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 at vineyards.
CSIRO and University of Melbourne scientists who studied vintage records from ten vineyards in five regions found that between 1946 and 2009 grapes continued to mature earlier at all but one site. The trend towards earlier maturation accelerated between 1985 and 2009.
The regions included in the study were from Victoria: Mornington Peninsula, Rutherglen and Central Victoria; South Australia: Eden Valley; and Western Australia: Margaret River.
Only at a vineyard in Margaret River did the grapes ripen later over the whole period, but the researchers described that change as “insignificant”.
The results of the study are published in the online journal Nature Climate Change.
Leanne Webb, a Postdoctoral Research Fellow at CSIRO, said that “early ripening of wine grapes, as seen in Australia in recent years, often has undesirable impacts on wine quality”.
The wine industry was intimately wedded to the concept of “terroir”: 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 also likely contributed. “Many of these practices would have improved the health and photosynthetic capacity of the grapevine, perhaps 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.