Wine grapes in Australia’s south are ripening on average 20 days earlier than they did in 1985, according to a study that attributes the trend to climate change, smaller harvests and improved technology in vineyards.
The study concentrated on 10 winegrowing sites in four states in Australia, which had records of observations for periods of more than 25 years. Nine of the ten sites studies had trends of earlier ripening, varying from 6-34 days. Margaret River in Western Australia was the lone exception where the grapes ripened later- the scientists found this result ‘insignificant’. The most common driver of earlier ripening was higher temperature, deemed a significant factor at seven sites. Previous studies have linked Australian temperature, and possibly rainfall to increasing greenhouse gas concentrations.
The study led by Dr Leanne Webb, a University of Melbourne- based viticulturist specializing in Climate, and CSIRO was published last Sunday in the journal Nature Climate Change. She claims that while trends towards earlier ripening have been widely reported, a detailed study of the underlying causes of these shifts had not been undertaken so far, making it first of its kind study in the world.
The authors of the study say that by increasing irrigation, or laying down mulch, growers can manage soil moisture and by changing their pruning regime, they can alter crop yields. “The study will give wine growers a head start in developing adaptation strategies to meet evolving temperature and soil moisture shifts by managing soil moisture content by increasing irrigation or mulching, vine rootstock choice, or managing crop yield,” she reportedly says, adding "Changes to the timing of biological phenomena such as flowering and emergence of butterflies have been noted on many continents over recent decades. In some wine-growing regions such as southern Australia, grape maturation dates have advanced about eight days per decade, with earlier maturing potentially impacting wine-grape quality and regional branding.”
Lower soil moisture, particularly in the drought-stricken areas, was a major factor for earlier harvests at five sites. Drier soils lead to higher levels of a stress hormone in vine roots, driving the fruit to earlier ripening. Vineyard management was also important. In four sites, pruning and fertilization methods that lowered crop yields contributed strongly to earlier maturation. And other technological innovations, such as improved disease and pest control, could also have been a factor, the study indicates.
The wine industry has been intimately wedded to the concept of “terroir”- i.e., matching grape varieties to unique combinations of climate and soils to produce wines of distinctive styles. Global warming and changing rainfall patterns were altering those terroirs, she said.
“This study set about to test the assumption that this earlier ripening of wine grapes was due to observed regional warming. What we found was that only about a third of the shift was driven by regional warming. Other factors were also affecting the timing of wine grape ripening. Those factors included declining soil water content, smaller harvests and evolving management practices. Smaller harvests would mature faster because sugars accumulated through photosynthesis would be distributed more rapidly into a lower volume of grape tissue.”
Management practices, which had evolved considerably in vineyards in Australia since the 1980s – with changes to trellising, irrigation and pruning, and improved nutrition and disease and pest control – had perhaps also contributed. “Many of these practices would have improved the health and photosynthetic capacity of the grapevine, inadvertently leading to earlier maturity,” the report said.
Wine growers could respond to the trend towards early ripening by planting new varieties or providing artificial shade to reduce temperatures, but these measures would be expensive. It would be more practical, the researchers suggested, to manage soil moisture by increasing irrigation or using mulch, increasing the crop yield by pruning less, selecting root stocks that were less sensitive to plant stress hormones, or removing leaves to reduce photosynthesis.
By choosing root stocks that are less sensitive to plant stress hormones, or trimming leaves, growers can also alter the response of the vine to lower humidity, the paper suggests.