Adapting to Higher Temperatures
The recent frosts in Europe are a reminder that some of the effects of climate change can be counter-intuitive, due to the complex way in which the Earth’s climate system responds to global warming. However, for many winegrowers, especially for those in warmer environments, such as the US West Coast, adapting to higher temperatures and more frequent droughts remains the priority. Fortunately, modifying vineyard design and taking advantage of new technologies can help growers to adapt to new climate patterns.
Wildfires have also emerged in the past several years as a serious threat in many wine regions, although there’s an important difference between wildfires and other climatic challenges. While meaningful measures to mitigate the latter can be implemented in individual vineyards, truly effective measures to deal with wildfires require action by the broader community and are beyond the scope of this article.
Higher temperatures affect the vines in two ways. Higher average temperatures accelerate phenology so that berries ripen under warmer conditions resulting in higher alcohol, lower acidity and greener tannins in the wine made from them. Heat spikes can delay, or even stop, the ripening process as well as leading to sunburn and berry shrivel. The berries are particularly susceptible to damage after sugar accumulation has stopped as there is then no longer any water being supplied to them.
Avoiding these unpleasant outcomes can be accomplished in two ways: cooling the vines when temperatures are overly high or preventing temperatures from reaching that point in the first place. A common way to cool the vines is by applying water to them, which then absorbs heat as it evaporates. One frequently used practice is applying extra irrigation in anticipation of a heat spike so that the leaves have enough water to continue transpiring as the temperature rises. There is some controversy, however, about how well this actually works in practice.
Water can also be applied directly to vines via sprinklers or misters, again resulting in evaporative cooling. There is even a clever technique still very much in the R&D stage for using microbubbles to deposit a thin layer of water on the surface of the berries and the leaves for that purpose.
The fundamental shortcoming of these approaches, of course, is the need for additional water, especially in years that are particularly hot and dry when water is in short supply. These conditions are occurring increasingly more frequently due to global warming and, unfortunately, it appears that 2021 is shaping up to be the second consecutive year in which that is the case on the US West Coast. How, then, can the temperatures the vines experience be prevented from reaching the point where cooling is needed to avoid adverse consequences?
The simple answer is to shade the fruit, or the entire vine. Vertical shoot positioning is a commonly used trellis system but it has the disadvantage of leaving the clusters overly exposed to the sun. Some winegrowers have begun adding cross arms at the top of the stake to spread the canopy out horizontally and provide greater shade to the fruit zone and limiting leaf removal on the side of the vines that receives afternoon sun. Cover crops can also help by lowering the albedo of the soil to reduce the amount of thermal energy radiating from it upwards toward the fruit zone.
In a new or redeveloped vineyard, the winegrower also has the freedom to select the row direction that provides the best protection for the fruit from the sun. Historically, many vineyards were planted with rows running east to west. The disadvantage of this orientation as the climate warms is that the berries on the south side (in the Northern Hemisphere) are exposed to much more sunlight and higher temperatures. There are a number of factors that need to be taken into account when deciding on the best row orientation, including operational considerations, but a general rule of thumb is that an orientation closer to north to south will likely be the best choice.
There has been research on artificially shading the vines in recent years as well as its use by some winegrowers. In most cases, various kinds of shade cloth or netting have been used to reduce the amount of sunlight reaching the berries with the leaves remaining fully exposed to the sun. The results have included lower temperatures in the fruit zone, reduced degradation of organic acids and anthocyanins, a slower increase in Brix following the end of sugar accumulation as a result of less berry transpiration and reduced sunburn. Since the leaves are not protected, however, they are still vulnerable to the effects of high temperatures, especially during heat spikes.
An exciting recent development is the advent of engineered plastic films that shade the entire vine. These are designed to filter out specific wavelengths of light, since having sufficient exposure to UV-B is important in the synthesis of some secondary metabolites by the berries. Not only do the films protect the fruit from higher temperatures and overexposure to sunlight, because they shield the leaves leaf transpiration is reduced lowering the amount of irrigation water required. Since, as noted above, hotter temperatures and drought frequently go hand in hand, the films may also be able to play a role in mitigating the effects of drought.
Putting all of these pieces together creates an opportunity for an incremental adaptation strategy that can be implemented in several steps over time as temperatures rise in a vineyard. The first step would be trellis modifications, accompanied by the appropriate changes in canopy management, to optimize shading of the fruit by the leaves. Cover crops could also be considered at this stage if not already employed. A second step, if the vineyard is redeveloped, could be a change in row direction to further optimize sunlight exposure. And, finally, shade nets or films could be deployed to provide additional protection and, in the case of the films, reduce water use.
At Argos Analytics, we’re focused on quantifying climate impacts on winegrowing and evaluating the effectiveness of vineyard design and cultural practices to mitigate them. If you’d like to learn more, we invite you to contact us.
By Robert Dickinson for The Porto Protocol Foundation
President of Argos Analytics, LLC