Untangling Terroir

a dirt row in a vineyard
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Terroir is one of the most widely used wine words, but it’s also probably the most ill-defined wine word. It’s a classic example of a French concept that doesn’t translate directly into English. Certainly, this leads to confusion, but I can tell you from experience that it is a poorly understood word even among French speakers.

The term terroir comes from the Latin word terra, meaning land or earth. In a sense, the word means “of the earth,” but in today’s vernacular, it has a more complex meaning. Whereas the vintage effect is the change in a wine’s quality and typicity from one year to the next based on weather, the terroir effect is the change in wine quality and typicity based on where the wine is grown.

Humans: A part of or apart from terroir?

a hand holding a bunch of grapes
Photo by Jodie Morgan on Unsplash

Terroir is defined in the Merriam-Webster Dictionary as “the combination of factors including soil, climate, and sunlight that gives wine grapes their distinctive character.” While this definition could include other factors, like human influence, its omission represents the major debate in the wine world. Are humans a part of or apart from terroir?

In my Introduction to Viticulture & Oenology class at the University of Bordeaux’s ISVV, I was taught that terroir can be considered from two angles. There is the agro-terroir angle, which is everything that concerns nature (i.e. hydrology, topography, soil type, etc.). Then there is the socio-terroir angle, which recognizes humans as an integral part of the quality of the wine. From this perspective, humans work to reveal the wine’s typicity, leading to high-quality wine.

As you can see, it’s impossible to separate human influence from terroir because there is no wine without human intervention. A Jura vin de paille wine has Jura terroir because humans put grapes outside to dry in the sun (a process known as passellirage), which increases their sugar content and leads to a wine with notes of dried apricot and peach. The typicity of Jura is because of the winemaking style in this region.

In other words, terroir is made up of the aspects of the natural world that grow the grapes AND the influence of humans both in the vineyard and the cellar. If it seems like terroir is a loaded word, it’s because it is. It means everything that influences the wine based on where and how the grapes are grown, from the climate to the soil to the way the vines are pruned.

What is “terroir wine”?

Most of the time, when we talk about terroir, we talk about the agro-terroir angle. We talk about how the soil and the climate influence the taste and smell of the wine. We talk about how limestone soils produce a terroir ideal for Pinot Noir. We even talk about certain wines being “terroir wines.” But aren’t all wines terroir wines?

Researcher Cornelius van Leeuwen from Bordeaux’s ISVV doesn’t think so. In his article, “The Concept of Terroir in Viticulture,” he makes a clear distinction between “terroir wines” and “branded wines.” He says that terroir wines are made from vineyards of a traceable origin, year after year. Their characteristics come from influences of climate and soil on vines. On the other hand, branded wines are produced at a larger scale, and the grapes cannot be traced back to the same vineyards year after year. Their characteristics come from influences of blending and “oenological processing” in the cellar.

How big is terroir?

Dr. van Leeuwen touches on an important issue in the conversation of terroir: scale. Is terroir regional? Certainly, Oregon winemaker Sam Parra from Parra Wine Co., who makes exclusively single-vineyard wines, would say no. He would say that the scale of terroir is at the level of the vineyard. That each vineyard has a different terroir.

But if we go over to J Vineyards & Winery in California, we find some single-vineyard wines and some wines that are blended from fruit all over the Russian River Valley. Their 2016 STRATA Pinot Noir is the latter, but they still sell this wine as a terroir wine. To J Vineyards & Winery, terroir is perhaps not only the single-vineyard scale, but on the regional scale or the appellation scale.

And the guys at Barefoot? The word terroir isn’t even mentioned once on their website.

Microbial terroir

Researchers from the University of California at Davis published a controversial article in 2013 that coined the term “microbial terroir.” Their work showed that there were regional distributions of grape microbiota in California’s Napa, Sonoma, and Central Coast growing regions. They suggest that regional differences in terroir may actually be explained by these regional microbiota distributions. In 2016, this same team found that they could actually correlate the vineyard microbiota with the chemical composition of the finished wines.

Not everyone is convinced. A review in 2017 highlighted the fact that the 2016 UC Davis study didn’t take into account the sensorial perception of the wines. So, while we can say that the chemical compositions of the wine are influenced by vineyard microbiota, if these microorganisms aren’t actually influencing the smells or tastes of the wine, can we say that they are a driver of terroir expression?

Terroir’s impact on wine aromas

This year, perhaps the most comprehensive article ever to be written on the topic of terroir’s impact on wine aromas was published by a big team of researchers from France, Texas, and Germany. Led by the terroir guru himself, Dr. van Leeuwen, this article summarizes the four main ways in which terroir acts on aromas: 1) air temperature, 2) solar radiation, 3) vine nitrogen status and 4) vine water status. This fourth point results from soil water holding capacity, evapotranspiration, and rainfall.

a diagram showing how air temperature, solar radiation, vine water status, and vine nitrogen status influence wine aromas
van Leeuwen et al. (2020)

As you can see, various factors of climate and soil are what drive the terroir effect. For example, air temperature and solar radiation, two climate influences, drive green and peppery flavors. The compound responsible for green pepper smells, IBMP, generally decreases as temperature and sunlight increase.

Soil nitrogen is also very important for the expression of volatile thiols in wine, the compounds responsible for grapefruit and passionfruit aromas in Sauvignon Blanc. When there is not sufficient nitrogen, these aromas decrease. The main reason is that nitrogen is necessary for the formation of glutathione, a precursor to volatile thiols. While glutathione itself is non-aromatic, during alcoholic fermentation, the yeast break the bond between the glutathione and the thiol, which reveals the thiol aroma. Thus, low nitrogen in the vineyard means less formation of volatile thiol precursors, and eventually, volatile thiols.

This article also suggests that by characterizing the four main drivers of terroir expression on a vineyard site (air temperature, solar radiation, vine nitrogen status, and vine water status), it is possible to predict aroma typicity from different terroirs. For example, since the literature shows a positive influence of solar radiation on dried fruit aromas, the general conclusion would be that vines that get a lot of sunshine will likely have higher dried fruit aromas. (Remember the vin de paille from Jura with dried apricot notes?)

Since terroir is one of the most used wine words out there, defining terroir and understanding how it impacts wine is one of the most important areas of research in wine today. The most important thing to remember is that place matters; where the grapes are grown undeniably changes the wine’s expression. And in a changing climate, understanding what factors influence terroir expression is paramount.

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The Vintage Effect

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Around late August in the Northern Hemisphere, grapes start to make their way into cellars. If they’re white, they start to be pressed, or if they’re red they start to be loaded into tanks to macerate. Thus begins a long series of steps that will turn juice into wine. There will be hoses and pumps, transferring liquids from one tank to the next. There will be pieds de cuve, fermentation starter packs full of nutrients and yeast. There will be big plungers, used to punch down the floating skins in a red wine tank in order to extract more color pigments, tannins, and aromas into the fermenting wine. And before winter, the reds will be put in barrels, ready to go into the long stretch of hibernation we call “aging.”

All of this might lead to good vintage. But that is dependent on one one factor alone: the grapes. A good vintage means, quite simply, good grapes.

As one winemaker from Alsace told me this fall, “If the grapes are grown well, my job in the cellar is trivial.” Sharing his sentiments is Stéphane Ogier of the Northern Rhône’s Côte-Rôtie. “The wine is made in the vineyard. We can do nothing without this basis.” These winemakers understand that the output is only as good as the input; no amount of Mega Purple or chaptalization can make low quality grapes into high quality wine.

What drives high quality grapes?

Certainly, the grape variety must be a good fit for its soil and climate. If the variety tends to be very vigorous, it should be placed in low fertility soils to balance this vigor. If it tends to have very low vigor, it should be placed in nutrient rich soils to give the vine enough energy to produce high quality fruit. Concerning climate, the goal should always be for the grapes to achieve ripeness as late as possible in the growing season in order to achieve varietal aroma development. For instance, if a grape tends to ripen quickly, such as Chardonnay or Pinot Noir, it should be planted in a cool climate in order to try to draw out the ripening period and make sure those secondary metabolites are developed.

Assuming that the plant material is well chosen for the climate and soil, grape quality variation is mostly influenced by two different effects: the vintage effect and the terroir effect. The vintage effect is the variation in the quality and typicity of a wine from the same vineyard year after year due to weather. The terroir effect is the variation in the quality and typicity of wines based on where they are grown (i.e. according to their terroir).

The Vintage Effect

When considering the same wine producer who is growing grapes on the same vineyard year after year, the variation in vintages will be predominantly from the weather. Here are the five main weather parameters that influence the quality of grapes, and thus the finished wine.

Spring frost

Spring frost is a major determinant of the quality of the season’s harvest. If frost occurs after budburst, it can damage the tissue of the developing bud. This can make the buds unviable, meaning that they won’t continue to develop. If this is the case, sometimes the secondary buds will develop. These buds will produce only about 25% of the fruit that the primary buds would have produced, and the fruit will be of lower quality. But remember, as I wrote in my blog on the fundamentals of vineyard science, these buds were intended to enter the game in the following growing season. So to top it off, spring frost could also impact the next vintage because the current vintage is “stealing” the buds intended to be developed for the next year.


In the early summer period between flowering and fruit set, it is crucial that the vine get enough water. This is because water stress at this stage can lead to poor fruit development, lowering the harvest quantity and quality.

But a little water deficit around the period of véraison, when the berry skin starts to change color, can actually be a good thing, since this stress signals to the vine that it should work on creating ripe, attractive berries for seed dispersers if it wants to spread its genes. This accelerates its sugar accumulation and switches on the grape’s secondary metabolism, which produces those key elements to wine quality, like phenolics (tannins, anthocyanins, and flavonoids) and aroma precursors. However, it’s important to note that summer water stress is, in general, more tolerated by red varieties than by white varieties.

Once the berry starts to ripen, it’s important for it to be dry. We want a dry harvest — not only because picking in the rain is no fun — but because mold at this stage in the game is devastating to wine quality.

Also, while irrigation can be used in some countries, like the US, it is forbidden in other countries, like France. Thus, the timing of rain is extra important where irrigation is unpermitted.


Temperature is another key factor that leads to a good vintage. In the spring, the air temperatures around the vines need to remain around 10°C for three days in a row in order for budburst to occur. After budburst, it’s extremely important that the temperatures don’t drop below freezing (see point 1 on spring frost).

Grapes grow in regions around the world whose growing season temperatures are between 13–21°C (55–70°F). Prolonged temperatures above 30°C (86°F) are problematic, and photosynthesis can stop all together when temperatures exceed 35°C (95°F). If photosynthesis is reduced or halted, this would drastically reduce the sugar accumulation in the berries, leading to an unripened, withered grape.

Beyond sugar accumulation, it can impact the development of phenolic compounds responsible for color pigmentation in red grapes. One study has shown that high temperatures around 35°C, applied one week before veraison, reduced anthocyanin concentrations in the skin of the Cabernet Sauvignon to less than half of what accumulated at 25°C.

And in general, higher temperatures lead to more jammy, cooked fruit aromas and flavors rather than herbal or vegetal aromas. One study tried to mimic what Bordeaux wines would taste like in 2050 due to higher temperatures driven by climate change. It found that the wine was denser and more bitter, with overripe fruit aromas.


Photo by Rui Marinho on Unsplash

Sunlight also drives good vintages. The right amount of sunlight is crucial for the sensory profiles of the berries, and thus the finished wines. One study showed that shaded Syrah grapes made wines of a less intense color, lower flavor compounds, and lower phenolics, leading to a less tannic and less fruity wine.

For white varieties, higher exposure to sunlight can actually induce a natural sunscreen, carotenoids, which break down into volatile aromas. In Riesling, for example, higher sunlight in the vineyard can lead to the breakdown of carotenoids into norisoprenoids, leading to a stronger petroleum smell. In moderation, this odor is a desirable trait of Riesling, but in excess, it can lead to an unpleasant medicinal smell.

It’s also been shown that for Sauvignon Blanc berries, higher sunlight exposure before véraison can lead to lower levels of methoxypyrazines, the family of aromas responsible for the green bell pepper or pea pod smell. In other words, shaded berries lead to a wine marked by vegetal aromas.


Photo by Cody Angus on Unsplash

A bad vintage could also be due to crop damage brought on by hail. Summertime hail is caused by drafts of wind that push raindrops higher into the atmosphere where temperatures are colder, causing the rain to freeze and drop as hail. For grapevines, hail is disastrous. In moderate hail storms, it can damage the fruit, leading to bruising or open wounds that are more susceptible to disease and pathogen infestation. In extreme hail events, the vines can be totally stripped of their fruit or flowers, leading to a lower harvest and loss of money. Nets can be used to cover vines and prevent hail damage, but these nets can be costly and time consuming to put up.

While the winemaking surely plays into the final product, what happens in the vineyard limits the winemaking’s effect. The best winemaker can do little in the cellar to turn low quality grapes into high quality wine. There are many vineyard practices that can lead to higher quality grapes, but sometimes, a little bit of luck is needed when facing Mother Nature and her weather whims.

4 Fundamentals of Vineyard Science

In order to get where we’re going, to the intriguing world of wine science, we have to start before the wine. We have to start before the grapes make their way into the cellar. We have to start even before the harvest. We have to start by understanding the vine.

Origins are revelatory. Trying to understand wine science without understanding vine science is like trying to fix your car without understanding how cars are put together. Only once we have understood several truths about the vineyard can we start to understand and appreciate the fuller picture of wine.

With that, here are the four most important takeaways of vine science that will guide our understanding of wine science — even wine ratings — down the road.


Humans train vines to do what they would not normally do in nature.

The vine is a liana. This means that it is rooted in the ground, and its woody limbs climb to the sky, clinging to whatever it can on the way, in search of the most unencumbered sunlight it can find.

Undomesticated vines do not seek to make perfect berries by winemaking standards. They seek to spread their scraggly arms as far as they can reach, and to photosynthesize. This shows us that what a vine wants to do in its natural state is to privilege its vegetative growth. Yes, it seeks to make berries in order to propagate its seeds. But an undomesticated vine does not produce the high quality fruit that we see on trained grapevines.

Thus, when humans domesticate vines in order to produce high quality fruit, they are putting it out of its natural state of existence. Because the vine privileges its vegetative growth, this necessarily comes at the expense of its reproductive system. We have to manipulate the vine to give less energy to its vegetative cycle and more energy to produce the fruit that we want to harvest.

This brings us to point number two.


The vine’s annual growth cycle is made up of two sub cycles, the vegetative cycle and the reproductive cycle, and the latter is dependent upon the former.

The vegetative cycle refers to the processes of growing roots and shoots. The reproductive cycle refers to the processes whereby fruit is grown and ripened.

A vine, just like a human, spends the juvenile portion of its life with an inactive reproductive system. It will spend its precious energy in its first years of life trying to grow vertically, from its roots growing down into the soil to its apex growing tall to the sky. Only around the third year will we see the vine start to produce some fruit.

the vegetative growth of a vine
Vegetative Growth (Photo Source)
the reproductive growth of a vine
Reproductive Growth (Photo Source)

How are the vegetative and reproductive cycles linked?

In the winter, the temperatures drop and the vine enters a period of dormancy whereby abscisic acid (ABA) inhibits cell growth. The vine still maintains a baseline level of metabolism to survive. In the spring, as the temperature reaches and maintains 10°C (50°F) for roughly three days, the plant hormones cytokinin and auxin are reactivated and ABA decreases. This initiates metabolic activity in the roots, which causes an increase in root pressure, and xylem sap is forced up the plant until the buds “bleed” i.e. the sap oozes out of the buds. Bud burst occurs in the spring, generally in March or April, and the vine resumes its vegetative growth.

After bud burst, clusters of little baby flowers, called “inflorescences,” are developed. These inflorescences shed their caps to reveal flowers, and those flowers are then fertilized to give fruit. What’s important to know is that buds that are developed in year 1 produce shoots that contain fruit in year 2. In other words, fruit production takes two years. First, the inflorescence is pre-formed in the dormant bud. It is only in the following year that the shoot containing the inflorescence will break from the bud, now the primary bud, and grow externally, giving rise to the fruit.

So, why does this matter?

First, we can see that the reproductive cycle is initiated by the vegetative cycle. We need the plant to grow because this provides the hormonal signals necessary to induce flowering and the bearing of fruit.

Second, if inflorescences are pre-formed in year 1 and we see them flower and yield fruit in year 2, impacts to the vine in year 1 can influence what we see during the harvest in year 2. This means that if the vine is stressed during the vegetative growth portion of the spring (say, due to not enough nutrients or water or because there is a hail storm), this could negatively impact the primordial inflorescence, and subsequently, the fruit that will set in the following year.

Now that we’ve covered vine cycles, let’s look at how humans can manipulate the vine to get high quality fruit.


Crop load is critical to quality fruit (and thus quality wine), and pruning is the tool used to achieve this quality.

a ripe cluster of berries
Photo by Zbynek Burival on Unsplash

Crop load is the ratio of fruit to leaves. There is an ideal crop load for each vine; this is called vine balance. Let’s break that down.

The vine has a limited amount of nutrients at its disposal. The amount to which it can photosynthesize and create life-sustaining energy is limited by the number of leaves it has. This, in turn, limits its capacity to support fruit. Of course, there are many other factors that will influence how much fruit a vine can carry, such as soil water holding capacity, organic matter content of the soil, temperature, etc. But the most fundamental idea is that the vine has a bank of energy, and that energy is divided into different bins: the vegetative bin or the reproductive bin. Finding the right balance of energy in each bin is the central idea behind vine balance.

So how can we achieve the right vine balance?

The first place to start is with pruning. In the winter, vines are pruned in order to set the stage for the crop load the following summer. The central idea to pruning is that you are determining for the vine how many buds it will maintain and in what arrangement it will maintain them. In other words, there are many different pruning methods, but the reason we prune vines is to tell the vine how much fruit we want it to carry.

Over-pruning (i.e. leaving the vine with less fruit than it should carry) means the vine will favor its vegetative growth, leading to a large leaf canopy. Under-pruning (i.e. leaving the vine with more fruit than it should carry) means the vine will struggle to provide enough nutrients to the fruit, leading to a reduction in fruit quality. Plus, it might need to dip into its carbohydrate reserves to “feed” all that fruit, meaning there are fewer reserves left for winter dormancy and the development of dormant buds.

Remember, what you do in year 1 will impact the harvest in year 2. If the vine is pushed to its limit to just support the fruit that it has to ripen in year 1, being forced to dip into its energy savings, it will have less energy to give to the developing inflorescences of year 2. This can lead to either lower bud fruitfulness or lower fruit quality in year 2.

How do you know what the ideal vine balance is?

There are two basic schools of thought here: trial by error or technical. The trial by error route assesses visually whether the vine seems too vigorous (i.e. lots of leaf area) or too tired (i.e. poor quality fruit, thin branches) and adjusting the number of buds retained accordingly. The technical route compares vegetative biomass to reproductive biomass. Concretely, this means measuring the weight of all the pruned branches from the vine in the winter and comparing it to the crop load in the summer. The vine is then assigned a score on the Razav Index which helps you to know if you should leave more or fewer buds.

So, now that we have our ideal crop load, we have to consider what happens to the berries during ripening that impacts the finished wine.


Secondary metabolites, the things that give wine their desirable aromas, texture, and color, develop during the ripening phase because they serve an evolutionary role.

a bird sits on a branch surrounded by berries
Photo by Chris on Unsplash

It’s late summer now, and the berries begin to change from green to their final color of deep red or golden yellow. This is the stage, called véraisonwhen the fruit starts to look edible. The flesh softens and the pulp gets juicier as the berry ripens. Things like aroma compounds and color pigments develop. The tannins mellow out. In short, the berry becomes more of a pleasure to consume. What’s critical to understand is that nature is not whimsical; there is an evolutionary reason for these berry traits to develop.

Before ripening, the grape berries are firm, green, and bitter. Acid levels are higher than sugar levels. There is little juice in the berries at this stage, and aroma compounds are predominantly pyrazines (the family of aromas that give Cabernet Franc their green pepper smell). All of these unpleasantries mean that the berry seed is protected as it forms, and eating the berries at this point would be a downright disappointment. Humans surely know better — and so does the animal kingdom. If nature is all about survival of the fittest, then grape vines need to figure out how to make their fruit more attractive to seed dispersers (i.e. birds and mammals).

How do they do this? The key lies in a metabolic pathway that is switched on at véraison.

You can think of the vine as having two metabolisms: the primary and the secondary. The primary metabolism is when the grape consumes sugars, acids, and nitrogen in order to give itself energy. The primary metabolism is vital; without it, the plant dies. The secondary metabolism is when the grape creates products (metabolites) that are not necessary for the growth and function of the plant, but rather are in response to some sort of stress or interaction with its environment.

So, after véraison, this secondary metabolite pathway gets activated. In red grapes, things like anthocyanins start to form, the pigment molecules with antioxidant properties. In the white grapes Muscat or Riesling, for example, their signature floral aromas (part of the terpene family) start to develop in the skin and the pulp. In other grape varieties, something called “ glycosidic aroma precursors” develop. These odorless molecules are bound to sugars, and it is during the process of fermentation when they will break free and volatilize so that we are able to smell them.

So, during ripening, we have these secondary metabolites that start to form. At the same time, we have sugar accumulating and acid levels dropping. What’s the raison d’être? Now, the berry is attractive to seed dispersers. The secondary metabolism is switched on at véraison so that the vine may pass on its genes to offspring.

It is this biological impulse for things in nature to spread and reproduce that gives us wines with all of their color, texture, and aromas.

Wine, then, is fundamentally a wonder of nature.