MODELLING THE TIMING OF LEAF UNFOLDING IN PEDUNCULATE OAK (Quercus robur L.) CLONAL SEED ORCHARD

For ten years, 53 genotypes of Pedunculate Oak were monitored in a clonal seed orchard. The aim of the study was to clarify effects of forcing and chilling temperature as background of leaf unfolding of pedunculate oak through the different kind of prediction models. The Pedunculate Oak is known to have three phenoforms of flushing phases, i.e., early, intermediate and late, and substantially different Growing Degree Days (GDD) requirements were found among these phenological groups. For each of the phenological groups, the GDD interval value required for leaf unfolding was determined, but the values varied between years, rendering the development of a simulation GDD model difficult. For these reasons, other variables - day of year, precipitation and insolation - were included in the assessment. The aim was to examine how these environmental factors determine the GDD requirement for leaf unfolding in the three Pedunculate Oak phenological groups. The results suggest that for all three flushing groups, insolation and the day of year are statistically significant predictors for GDD. Insolation was demonstrated to be the primary factor with the largest influence on the GDD values. In the early flushing group, insolation accounted for 74.1 % of the variation, 90.6 % in the intermediate group, and 78.7 % in the late group. We recommend using the univariate GDD model for the early and intermediate flushing groups (GDD(early) = -27.651 + 0.539 (star) insolation; GDD(inter) = -48.084 + 0.690 (star) insolation) and the multivariate model for the late flushing group (GDD(late) = -237.839 + 0.559 (star) insolation + 2.479 (star) day of year). To break the dormancy, chilling is required to be in the range of +/- 40 chilling units, and if the values are higher or lower, the trees require significantly higher forcing temperatures.