Cost-effective viticultural strategies to adapt to a warmer, drier climate
Abstract
Climate change is already impacting the grape and wine sector in Australia. This project evaluated two vineyard management strategies to adapt to a water–constrained and warmer climate. One of the measurable effects on viticulture is that hotter temperatures are causing earlier and compressed ripening of grapes. This places a strain on harvesting and winery infrastructure and changes berry chemistry and resulting wine quality.
This project showed that delayed pruning is a practical, environmentally sound and economically viable tool to shift grape berry ripening. Experiments at two trial sites, including a commercial vineyard, showed that late pruning can delay grape ripening in Barossa Shiraz by up to two weeks, potentially spreading harvest with no deleterious effects on vine vigour, yield, grape quality or wine quality.
The second part of the project investigated the use of evaporative cooling in vineyards to counteract intense heat events. It appears that temperature effects of sprinklers on grapevine meso-climate are minimal and transient. Nonetheless, results from two Riverland trial sites showed a significant improvement in yield, a positive cost/benefit, reduction in grape sunburn and early indications of improved wine quality in “cooled” vines. Clearly another, unknown plant physiological response occurred.
Summary
It is well-acknowledged that climate change is impacting the grape and wine sector in Australia. Specific challenges due to warming include: (a) compressed harvests, (b) disruption of the balance between sugar, colour and flavour in ripening grapes, leading to high alcohol wines, and (c) potential loss of regional character due to warmer ripening conditions. Wineries lack the harvesting and crushing capacity to deal with a narrowing harvest window, leading to fruit becoming overripe and a reduction in quality. The expansion of processing facilities places more strain on a capital intensive industry. Viticultural practices are needed to deal with these challenges.
The aim of the first part of the project was to test delayed pruning as a tool to counteract warming effects. Delayed pruning seeks to spread maturity, hence decompressing harvest, and shift berry development into cooler conditions, hence preserving the balance between sugar, colour and flavour and regional wine styles. Delayed pruning is environmentally and economically superior to double pruning and chemical treatments.
Two trials were established with Shiraz in the Barossa Valley. First, using the open top chambers constructed in previous industry-funded projects, we increased vineyard temperature and evaluated the interaction between delayed pruning and temperature in SARDI’s experimental vineyard at Nuriootpa. Second, using large scale trials, we explored the feasibility of this practice in a commercial vineyard at Marananga. This trial further tested the carry-over effect of repeated late pruning. Three pruning dates were tested in both trials: winter control (current practice), budburst, and 2-3 leaves.
Delayed pruning was shown to satisfy four criteria. It shifted phenology and delayed grape maturity (by up to two weeks), was neutral or positive for yield, was neutral or positive for wine quality, was neutral for cost and fitted well with standard commercial vineyard practices. It is therefore a feasible practice to counteract the negative effects of global warming on viticulture and winemaking.
Another consequence of a changing climate is the increasing occurrence of extreme heat events. Several successive hot nights, rather than hot days, appears to result in permanent damage to vine canopies and crop (leaf and berry scorch or sunburn). One strategy to reduce the impact of heatwaves is therefore evaporative cooling of vineyards during hot nights. Such use of water is a trade-off with water use efficiency as this water is specifically applied to evaporate, however, if successful, this technique may a have positive cost-benefit ratio.
Three large-scale field experiments were established, two in the SA Riverland and one in Coonawarra, to investigate the effectiveness of under- and over-canopy sprinklers on lowering vineyard temperature during hot nights experienced during a heatwave. Large temperature data sets were generated during several heatwave events during the 2015/16 season, however there was little effect on temperature and humidity within the meso-climate of the bunch zone and vine canopy. At the two Riverland sites there was a 20-30% increase in yield as a result of activating under canopy sprinklers on just six nights between December and harvest in late January/early February. The value of the additional crop far outweighed the cost of the water used for cooling. In addition, there was a significant increase in yeast available nitrogen in must, which resulted in more rapid fermentation. Early indications suggest differences in wine sensory attributes; these will be reported later.
In summary, this work has demonstrated that activating under canopy sprinklers only at night during heatwaves had only a minor impact on temperature within the canopy, however there was a large, yet unexplained, increase in yield and positive impacts on grape must and potentially wine quality.