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ŠUMARSKI LIST 3-4/2018 str. 25     <-- 25 -->        PDF

of bud growth to forcing temperatures and/or to chilling temperatures, and this response has been described with different types of mathematical functions (Chuine 2000). The leaf unfolding process is often considered to involve two stages (Cannell and Smith 1983, Hänninen 1990, Kramer 1994, Chuine 2000, Chuine et al. 2003). First, bud dormancy needs to be broken by exposure to cold temperatures during winter, referred to as chilling. Once sufficient chilling has accumulated, the bud enters the second phase of dormancy, during which sufficient warmth is needed to trigger bud burst, referred to as forcing. The accumulation of forcing is based on a linear temperature relationship above a given temperature threshold, and the interaction of chilling and forcing requirements with temperature determines the timing of leaf unfolding.
This work is based on ten years of data recording the leaf unfolding of Quercus robur L. in a clonal seed orchard and meteorological data. The parameters are start time (day of year), base temperature, cumulative GDD, precipitation and insolation. From the start time, the daily temperatures above the base temperature are summed, and the model predicts that the phenological event will occur once the cumulative GDD requirement has been met. A number of approaches exist for identifying the best parameters under the GDD model, but most involve the iterative application of combinations of parameters to the temperature data and estimation of one of several related metrics that quantify the difference between the predicted and observed phenology (Bonhomme 2000). The aim of the study was to determine requirements of the GDD values, precipitation and insolation by each phenological group of pedunculate oak, as well as chilling requirements for breaking dormancy of vegetative buds and mutual relationship between chilling and GDD. Thus, the application of GDDs in the future could contribute to better and easier predictions of how phenoforms and pathogens are related, based on their biology and ecology. The values obtained by chilling could be used for the prediction of the required amount of forcing temperature and the occurrence of the leaf unfolding phase.
Study Area – Područje istraživanja
A clonal seed orchard of Pedunculate Oak (Quercus robur L.) was established in the spring of 2001 at the Forest Office Čazma in the lower Sava river provenance region of Croatia. The seed orchard has an area of 26 ha, contains 53 clones (genotypes) selected in seed stands, and features a total of 2,150 ramets (grafts) randomly planted in the orchard. The genotypes belong to a single seed region that includes seven subprovenances. The pruning and training systems of the grafts in the clonal seed orchard should produce a strong framework and good light penetration in the canopy (Kajba et al. 2008). An oval spindle training system is used after establishment, and the spacing between ramets is 10 × 8 m. The total height of the training shape is up to seven meters with six scaffold whorls.
The clonal seed orchard is located at 45º 45’ 55’’ N and 16º 35’ 32’’ E in the northwest part of Croatia. The climate in this region is moderately warm and rainy with a two short dry period (April and July). The average value of coldest month temperature is between - 3°C and 18 °C, while the mean temperature of the warmest month is not above 22 °C.The mean annual temperature is 9.8 °C, and the mean annual precipitation of 810 mm is evenly distributed throughout the year.
Phenology measurements – Fenološka motrenja
During the 10-year study period (2004-2013), the phenological data of leaf unfolding and meteorological records were studied. Only well-developed individuals (grafts) were chosen for observation. The start date of the leaf unfolding stage is defined as the point at which the entire leaf blade and leaf stalk were visible.
Each year, observation began before any buds began to break, and observation was made every 10 days until the 10th of May. The meteorological data were supplied by the Croatian Meteorological and Hydrological Service, based on data recorded at the Čazma meteorological station, which is located 1 km from the orchard site (DHMZ). Observations were made on ramets of 53 clones randomly selected across the entire area of the orchard. The start of heat accumulation has been taken from the 1st of January. Only positive values of GDD were taken into account to calculate cumulative sums. Insolation is expressed in hours (h) and precipitation in mm. The ten years of leaf unfolding data and the corresponding weather data were used to build the phenology models.
Statistical analysis – Statistička analiza
Growing Degree Days (GDD) were calculated using the Hand Calculating Degree Days method described by Snyder (1985):
If: GDD from the days included in final sums;
If: GDD from the days not included in final sums.          
Here, Tmax and Tmin are daily maximum and minimum air temperatures, respectively, and Tbase is the base temperature.