Phenology of leaf development in European beech (Fagus sylvatica) on a site in Ljubljana, Slovenia in 2020

In spring of 2020 we observed leaf phenology in mature European beech (Fagus sylvatica) trees in Tivoli, Rožnik and Šišenski hrib Landscape Park in Ljubljana, Slovenia (46.05°N, 14.49°E, 310 m a. s. l.). A group of 11 trees was selected for daily monitoring of leaf development. We recorded seven phases from dormant buds, through leaf unfolding till development of mature leaves. At the same time, photos were taken to illustrate the leaf development. First developing leaves were observed on 4 April in one tree. General leaf unfolding, as defined by International Phenological Gardens (IPG), was observed in different trees between 7 and 25 April 2020. The occurrence and duration of individual phases of leaf development showed great variation within and between trees. General leaf unfolding of the tree included in the long-term monitoring program of the Environmental Agency of the Republic of Slovenia (ARSO) occurred on DOY 105 (14 April 2020), which is 4 days earlier than long-term average of the same tree/location, and this is ascribed to above average temperatures in the winter of 2019/2020.