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ŠUMARSKI LIST 3-4/2018 str. 24     <-- 24 -->        PDF

The Pedunculate Oak is an allogamous monoecious species, and the flowering and flushing are largely determined by climatic conditions. Distinct variations in the flushing of the Pedunculate Oak have been noted, and different authors have divided the species into different forms (Mátyás 1967, Stojković 1991) based on differing numbers of categories from very early flushing (super praecox) to very late flushing (tardissima). The late flushing form (Q. robur var. tardissima) develops leaves approximately five weeks later than the early form (Q. robur var. praecox), and both forms occur in the all parts of the species’ distribution. This phenomenon is most pronounced in the south-eastern part of the Pedunculate Oak range, where the early and late trees are easily distinguishable (Bacilieri et al.1994, Wesolowski andRowiński 2006, Perić et al. 2000). The late flushing types of Pedunculate Oak have a smaller length of crown, are more cylindrical with good straightness of the trunk and are less damaged by the late spring frost than the early types.
The genetic structure and variability of clonal phenoforms in clonal seed orchards were analysed, revealing that there were no significant differences in genetic differentiation among the phenoforms (Katičić et al. 2010). The analysed neutral markers were apparently not associated with adaptive differences between the phenoforms. The variation in flushing phenology is presumably one of the species’ defensive strategies against late frosts. There are several reports of significant differences in genetic variability and differentiation among phenoforms within temperate tree species, e.g., late flushing forms of European beech had a higher intrapopulation genetic variability than early flushing forms. Oak trees were found to be more sensitive to temperatures with regard to leaf unfolding over an elevation gradient and were also found to have a lower chilling requirement for dormancy release than European beech (Chmura and Rožkowski 2002, Jazbec et al. 2007, Gömöry and Paule 2011, Dantec et al. 2014). Genetic variations in leaf unfolding timing between and within oak populations are likely due to differences in heat requirement rather than differences in chilling requirement (Dantec et al. 2014).
The technical quality of late flushing Pedunculate Oak trees is likely slightly better due to the delayed budburst and flushing of leaves, which is out of synchrony with eclosing caterpillars and avoids defoliation (Schütte 1957, Hunter 1992, Tikkanen and Julkunen-Tiltto 2003). In addition to the early and late types, there are also intermediate phenotypes, which are moving towards one or the other direction.
Because the late flushing Pedunculate Oak (Q. robur var. tardissima) is more desirable for cultivation in many European countries, the continued selection and collection of acorns from very late flushing trees from natural stands in the beginning of May is recommended. The establishment of clonal seed orchards with late flushing genotypes would certainly ensure higher quality forest reproductive material with the most advantageous genetic type.
The establishment of clonal seed orchards began with the goal of controlling more regular yield periodicity and obtaining seeds (acorns) with high genetic quality. The leaf unfolding of Pedunculate Oak is largely genetically controlled (Mátyás 1967, Stojković 1991, Baliuckas 2001), which provides an opportunity for legitimate selection of plus trees to achieve lasting phenological uniformity of leafing genotypes from which to establish clonal seed orchards. The phenology of trees is strongly driven by environmental factors, such as temperature, that have already altered the vegetative and reproductive phenology of many forest species. However, the impact of adverse weather conditions, such as heavy rainfall, high relative humidity, frost, low and high air temperatures, and hail, can sometimes disturb favourable phenological uniformity of phenophases in forest stands and within seed orchards (Wolgast 1972, Cecich 1997, Garcia-Mozo et al. 2001). Temperature and photoperiod are widely considered to be a major factors controlling the phenology of boreal and temperate tree species (Sarvas 1972, 1974, Heide 1993, Schwartz 2003, Chuine et al. 2003, 2010, Hänninen and Kramer 2007, Basler and Körner 2012, Fu et al. 2013).
Phenology is the study of the timing of life-history events that occur in a seasonal and repeated pattern. Understanding the processes responsible for macro-scale spatial and temporal phenological patterns is a critical step in developing predictive phenological models (Phillimore et al. 2013). While phenological responses may involve the integration of multiple environmental cues, the spring phenology of many plant species appears to be especially sensitive to temperature. The timing of leaf unfolding is mainly regulated by temperature in cold winter environments. Chilling temperatures break winter dormancy and subsequent warm temperatures induce leaf unfolding, which is why the phenological onset of spring correlates very well with the air temperature of the preceding months (Menzel 2002, Davi et al. 2011). Heat units, expressed in growing degree-days (GDD), are frequently used to describe the timing of biological processes. Approaches that use growing degree-days (GDD) assume a linear effect of temperature on the development rate via enzyme activity (Bonhomme 2000). In the areas of crop phenology and development, the concept of heat units, measured by GDD (°C-day) has vastly improved the description and prediction of phenological events relative to other approaches, such as time of the year or day of year (McMaster and Wilhelm 1997). Growing degree days (GDD) models predict the day on which a phenological event should take place.
Phenological models, used to simulate leaf unfolding, i.e., the start of the growing season, are based on the response