Has soil drying contributed to earlier grape ripening in wine regions of southern Australia?

Abstract

Background and Aim A long-term trend in early ripening of winegrapes in southern Australia has been attributed to an increasing growing season temperature, soil drying linked to global warming and crop management. However, the 5 × 5-km cell size used used in the continental-scale model to derive soil properties was too coarse for the study vineyards of 0.2–16 ha. This paper aims to test the modelling conclusions using long records of annual rainfall for sites as close as possible to the experimental vineyards. Methods and Results If prolonged soil drying has occurred, it should be correlated with a decrease in annual rainfall, given that actual evapotranspiration should change little as the effect of increased atmospheric carbon dioxide concentration compensates for the effect of temperature increase on evaporation. Analysis of the longest, most complete Bureau of Meteorology records showed a highly significant decrease in rainfall (27 mm/decade) at Margaret River, but paradoxically no significant early ripening. Conversely, significant earlier ripening on the Mornington Peninsula was associated with a significant increase in rainfall (8.5 mm/decade). Conclusion Although growing season temperature and crop management to reduce yields may have contributed to earlier ripening, the case for an effect of prolonged soil drying was not supported. Significance of the Study The output of simulation models describing complex biological systems needs to be tested wherever possible against experimental data and observations. Also, the scale of modelling should be appropriate to the system being studied, especially for soil, which has marked spatial variability.