Photosynthesis, Carbon Dioxide and Higher Alcohol Wines

Let me preface this piece by saying that I'm a fan of lower alcohol wines. I was first introduced to wine in 1980s Upstate New York, drinking 11.5% Rieslings and, yes, Cayuga Whites. Red wines from the area, like Marechal Foch and De Chaunac (for an overview of hybrid varieties like Cayuga White, Marechal Foch and De Chaunac, see here) were in the range of 12-12.5%. At the time, this wasn't considered low alcohol, it was considered the norm.

Perhaps because of this early exposure to lower alcohol wines, I've always preferred them, and I actively look for them when considering a purchase. New Zealand, despite being a cool climate winegrowing region, now regularly produces 13-13.5% whites and Pinot noirs are often seen with 14.5%. This goes against my, personal, ideas of what these wines should be.

I look forward to wines from the really challenging vintages where grapes struggle to get as ripe as the winemakers want them. The 2012 vintage in Hawkes Bay had a really cool and extended ripening period and grapes were brought in at much lower sugar levels (measured in degrees Brix in New Zealand and other places) than usual, but it resulted in some very good, and more elegant, in my opinion, wines as a result, with the alcohol being in balance with the other aspects of the palate. New Zealand's most recent vintage was also a challenge due to rainfall in the ripening period, resulting in grapes being brought in before target Brix were hit. I tasted my first of this vintage's wine in August, a Marlborough Sauvignon blanc, that had all the hallmarks of the region, but with only 12% alcohol: it was a much more harmonious assemblage than with the usual higher alcohol wines of the region.

Having got that out of the way, what about climbing alcohol percentages in wine?

Rising average alcohol levels in wines has been a topic of discussion for some time, and various reasons for this have been put forth over the years. A useful study to look at was published in 2011 by Alston et al. where data from California was examined to show that average harvest Brix levels between 1980 and 2010 increased. Sugars in red wine varieties increased by an average of 0.23% per year over that period, and notably, in their Figure 1, Brix for red varieties was pretty much flat from 1980 to the mid 1990s, rising from there to the 2010 average of around 23.8°. This was particularly noticeable for the North /Central Coast and Delta regions, where the rate of increase was 0.72, 0.75, and 0.96%, respectively, between 1990 and 2008, compared to 0.53% for California as a whole (Alston et al. 2011 Table 1).

So fruit sugars are going up (at least in California) and the wines made therefore have higher alcohol. But why are the sugars going up?

A different take on this has come about recently, where people are starting to look at rising carbon dioxide (CO₂) concentrations in the atmosphere and linking this to increased plant productivity (e.g. here and here). This isn't a difficult link to make, as the process of photosynthesis takes CO₂ and water and with the help of the enzyme Rubisco, releases oxygen and sugar (in the form of glucose). It stands to reason that if you increase the availability of a starting material, you can end up with more product. This is assuming that other starting materials (Rubisco, water, sufficiently warm temperatures and light energy in this case) aren't limiting, and that the products don't start piling up in the area where they're being produced - if glucose and oxygen keep building up in the cell, the rate of photosynthesis will slow through a process called feedback inhibition.

So increasing CO₂ should mean more efficient photosynthesis and more sugars to go around? As we usually find, things aren't that simple.

There has been plenty of research into the effects of raising CO₂ concentration and its positive effects on plant growth and productivity, however, much of this has been with relatively short duration experiments. When plants have a longer time and a chance to adapt to the changed conditions there is more talk of photosynthetic down-regulation, or acclimation, resulting in relatively little change.

A review by Makino and Mae notes that longer term plant adjustment is a complicated system. For example, if sugar is being produced more quickly, but the plant does not have the capability of moving the sugars out of the cell fast enough, photosynthesis will be slowed by feedback inhibition. This kind of makes sense, too, as the plant would change things so that a balance remains between production and utilisation of photosynthetic products.

There is also a suggestion that seedlings have a greater response to high CO₂ compared to older plants, possibly because seedlings are generally carbohydrate supply limited, whereas older plants have a store of carbohydrates that are used when needed. Grapevines, being perennial plants, have decent carbohydrate stores even from a reasonably young age.

It's not just photosynthesis that can change, either - under climate change scenarios, increasing temperatures will also increase the respiratory activity of Rubisco. Yes, this enzyme goes both ways: it can help convert CO₂ into sugar, but the same enzyme also latches onto oxygen in the process of photorespiration. This opposes photosynthesis and makes the process less efficient. Photorespiration increases faster than photosynthesis as temperatures increase, so photosynthetic efficiency suffers.

And as Jamie Goode has pointed out (here in an article where he points out a whole bunch of interesting things on the subject) with higher CO₂ plants don't need to open their stomatal pores as much, because a lesser amount of air holds the same amount of CO₂. This can lead to less water use, as with less air movement in and out of the leaf, there is less water vapour lost, too. A side effect of this, however, would be a rise in leaf temperature due to less evaporative cooling (you can experience this by spraying your arm with water - it feels cooler right away because the water is evaporating, and to do that your body heat is used). Higher leaf temperatures could mean more photorespiration, and more time when the leaf gets too hot to keep the enzymatic machinery going. Higher temperatures, associated with climate change, will only make this problem worse.

The multiple changes to the environment will cause plants to respond, but exactly how they respond is really too complex for us to say at the moment, especially when you start to take into account that these changes will have an influence on all the other living creatures around and on the vine (disease organisms, insect pests, and don't forget the soil ecosystem!).

So the overall effect on grapes and wine gets hazy pretty quickly, with lots of factors, and responses, involved. Having said this, I agree with Jamie in that the rise in CO₂ concentration is not really what's responsible for increasing wine alcohol - that has more to do with consumer preference and technological advances.

For those that are thinking about strategies for dealing with high Brix and high alcohol wines, we have a number of tools in the viticultural toolbox, but this is a topic for another article!

Climate change and its interaction with winegrowing

A Radio NZ article about University of Waikato student Electra Kalaugher and her work on climate change and dairy farms, and the accompanying video was particularly timely, because at about the same time I was answering a questionnaire sent to me by a student studying an MBA in Bordeaux. Her thesis is on climate change on winemaking and how it concerns the French legislation relating to the wine industry.

Below are her questions and my answers to them. My thanks to her for permission to post the information here...

1-Climate change has been affecting the wine industry, like all fields in agriculture. What is your experience in the vineyards of New Zealand so far?

Of those events that are supposed to be altered with climate change, in recent years I think we have seen wider swings in weather events, such as rainfall, snow, frosts and the like. We have had some periods of significant drought, as well as highly unusual heavy rainfall. We have seen some late season frosts, and unusually large amounts of snow in some areas. However, it remains to be seen whether these events are considered to be abnormal in the longer term.

2-Do you think New Zealand will benefit from the climate change since the warmer areas expand?

It is possible that new geographic areas will open up to winegrowing as a result of the overall warmer temperatures. However, this will have implications for the existing areas, where the warmer climate may mean that making wine styles associated with a particular region could be made more difficult. For example, the Marlborough style of Sauvignon blanc is associated with the cooler ripening period that the region has experience. If temperatures rise, the flavours in the wines will also change, and so the wine style.

This is one challenge, and another significant one is the chance of more extreme weather events. Of the possibilities, early season or late season frosts are a particular concern for the wine industry, as many areas are already prone to damaging frost events, so having them occur later into the growing season, and earlier as the season ends and harvest approaches, will have a direct impact on profitability.

Some forecasts for seasonal changes in precipitation point to less overall rainfall as a result of climate change in the eastern parts of New Zealand. Therefore, water will become an even more valuable resource, potentially limiting grape production.

Overall, I do not think that climate change will be a beneficial thing for the New Zealand wine industry, but the reality is that we will have to deal with it.

3-What are the challenges that the wine producers have been facing due to climate change effects?

I've mentioned some of these already - the possibility of frosts happening later in the beginning of the season and also happening before fruit is harvested. Water availability has been an issue with the establishment of newer vineyards, with water schemes needing to be developed to ensure a reasonably reliable water supply. Increased heat means that some grapevines will need to be grown slightly differently in order to retain the flavour profiles that are wanted in the wine. Severe flooding has had minor impacts on vines so far, but this will probably be more of an issue in the future.

4-What kind of changes should be made in the vineyards in order to adapt the shift in climate?

Viticulturally, it will be necessary to change the management of the vines to retain flavour profiles - for example, doing less leaf removal, or changing its timing. Trellising systems may need to be changed to help with this, as Vertical Shoot Positioning, which is the most widely used system in New Zealand, may give the fruit too much exposure.

Irrigation management (and linked with that, cover crop management) will have to be tweaked to ensure vines don't get water stressed at inappropriate times. More efficient ways of delivering water to the vines, and measuring soil and grapevine water status, need to be developed.

With frost events potentially happening when vines have more canopy on them, more efficient ways of dealing with frosts will be needed. If enough water can be found (which is unlikely for large vineyards), sprinkler systems will work, but most are using fans or helicopters at the moment, which rely on the presence of an inversion layer, which holds warm air. As well, it isn't certain how climate change will alter the occurrence and strength of inversion layers...

New vineyards should be planted with future shifts in climate in mind. This encompasses most aspects of vineyards, but also variety choice and potential wine styles to be produced.

5-As a viticulturist, what are your conclusions for the future concerning the climate change effects, for New Zealand and also globally?

In my mind at least, climate change is a reality that we should be ready for. Planning for its occurrence, using the latest forecasts (e.g. NIWA's Climate Change Scenarios for New Zealand), is the best we can do. 

The wine industry will be able to cope with climate change, but it will likely have an impact on the financial planning, with increased risk of crop loss and increased management costs. The possible changes to wine style also need to be considered carefully, as the consumer may want to stick with the current style, but it may not be able to be grown in the same area or it wouldn't be cost effective to do so given the extra labour inputs.

Seasonal update

As I'm back in New Zealand and haven't been here since the beginning of the growing season, one of the things I did was catch up on growing degree day (GDD) accumulation for Lincoln, Canterbury.

The concept of heat accumulation (one way of measuring which is GDD) and how plant growth corresponds with it is an important tenet of modelling the phenology, or seasonal development of plants, and in this case, grapevines.

It goes a little something like this:

Plants need heat to grow. At its most basic, this is because the enzymes that do the chemical work in plants can't function when it gets cold (in fact, this is true for animals as well -it's one of the reasons cold blooded animals hibernate in the winter. Warm blooded animals use energy to generate heat, which keeps the enzymes working). So below a certain temperature threshold, plants won't grow. As the temperature rises above that threshold the enzymes work faster and faster, up until the temperature gets so high as to prevent the enzymes from working again (around the mid 30s in celcius).

So measuring the accumulation of heat during the growing season results in a pretty good match of how far along the vines have come, or what stage they have gotten to. One question, though, is what do you set the temperature threshold at?

Based pretty much on enzyme activity in relation to temperature, 10C is the most commonly chosen base temperature (the temperature below which there is no plant-active heat accumulation). However, this may differ for different types of plants, and even for different times of the season (e.g. it seems that for the process of budbreak, a base temperature of 4C is omre appropriate, and for the first leaf appearances, 7C, Moncur et al. 1989).

Setting aside those special circumstances, a pretty straightforward way to quantify heat accumulation is to take the average temperature for a month (which falls within the growing season) and if it is greater than 10C, subtract 10 from it. That result is then multiplied by the number of days in the month, which gives the number of growing degree days for that month:

[ (average temperature for the month-10) * (number of days in the month) ] = GDD

In other words, on average each day was X amount over 10 degrees, and over the whole month, X times the number of days equals the amount of plant-useful heat that was experienced. Note that negative GDDs are not counted (though it's cold, the plants don't regress - they just sit there until it warms up again)

So to put some numbers in there, if the average temperature of November was 12.6C (as it was at Lincoln in 2009), 2.6C times the 30 days in the month equals 77 GDD accumulated.

If this value is calculated for each month of the year, we can follow the heat accumulation in a useful way, especially when comparing one year to another, or comparing on location to another.

In the case of the former, the graph for the 2009-2010 season looks like this, given the data collected up through November:

Growing degree calculations for the 2009-2010 season (up through November, the orange line) and for the long term average (LTA, the blue line), which is the average over the last 40 years.

The Long Term Average (LTA, blue) is the smooth sigmoidal curve, which signifies that heat accumuation is slower (the line is more horizontal) in the spring and autumn, and quicker (an more vertical line) during summer. The number in parentheses is the LTA GDD accumulation for Lincoln - a paltry 924GDD.

You can see that up to this point in the season, the orange line (current season) is below the LTA line, which means that the season has been cooler than average. If you squint just right, you can make out that the last orange dot corresponds to 77GDD, which is what we calculated above. This also means that there was no accumulation of plant-useful heat in October, or earlier in the season (at least, when based on monthly averages - more on that in another post!).

If you want to compare seasons, this way of looking at the data is fine, but if you want to see the differences more clearly, you can plot the current season's GDD relative to the LTA, which looks like this:

Growing degree calculations for the 2009-2010 season (orange) and for the LTA (the blue line, which is the X-axis). This figure is showing the same data as the previous one, but in a slightly different way.

Now it is pretty plain to see that we're veering away from the LTA. If the line is below zero (the LTA blue line), then there has been less heat accumulation than the LTA, and if it's above, there has been more, and it's been a warmer season.

You can also see more clearly what's happening for each month. If the line moves down, then the month has been cooler than LTA; if it slopes up, then it's been warmer. If the line is parallel to the LTA, then the average temperature for the month has been the same as the LTA.

So what we'd like to see is the line above the LTA - sadly, up through most of December, this has not been the case!