Sunday, 16 June 2013

Terroir and the Côte de Nuits Part 2: Hydric Stress and Pinot Noir



Bill Blatch’s excellent Bordeaux vintage report of 2010 gave a fascinating account of the vagaries of Bordeaux’s weather through the spring, summer, autumn and winter. The vineyards were, by varying degrees, subjected to heat, drought and deluge, but at the end of all the climatic involutions, the vintage is one of “embarrassingly good quality”.  In 2009, the ripening curve for both Cabernet and Merlot had been smooth and predictable, but in 2010 the tracks of decaying hurricanes, and the switch from El Niño to La Niña mid-summer, reacquainted the Bordelaise with the anxieties of grape growing in the mid-latitudes. Autumnal showers and a burst of unseasonal warmth in October finally blunted the sword that had hung threateningly over the region since high-summer. It will be fascinating to follow the development of this tannic vintage, both in terms of the absolute quality achieved by the very top estates, and its consistency across appellation boundaries. Really great claret vintages attain an imperishable level of perfection, and their finest wines gather within themselves, uniquely it seems, the silty scent of time’s gentle motion. 

 One of the consequences of climate change is, we are told, more weather, when “weather” is understood as a measure of deviation from climatic means. Climate and weather are very different and, as Bill Blatch’s commentary makes clear, at close quarters the latter’s progress can appear both causal and chaotic. Those who still think that the warming trend in global temperatures will be a gentle notching-up of just one climatic variable are in for a shock.

It is against the shifting backdrop of weather that “Terroir and the Côte de Nuits” is intended to be read. The article makes a strong case, I hope, for the role soil composition plays in buffering the vines against the uncertainties of weather…..”quantifying over individual cloud bursts and droughts, and delivering the vines into a realm, that at root level at least, more closely resembles the smooth curves and plotted trends of climatic means.”

Soil composition and depth show significant variation along the Côte de Nuits, but according to Seguin and Gadille, similarities do exist within appellation hierarchies.  In the Grands Crus and better Premiers Crus, the limestone bedrock is covered by an average 40-50 cm of clay, silt, sand and gravel, and the high levels of calcium within the soil aid drainage and root colonisation. Thus, while the bulk water storage capacity for these soils maybe as low as 50mm for some vineyards, these reserves are readily available to the plant. Supplemental to this, the soil’s load of porous stones together with the permeable bedrock, provide another water source that the vines can draw upon during times of drought. We need to remind ourselves that water movement can be multi-directional within certain soils.
Many researchers have explored vinifera’s response to stress, and there is general recognition of vines’ inherent tolerance of water deficits. Vine water loss is not a constant. Each plant species has its own evapotranspiration coefficient, and for vines the peak period of water loss is likely to coincide with mid-summer, when temperatures and solar irradiance reach their respective zeniths.  In Burgundy, the monthly rate of evapotranspiration is at its highest in July, when it is equivalent to about 120mm of water. For June and August, the months either side of this peak, the value of evapotranspiration (ET) is approximately 100mm of water. The significance of these values becomes more apparent when we compare them with the rainfall means for the corresponding months.


Table 1

Precipitation as % of ET


Plant water status is the difference between a vine’s water need (ET rate) and the water that is available through its root system. Soils with a high water storage capacity can buffer the vine from stress, as will a water table that is within easy reach of the roots. Both possibilities are met with on the Côte de Nuits, and when they occur, the wines they produce are most likely sold under the generic Bourgogne label. Where the vine roots have more limited access to water – the shallower soils of the Premiers and Grands Crus vineyards – stress occurs earlier in the season, and recurs more quickly after rains.

The effect of hydric stress upon the vine depends both on its intensity and the point at which it is exerted along the ripening curve. Experiments conducted with irrigation have found that the consequences of excessive stress before fruit set are best avoided: flowering may be compromised, and the vine might only develop a limited canopy that is insufficient for crop ripening.  Between fruit set and harvest, however, if precipitation only replenishes at a rate equivalent to 30%-50% of ET, then the quality of black grapes is generally improved. The benefits are physiological, inasmuch as berry size is reduced and bunch exposure improved, but there is also a hormonal impetus towards more rapid colour change and the preferential partitioning of carbohydrate into the fruit. For those interested, the unequivocally titled “Water deficits accelerate ripening and induce changes in gene expression regulating flavonoid biosynthesis in grape berries” (Castellarin et al, Planta 2007) provides further reading.  Importantly, hydric stress induces no benefits if precipitation is greater than 60% of the ET value, or if drought occurs and values for precipitation fall below 20% of ET.

How these scenarios play out on the Côte de Nuits is illustrated by table 1. When set out next to each other, we can see that through the critical pre and post-veraison months of July and August values for precipitation and ET lie just at the humid threshold of inducting beneficial stress. The table also highlights what Cornelius Van Leeuwen has called “the vintage effect”: if rainfall in July and August was +15mm above the mean, which is well within the standard deviation for the region, then the level of hydric stress (≥60%) would be insufficient to bring about the crop benefits outlined above. In addition to “the vintage effect”, we can also identify “a terroir effect”: vines grown on soils with low bulk storage capacities will be subjected to stress more frequently than vines grown on deeper and more humid soils, whilst the close proximity of porous limestone, either as bedrock or fragments, will limit the incidence of severe water deficit.

An understanding of hydric stress also helps to explain an apparent anomaly that exists between two of the world’s coolest climates: Southern England and Otago. Seasonal heat summation for the two regions is similar, 900-1000 growing degree days, but where Otago attains 14% of alcohol in its Pinot Noir, England regularly struggles to achieve the minimal level of concentration that is necessary for the production of sparkling wine. The month that precedes veraison in the UK’s maritime climate is not July (as in Burgundy), but August, when irradiance, if not temperature is falling away from its high-summer maximum. As a consequence, the ET rate is equivalent to about 70mm of water, or, if you prefer, equal to the mean value of August precipitation. By contrast, Pinot reaches veraison earlier in Otago, and through the months of January and February ET, under the luminous alpine sun, can be as high as 200mm, with precipitation trailing in at a meagre 30mm for both months. In most years, Otago needs irrigation to limit stress, whilst the UK rarely benefits from the (much needed) changes in carbohydrate partitioning and berry morphology that limited hydric stress might confer. In comparison with Southern England, Champagne is significantly warmer through the months May-August and drier across July-August-September. The convergence between Champagne’s climate and its thin chalk soils is, in most years (or else in reserve wine), sufficient to induce some stress into its vines, adding an extra element of concentration, and, arguably, an elevation in autolysis.   

There is an argument, a very good argument, that science lacks the subtlety and nuance to properly evaluate the quality descriptions we give to wine. I believe we can be sympathetic to this point of view, without accepting that we need three separate and exclusive disciplines called enology, viticulture and sensory evaluation. How we might advantageously fit these three elements together has been the real subject matter of my discussions of terroir.