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Column Article from the November 2009 Magazine Issue

The Science Behind Canopy Management

by Cliff Ohmart

  • Dr. Mark Kliewer’s research is responsible for the contemporary understanding of sunlight’s role in winegrape quality. The recent canopy management symposium was held in his honor and drew an international cadre of speakers.
  • Several scientists speaking at the symposium noted that revised trellising, spacing and orientation could be critical to offsetting the effects of climate change in the vineyard and protecting fruit.
  • At least one vineyard expert predicted that robotics would become a driving force in canopy management technology, with software programs being developed to carry out pruning, thinning, positioning and harvesting.

Whether you are interested in maximizing winegrape yield, wine quality or both at the same time, understanding vine canopy management is essential to being consistently successful from one year to the next. A properly balanced vine, with the right ratio of shoots and leaves to fruit, is the goal, as well as striving for the right fruit exposure to light and maintaining the fruit within an optimum temperature range. We tend to focus on fruit production when thinking of vine balance, but two other critical elements are production of adequate fruit buds and production of sufficient carbohydrate and nutrient reserves for the following year.

Many things affect vine balance, such as trellis, spacing, pruning, irrigation, nutrition program, rootstock and scion. Furthermore, it is critical to match these to the vineyard site and regional climate. With such a complex set of variables affecting vine balance, it is not surprising that it has taken many years of basic and applied research by many top-notch scientists, as well as on-the-farm trial and error, to understand the relationships among them.

On July 16 of this year, many of the world’s top vine canopy management scientists gathered at University of California, Davis, to honor Dr. Mark Kliewer’s life work by delivering a series of lectures on the state of the science. I will present some of the highlights of these lectures in the following paragraphs. Jon Tourney also provided an excellent summary of the event for, but the importance of the event and depth of scientific content warrant a report here as well.

A group of experts
The day-long symposium was a part of the 16th GiESCO Congress (Group of International Experts of Vitivinicultural Systems for CoOperation) held at UC Davis this summer. It marked the first time the congress has met in North America. GiESCO was formed by the French viticulturist professor Alain Carbonneau, who was present at the symposium. Initially, the organization has focused on grapevine training and trellising systems research.

Kliewer joined the Department of Viticulture and Enology at UC Davis in 1963. He was a biochemist by training, and began a program focused primarily on the compositional quality of grape berries as affected by temperature, light and the effects of canopy shading.

At the same time, other scientists around the world—particularly Dr. Nelson Shaulis at Cornell University—and several scientists in Australia were systematically documenting the influences of light, temperature and vine balance on vine physiology and productivity.

Kliewer’s earliest research was focused on the temperature and light dependent synthesis and breakdown of amino acids, sugars and anthocyanins in vines and berries. By the late 1980s, results of his work and others’ made it clear that the development, physiology, fruit yield and fruit quality of a grapevine are all governed principally by the canopy light environment. One of the products of the research and thinking going on at this time was the 1991 publication of the very influential book, Sunlight Into Wine, by Smart and Robinson.

Many things make canopy management science difficult. An amazing variety of canopy management styles is used around the world. In 2003 Carbonneau and Cargnello produced a dictionary of 50 basic canopy management forms.

As scientists have learned more over time about the way vine canopies function and the complexities involved, computer simulation models have become the main tool to integrate the activities of individual leaves and their responses to the natural environment. That is due in large part to the great variation in sun radiation within the vine canopy over both space and time.

A simulation model is a series of interacting mathematical formulas  based on theories of how something behaves: a chemical process like carbon assimilation, or how a vine grows in terms of its leaf area and its distribution. Scientists compare the output of the model with real life data from the vineyard, and if they match up within a given acceptable level of statistical probability, then you can conclude the model is accurate, meaning the theories of how the vine is operating are likely correct. It is a methodical process, but with persistence it can lead to a much better understanding of how vine canopies behave.

canopy management
Developing new models
Professor Hans Schultz from the Geisenheim Research Center in Germany is one of the foremost authorities on canopy management modeling, and he updated the audience on the latest work in this area. The newest models can integrate structure components of a canopy such as shape, orientation and location of shoots and leaves (which influence light interception and therefore canopy energy balance) with detailed vine physiological functions such as stomatal aperture (how much a stomata opens), photosynthesis and other metabolic processes.

Schultz made the case that because vine canopy structure, functioning and management are important in determining yield and quality, the new models may be able to help predict the effects of vineyard management on yield and quality. Moreover, these models help our understanding of carbon assimilation and water loss in vine canopies—two very important topics that are essential to understand if we are to deal successfully with pressures brought about by climate change and its inevitable effect on water availability.

For example, Schultz showed the output of a model comparing water consumption for north-south oriented vine rows to that of east-west oriented rows in Bordeaux, France. For most of the season, vines with a north-south orientation had greater water consumption than vines with an east-west orientation, but late in the season this trend was reversed.

Dr. Stefano Poni from the Università Cattolica del Scro Cuore in Italy discussed how some models of vine canopy dynamics are being used in Italy. Until recently, quantifying crop load relied on yield-to-pruning ratios and total leaf area ratios, because they are relatively easily to determine with some simple vineyard measurements.

However, he pointed out that a pruning weight uses 1-year-old cane tissue for part of the analysis and may not be related to actual vine capacity. He and his colleagues use modeling to estimate seasonal variation of canopy assimilation, i.e., a measure of tissue production as a result of photosynthesis and dry matter partitioning, or how this photosynthate is distributed among the various types of vine tissue such as leaves, fruit and canes.

They have gone as far as attempting to model the fraction of the foliage that is well exposed to light, thereby assessing the photosynthetic efficiency of the vine canopy. These models depend in part on the ability to measure photosynthesis directly in the grapevine canopy. Traditionally this was done by measuring the gas exchange of a single leaf and then extrapolating the results to a whole canopy. However, recently they have been able to use custom-built systems to enclose the entire vine canopy and measure the gas exchange of the whole vine.

Feel the burn
Poni described another interesting study assessing the effects of what he termed “summer pruning”—removing excessive shoot growth or canopy density via leaf removal on varieties like Sangiovese, Trebbiano, Barbera and Lambrusco. Poni and his colleagues, for example, have shown that pre-flowering removal of an average of six basal leaves has consistently decreased fruit set, which in turn increases cluster looseness and reduces bunch rot. Crop level is controlled through physiological mechanisms in the vine that are different than those achieved through traditional cluster thinning. They also have shown improved grape and wine quality as a result of this very early leaf removal.

Poni and other presenters at the symposium referred to the importance of vine canopy management in ameliorating the effects of increased temperatures as a result of climate change. He said scientists are forming a consensus indicating that in warm climates characterized by hot summers, a cluster microclimate where the prevalent regime is diffuse light broken by occasional sun flecks is most advisable.

On a field trip to Lodi two days prior to the symposium, several visiting scientists concluded that a trellis type likely to provide this microclimate would be a modified California sprawl that allows for fruit exposure to some dappled sunlight, a strategy already being practiced in the region.

Embracing technology
Dr. Richard Smart of Smart Viticulture gave the last presentation before Kliewer took the floor to make closing remarks. Never one to mince words, Smart implied that many growers are not taking advantage of the latest information about canopy management, in part because many seemed frightened of change. He does not foresee any significant new ways to divide a vine canopy or manipulate it through shoot positioning, because most of the likely permutations of vine canopy division and shoot orientation have been thought of, proposed and tried.

He sees the future of canopy management in mechanization, particularly in robotics. This will involve video image analysis software that guides robotic “arms” to carry out dormant pruning, shoot and cluster thinning, shoot positioning and harvesting of individual clusters (not vine shaking).

Dr. Nick Dokoozlian, a former student of Kliewer at UC Davis and now with E. & J. Gallo Winery, commented in his presentation that for California to stay competitive in the global marketplace, yields must increase while maintaining or increasing key grape and wine quality parameters. He concluded that vineyard design and canopy manipulation—combined with irrigation and crop load management—must be better integrated to achieve this goal.

Dr. Cliff Ohmart is research/IPM director at the Lodi-Woodbridge Winegrape Commission, where he oversees the research and grower education program and helps growers implement sustainable practices in their vineyards. He has been writing on sustainable winegrowing issues for Wines & Vines since 1998, basing his observations and opinions on his experience as a research scientist, private IPM consultant and most recently with Lodi growers. Contact him through



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