbs), you would expect that the light coming from each of these light bulbs would be much dimmer. For a similar reason, the grape berries of the more highly cropped vine will have more character than those in the lower cropped vine, given the same amount of functioning leaf area per vine.

I emphasise the word “functioning” because some leaves may actually produce very few carbohydrates. Leaves that are shaded by other leaves (as in dense canopies) or leaves that are diseased are not effective in producing the stuff that drives character in grapes. Viticulturists have two options on how to maximise 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 vigneron's 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 and into an upright position using wires attached to the trellis. This almost Viagra-like approach is called “vertical shoot positioning” 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 because there is only so much that can be done to improve leaf efficiency by a vine.