This wonderful book is a book in two parts. I deal here with the Wine and Terroir part of the book, which is contained in the first 11 chapters and then again in chapter 14, 21st century viticulture and its terroirs: a summary. Chapters 12 and 13 deal with Climate Change and its potential effects on viticulture as we know it. I will deal with this topic separately at a later time.
The great contribution this book makes is that it carefully integrates what we know about the many elements of the viticultural environment with the physiological mechanisms of the vine’s response and their effect on grape composition and thence on wine style and quality. That’s the holy grail of knowledge for a vigneron trying to understand terroir.
John Gladstones works to a sensible and uncontroversial definition of terroir:
‘That is, simply, the vine’s whole natural environment, the combination of climate, topography, geology and soil that bears on its growth and the characteristics of its grapes and wines’ (p 2).
John Gladstones does not only set out to explain how terroir works. That happens along the way. A main purpose for the book is much less ambitious: to create a systematic template to measure that part of the growing-season heat of an individual location which affects the time of harvest and allows prediction of the date of harvest for the range of varietal maturation groups in that site.
For any location, given
- the average maximum and average minimum temperatures for each of the seven months of the growing season (northern hemisphere March to October, southern hemisphere October to March);
- the latitude;
- the altitude;
- the degree of slope and inclination to the sun;
- type of soil (depth, drainage, calcareous, stoniness); and
- the surrounding topography (isolated hill or ridge, inland, near a lake or the sea etc.),
a calculation can be made of the heat which affects the phenology of the vine, ie its growth rate and the timing of flowering, fruit set, veraison and, ultimately, harvest. John Gladstones calls this heat ‘the biologically effective degree days, E ºdays’. A prediction can be made of the average harvest date for each of the maturity groups of winemaking varieties from this calculation.
John Gladstones describes ‘Grape variety maturity groups and E°days needed for their maturation’ in Table 10.1 (p 100) based on empirical knowledge of grape varieties from many sources.
In a working example, John Gladstones sets out the site-temperature adjustment factors in Table 2.2 (p 15) and applies those adjustments to the base temperature data for a typical vineyard site in the Canberra region in Table 11.3 (p123). He then compares that vineyard heat accumulation data to the heat requirement to ripen each grape-variety maturity group to predict a harvest date for each maturity group in that location, Table 11.4 (p126).
Central to the estimation of harvest dates is an understanding of how temperature affects the rate of phenological development of the vine.
The Gladstones’ hypothesis of phenology
John Gladstones’ central hypothesis concerning the rate of phenological development of the grape vine is that this rate is maximised at 19 ºC. Hence only those months with a mean temperature of less than 19 ºC and in particular the night temperatures of those months, influence the timing of phenological events (pp 6-7).
For most locations it is the suboptimal temperatures for phenological development of the first two to three months of the growing season that determine the time of flowering and set. Cool conditions retard and warm conditions accelerate flowering. After the time of flowering is determined, mean temperatures are greater than 19 ºC, phenological development rates are maximised and the time between flowering and harvest is the same regardless of the temperature of the later ripening months.
This hypothesis has implications for sites with high daily temperature ranges. The low night temperatures of these sites act as a brake on the phenological development of the vine, predicating later flowering and harvest than would otherwise be expected (pp 7-8).
The understanding of the drivers, and the accurate prediction, of harvest dates for varieties and sites is a considerable contribution to viticultural understanding. However, that is a minor contribution in the context of the other insights this book provides.
The greater logic of the book lies scattered through the chapters on the various elements of terroir and how they have an impact on the physiology of the vine and grape to impart a terroir thumb-print on wine style and quality.
The Gladstones’ hypothesis of temperature equability
The central role of atmospheric temperature of a vineyard site being a (the) major determinant of wine quality and style is well understood (Chapter 2).
Further to this accepted wisdom, John Gladstones advances the hypothesis that a low daily range of temperature during the growing season defines the best viticultural sites and creates the best wine quality (pp 24-25).
Low daily range sites are usually maritime or at high latitudes (pp 38-39) and have intrinsically lower temperature variability and hence more immunity from frost and heat damage. They also have higher humidity with lower rates of transpiration and hence of sugar transfer into the ripening fruit (p 30). The lower rate of sugaring in humid climates allows flavour and tannin ripeness to occur at lower sugars and hence at lower wine alcohols.
Low daily range sites also have higher night temperatures allowing respiration and flavour and tannin synthesis to occur at night as well as during the day (p 25). This results in more rapid and complete ripening of the fruit again at lower sugars.
The corollaries of John Gladstones’ low daily range hypothesis are that:
- ‘the best fruit and wine quality almost invariably follows rapid flavour ripening’
- ‘the greatest vineyards are almost always in the topographic and mesoclimatic situations with best night air drainage and therefore lowest diurnal temperature range’
- ‘the best wines come from the warmest sites in the coolest climates’
- ‘on average the best wines also come from the warmest seasons, when the grapes ripen early’ (p 26).
Root control of terroir expression
Recognising the pivotal role of climate in the appropriate choice of variety for a site and climate’s influence on the canopy and fruit in the ripening process (Chapter 2), a more hypothetical case is made for the role of the edaphic (underground) environment and the health and response of the root system being central to the expression of terroir and the achievement of quality (Chapter 5).
Referencing the large and often contradictory literature on hormone activity in grape vines and their influence on phenology, John Gladstones hypothesises with conviction based on the experimental evidence that the roots are central in the achievement of quality and a unique expression of a terroir and for that to happen the vine’s root system must be mature (p 52). Mature means the vine’s roots are filling the whole soil space available to them, down and into the soil-rock boundary (lithosphere), where soil is being formed, and potentially into the rock itself in soils formed in situ on a local geology.
The Gladstones’ hypothesis on root control of ripening
‘Soils that warm readily, and to the greatest depth, are a universal part of good viticultural terroir’ (p 51).
John Gladstones explains the role of root-derived hormones in vine growth and grape ripening (pp 57-58). The soil temperature and moisture availability throughout the soil profile control the formation of hormones in and behind the growing root tips, which in turn control the growth, fruiting and ripening processes in the aerial parts of the plant. Initially the hormones are carried to the shoots, leaves and fruit by the sugar and nutrient-transferring phloem system. The evidence is that without a constant stream of these trace signal and control hormones from the roots, seeds and leaves, all growth and ripening processes stop.
As the soil warms up in the early spring, the fine roots begin to grow out from the permanent root architecture, ideally in a soil that is at full water capacity at that stage but has the composition and drainage structure to be aerobic. This condition of water luxury and gradual soil warming initiates the early spring flush of root growth and produces from the root tips a supply of the hormone gibberellin.
Gibberellin is the growth hormone of plants and the shoots emerging from the buds into the spring air are triggered by gibberellin to elongate and form the aerial architecture of the vine. If the condition of water luxury and relatively cool soil persists through the spring, gibberellin remains in control and the shoots continue to elongate rapidly with very long internodes and forming very big leaves, a condition of excess vine vigour.
If the right dynamics prevail in the soil system, the drying and warming of the soil from the top down through spring allows the balancing production and transport of another class of hormones from the roots to the canopy, the cytokinins.
The cytokynins are the fruiting hormones, which work against gibberellin and continued elongation of the internodes, forcing shoots to form nodes and leaves and causing shoots and leaves to thicken up. Cytokinins stimulate the fruiting structures (flowers) to form properly both on the current growing shoots (this year’s crop) and in the developing buds in the axils of the leaf and stems (next year’s crop). Cytokinins are critical to the health and fruiting process of the vine and also control the not so helpful branching or lateral shoot formation from the newly formed buds on vines growing too vigorously.
The ideal result of the continued warming and drying of the soil profile from the top down through the flowering and fruit-set period is to create increasing water stress on the surface soil roots, forcing the vine to rely increasingly on the more tightly held reserves of moisture deep in the clay subsoil and by capillary action from water-bearing rock structures accessed by the deep roots. This increasing water stress on the roots reduces the formation of gibberellin and cytokinins, stops shoot growth and stimulates the formation of abscisic acid.
Abscisic acid is the ripening hormone, which precipitates veraison in the fruit. At and from veraison a constant strong stream of abscisic acid from the roots is required to identify the ripening fruit as the strong sink for leaf produced-sugar and root-derived nutrient and to develop the colour, tannins and flavour of ripe grapes.
Excess crop acts to limit root growth by restricting assimilate transfer to the roots. This reduces root growth and the strength of the abscisic acid signal to the fruit to the detriment of the ripening process and grape and wine quality.
The ideal dynamics of soil moisture availability to the vine’s roots through the growing season is one of the defining characteristics of great terroir and is a unique circumstance difficult to emulate with the root-zone limiting actions of irrigation.
Importantly Gladstones explains the ideal rainfall pattern and soil moisture response to achieve the best growth and ripening response from the root-derived hormones through the growing season (p 29). This is followed by a discussion of the limitations of, and the best strategies for irrigation in irrigation-dependent climates (pp 65-67).
The successful application of irrigation to achieve best quality depends on a ‘reliable enough wetting of the full soil profile in winter-spring’, which allows the vine ‘to develop a fully comprehensive root system …which taps moisture to depth’.
‘… a heavy supplementary watering in winter or early spring may successfully mimic’ the ideal winter-spring rainfall pattern.
This is an important contribution to irrigation strategy in the globe’s many irrigation dependent vineyards.
John Gladstones examines the potential role of partial rootzone drying (PRD) in management of post-flowering irrigation (p 66).
The precarious moisture balance of the post-veraison ripening period requires enough soil moisture stress to keep the abscisic acid flowing from healthy roots and not so much stress that the roots lose activity and the canopy suffers desiccation as transpiration from the leaves exceeds water uptake by the roots.
John Gladstones’ hypothesis about the controlling role of the roots in the ripening process is not so controversial, more an insightful synthesis of established scientific observation into vine and terroir logic.
More controversially his hypothesis about the role of roots in defining grape and wine composition and quality extends to the defining role of roots and the influence of geology on grape and wine mineral composition. The mineral composition of wine is a signature of its origin and, he speculates, must therefore relate to the unchanging base geology as opposed to the more dynamic mineral status of the soil (pp 86-87). The hypothesis of the role of geology in determining the mineral composition of wine relies on the vine root system extending down to the parent rock material and the vine’s xylem system becoming the main conduit for hormones and minerals after the phloem has closed down, from the roots to the ripening grapes late in the ripening process when the vine is dependent on deep moisture reserves. Some of these minerals are important as cofactors in the enzymes responsible for the synthesis of colour and flavour compounds in the grape.
For John Gladstones, this hypothesis explains the widely empirically observed but not scientifically proven assertion of soil and geology’s major role in site terroir expression.
Gladstones’ hypothesis about late-ripening varieties
The hormone control of grape ripening is largely but not exclusively root generated. In cool, humid soil, high-latitude environments, the rapidly decreasing day length and temperature in the late autumn trigger abscisic acid production from the leaves which serves the same grape-ripening stimulus as root-derived abscisic acid.
The developing seeds of the berries after flowering and fruit set contribute a hormone class called auxins. The main role of the auxins is to delay the ripening effects of abscisic acid until the embryos in the seeds are fully developed a logical result of natural selection (p 55).
In describing the ‘Maturity rankings of grape varieties’ (Chapter 10), John Gladstones recognises that the differences in the time of bud burst between varieties is significant but that the main difference in harvest times between maturity groups is created by the length of ‘phase 2, the lag phase’ of berry development, the time between fruit set and veraison (p 102).
This is the period when seed-produced auxin is holding off the final ripening effects of abscisic acid. It is differences between varieties in this phase that are largely responsible for harvest-date differences.
Tannin synthesis in berries finishes at veraison so the later-ripening varieties with longer lag phases have more time to build up tannin content, an observation supported by the generally higher tannins of late-maturing varieties (p 105).
Implications of the Gladstones’ hypotheses
Adopted slavishly, Gladstones hypotheses would lead to the selection of sites only at higher latitudes or at lower latitudes in cool, foggy, low-rainfall maritime environments, anyway with low daily temperature range and low temperature variability.
The ideal vineyard would be on a moderately steep north-east slope with its back to the prevailing winds at an altitude of 200 to 300 metres.
The soil would be dark or red in colour to absorb sunlight heat and reradiate it at night. It would have a high incidence of rock at surface and throughout the profile for the best heat transfer to the vine’s deep roots.
The soil could be a duplex soil with a deep reserve tank of tightly moisture-holding clay and a free-draining, moderate fertility top soil or alternatively a free-draining top soil over a deep layer of water-holding limestone.
The winter-spring rainfall of the selected site would be sufficient to fully charge the soil profile but allowing the vine to deplete soil moisture by mid summer and stop growth before veraison. Then with small supplements of summer rain, the deep subsoil and underlying geology must provide barely sufficient moisture to achieve ripening with mild stress on the roots but not so much as to cause leaf senescence.
There are obviously many fine terroirs around the globe that don’t fully answer these criteria and especially many that have high daily temperature ranges (Mendoza in Argentina and Maipo in Chile). Such sites work because of the total absence of rain in the late autumn allowing the grapes to fully ripen despite the night temperature inhibition of phenological development and ripening.
What John Gladstones’s masterly work allows is an understanding of the reasons why the exceptions work as well as the terroir mechanisms of those sites defined by his courageous hypotheses with ideal environmental criterea.
Wine, Terroir and Climate Change has, more than any other modern viticultural treatise, consolidated and made sense of my viticultural experience and logic framework, gained from my life’s work as a vigneron.
In my estimation, Wine, Terroir and Climate Change sits alongside Dr A C Kelly’s classic 1867 publication, Winegrowing in Australia, as a significant contribution to viticultural thinking and practice.