Harvest dates – temperature relationships and thermal requirements of winegrape varieties in Greece: observed and future climate responses

Abstract

Context and purpose of the study Air temperature is arguably one of the most decisive factors for winegrape varieties developmental cycle, ripening potential and yield. Taking into account that predicted future warmer conditions will possibly impose challenges in global viticulture, it is of outmost importance to understand the adaptive capacity of each variety in the current and future climate conditions. Thus, the objective of this study was twofold: (a)to investigate the relationships between air temperature during the ripening period and harvest dates for eight principally cultivated indigenous winegrape varieties (one for each winegrape region of Greece) and (b) to assess varieties’ thermal demands (four varieties) using the standard growing degree day (GDD) formula and project harvest date in two future windows using a multi-Regional Climate Model ensemble dataset.

Material and methods Harvest dates were assembled from four white [cvs. Muscat of Alexandria (Limnos), Assyrtiko (Santorini), Muscat blanc (Samos) and Athiri (Rodos)] and four red [cvs. Moschofilero (Tripoli), Mavrodaphni (Pyrgos), Mandilaria (Crete) and Xinomavro (Naoussa)] varieties, covering a period from 11 to 44 years. Daily observations of maximum (TX) and minimum (TN) air temperature were obtained from the Hellenic National Meteorological Service (HNMS) in order: (a) to investigate the relationships between harvest dates and temperature conditions during the ripening period and (b) to o calculate growing degree days (GDD, C units) for each variety. In addition, high resolution ensemble datasets (derived from 5 model experiments) with the two representative concentration pathways 4.5 (RCP4.5) and 8.5 (RCP8.5) were employed to project harvest dates for two future time windows [future projection 1 (FP1): 2041-2065 and future projection 2 (FP2): 2071-2095]. Pearson’s correlation coefficient was used to investigate relationships between air temperature and harvest date. Statistical significance was set at p< 0.05.

Results Harvest dates showed negative trends in six out of eight cases (four cases statistically significant) while in two areas (Crete and Pyrgos) harvest occurs later. In addition, harvest date – temperature analysis showed significant negative relations in seven out of eight cases. Rodos (cv. Athiri) was the only case with a significant positive relationship. Heat requirement analysis revealed that two varieties (cvs. Muscat of Alexandria and Moschofilero) needed almost 1700 GDD to achieve full maturity while the other two varieties (cvc. Mavrodaphni and Xinomavro) exceeded 2000 GDD units (2021 and 2049, respectively). Future projection analysis showed that harvest will shift earlier for all varieties (ranging approximately from one to two months) and this shift in both time windows will depend on the variety and the selected emission scenario.