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(Roetzer et al. 2000). In climates with distinct temporal seasonalities, forest trees employ phenology to adapt to these conditions of growth. In case of abrupt climate changes in the future, forest trees will find it more difficult to adapt to newly-formed environments, unlike plant species which have much shorter life spans. In order to assess possible impacts of climate change on the growth of different forest ecosystems in Europe, it is important to understand environmental drivers which directly influence phenological manifestations (Kramer et al. 2000). Phenological phenomena, such as leaf unfolding, autumn colouration, leaf falling and others are influenced by several different factors. These include site competition, chemical soil composition, genetic constitution and age of an individual’s; however, the greatest source of activation of all these phenomena is contained in the environmental drivers to which a species/population is exposed. De Réaumur (1735) found that the greatest impact on flushing and flowering is performed by cumulative temperatures preceding the manifestation of phenological traits. Several more factors have been identified that influence the manifestation of phenological traits, such as the length of chilling period (Murray et al. 1989; Sogaard et al. 2008; Laube et al. 2014), photoperiod (Heide 1993; Körner and Basler 2010; Caffarra and Donnelly 2011; Basler and Körner 2012; Laube et al. 2014), temperatures in the preceding autumn (Heide 2003), soil nutrient availability (Jochner et al. 2013), precipitation (Penauelas et al. 2002; Estiarte et al. 2011; Andrić et al. 2016) and insolation regimes (Linkosalo and Lechowicz 2006). If temperature is the main driver of phenological manifestations in a species, then damage from late spring and early autumn frost is minimal for this species (Hänninen 1990; Häkkinen et al. 1998), but if the main driver is water availability in the soil, then damage from drought, such as embolism in the xylem, will be avoided (Magnani and Borghetti 1995). Higher precipitation quantities before the vegetation season may increase the need for temperature sums (Fu et al. 2014), which confirms that precipitation can also indirectly contribute to the beginning of leaf unfolding. The impact of precipitation on the beginning of leaf unfolding is more distinct after a dry winter, when afterwards the precipitation quantity in the spring period represents the only available water source for the plants (Shen et al. 2015). The mechanism of the effect of sun insolation on the phenological changes has not yet been fully clarified (Calle et al. 2010). Although the length of day as a separate variable is sufficient to explain physiological processes of leaf unfolding (Borchert et al. 2005; Borchert and Rivera 2001), more recent research (Calle et al. 2009) has indicated that insolation should also be perceived as an important variable in phenological studies. According to some earlier studies (Stewart and Lhryer 1994; Yuan et al. 2007), precipitation as one of the drivers of phenological manifestations has a much greater impact on the phenology of understory plants, but not necessarily in forest ecosystems of temperate regions (Dose and Menzel 2004; Morin et al. 2010; Sherry et al. 2007). These studies draw on the fact that forest trees have much deeper roots and are thus capable of satisfying their water need from deeper layers of the pedosphere (Sarmiento and Monasterio 1983). The objectives of this research were: 1) which of the three environmental drivers has the greatest influence on the budburst dates in narrow-leaved ash; and 2) is it possible to predict the beginning of budburst through the studied variables, and if so, with what accuracy. The results of this research could contribute to a better understanding of narrow-leaved ash responses to climate conditions in which it grows. MATERIALS AND METHODS MATERIJALI I METODE Study area and phenology monitoring – Područje istraživanja i fenološka motrenja Phenology monitoring was performed in the clonal seed orchard in Nova Gradiška Forest Administration (seed region of the central Sava valley) that covers an area of 3.53 ha. The clonal seed orchard (45.252463, 17.362132) was established in 2005 with planting distances of 4×4 m. Research comprised 168 plants (42 clones with 4 ramets) over four vegetation seasons (2012, 2014, 2015 and 2016). The target phase of phenological monitoring was the phase in which budburst and partial separation of bud scales was visible. All daily values of temperature (average values); precipitation and insolation were obtained from the meteorological station Gorica (DHMZ, Meteorological and Hydrological Service of Croatia) one km away from the clonal seed orchard. Statistical analysis – Statistička obrada podataka Three environmental drivers were included in the research: daily temperature sum (TEMP), daily precipitation sum (PREC) and daily insolation sum (INS). Three sub-variables were created for all the three variables based on the principle of different starting date of summing. The first summing date was from November 1st (TEMP_NOV; PREC_NOV; INS_NOV), the second from December 1st (TEMP_DEC; PREC_DEC; INS_DEC), and the third from January 1st (TEMP_JAN; PREC_JAN; INS_JAN), so that nine variables were obtained for the analyses. The first step was to determine the correlation between the beginning of budburst and nine different variables. The Spearman rank correlation (rs) was used for this purpose. Linear regression was applied to analyze all data sets separately for the entire research period (2012, 2014, 2015 and 2016). Multivariate regression was employed to test the best subset of variables for budburst date description. The three best subsets were tested using the leap function for each parameter, which were then ranked according to the R2 criterion. In the final part, models were |