Evaluation of Walnut Tree Flowering and Frost Occurrence Probability During 1961-2012

Strength and direction of the potential effect of climate change on walnuts is regionally specific (Gauthier and Jacobs, 2011) as climate change will probably affect the spatial distribution of the walnut. This paper evaluated the long-term phenological series (1961–2012) of the beginning of flowering, end of flowering and duration of flowering in walnut tree ( Juglans regia ) at two phenological stations located in different geographical locations of the Czech Republic but in the same climatic conditions (warm region). Phenological stages were analyzed in relation to growing degree days and to spring frosts occurrence. Onset of the beginning and end of flowering occurred earlier at Velké Pavlovice station (-2.1 and -1.3 days), and conversely occurred later at Doksany station (+1.8 and +1.0). Period of flowering shortened at Doksany station (-0.8 day) and prolonged at Velké Pavlovice station (+1.2 day). The occurrence of days with minimum air temperature < 0 °C during walnut tree flowering was more frequent at Doksany station (in total, 29 days) with absolute minimum value -5.5 °C. At Velké Pavlovice station 5 days with minimum air temperature below 0 °C were found during examined period with absolute minimum value -3.8 °C. The negative trend in number of frost days occurrence during flowering period was found at both stations. Pearson correlation coefficient between phenophase onset (and flowering period) and sums of growing degree days was stronger at Velké Pavlovice station, and the highest value was between period of flowering and temperature sums (0.782). The results confirmed our hypothesis of changes in phenophase onsets and duration of flowering including spring frosts occurrence according to west-east gradient (maritime climate-continental climate).