Using Shiraz vines enclosed in open top chambers, the impacts of warmer CO2-enriched air on vine phenology, leaf function, vine water use and grape and wine composition were assessed over four seasons. Over and above the seasonal influences, which were strong, warmer air advanced maturity and resulted in lower quality grapes and wine. Elevated CO2 caused a small improvement in yield. Elevated CO2 alone and in combination with warmer air increased photosynthesis but not water use by vines. As the climate warms, it is likely that water use in wine grape vineyards will remain static or diminish marginally.
The advancement of harvest dates over the last three decades has focused attention on what a future climate might mean for wine grape production in Australia, given the predicted rise in global atmospheric temperatures over the coming decades. Despite it being the root cause of warmer air and despite it being the substrate for photosynthesis, the direct effects of elevated atmospheric carbon dioxide concentrations have received far less industry attention. The effects of elevated [CO2] may be different from, and may modulate, the effects of a warming climate on vine growth, grape production and grape and wine quality. The relationship between the effects of elevated CO2 and the effects of warmer air have the potential to be very significant for wine grape production in the warm irrigated regions in the intermediate term, and more widely across Australia in the longer term.
Building on an earlier study of the effect of warmer air on the phenology of mature vines in the field using open top chambers (OTCs), a system to enrich the air with CO2 around mature Shiraz vines in OTCs was designed and built. A factorial combination of ambient/elevated CO2 and ambient/elevated temperature were imposed for four seasons, and the effects on vine phenology, physiology, grape composition and wine quality assessed.
Most of the variation in phenological development occurred after budburst. The biggest driver of phenology after budburst was season, but air temperature and the concentration of CO2 also contributed. The effect of the latter was sporadic and less important compared to the influence of air temperature. The supply of air warmed by 2°C relative to the ambient air temperature did result in significant changes in the temperature indices commonly used to relate grapevine phenology to climate across seasons and regions. But here, these indices were seen to have limited explanatory power where a temperature differential was applied across the same climatic backdrop as occurred in this project. In other words, although the steady rise in average air temperatures may result in a general advancement of phenology, it is the unique series of high and low temperatures at different times and for different lengths of time that is unique to each season that will be the ultimate driver of phenology. The widening of the spread in timing to various phenological milestones from a common starting point as each season progressed clearly implicates temperature, but in ways that the current application of climate data to viticulture cannot explain.
Leaf function measurements showed that assessing single components of climate (e.g. air temperature) in isolation from other components (e.g. CO2 concentration) gave an incomplete picture of likely leaf function in the projected future climate. Photosynthesis responded positively to elevated CO2, whereas +2°C air temperature alone had no impact. When the two factors were combined, photosynthesis was greater than that observed in response to elevated CO2 alone. Thus, projections of the likely impacts of climate change based on assessing the impact of one climatic variable without the other may be misleading.
Annual and perennial growth of grapevines were largely unaffected over the four seasons by either the concentration of CO2 or air temperature. Nonetheless, elevated CO2 was associated with an additional two kg of fruit per vine per season overall, but that effect was dwarfed by a factor of five by the seasonal effect. Leaf function measurements combined with whole vine water use measurements suggested that in a warmer climate with higher CO2 concentrations, whole vine water use may not be any different from the present. Combined with the observation that higher temperature did not carry a yield penalty, and that higher atmospheric CO2 may even result in a slight improvement in productivity, the volume of water the vines use to produce each tonne of grapes will most likely remain the same or diminish marginally as the climate changes to resemble the climate imposed experimentally here.
The single biggest influence on grape composition at maturity was season, but, over and above the seasonal factors, warmer air resulted in grapes with less tannins, anthocyanins and flavonols. Warmer temperatures also advanced maturity, and the effects of temperature on final grape composition may be due to a combination of less time to accumulate sufficient oenologically-significant compounds and greater thermal degradation of those compounds. For the most part, the effects of the climatic variables on grape composition at harvest translated through to the final wine. Wine stability during storage out to 24 months was not influenced by the grapes’ growing conditions. The Australian wine industry may need to consider the wider use of altering the microclimate in the grapevine canopy in order to counter these effects.
The project team gratefully acknowledges the financial support, through Wine Australia, of Australian wine grape producers and the Australian Government, and the co-investment by the Victorian Government and CSIRO. The project team also acknowledges the financial support of the same parties for project DPI 09/01, which enabled the development and testing of the heated OTC concept that was the platform on which the CO2 enrichment could be added.