Biological and taxonomical investigations of some oak species
Hermaphroditic flowers blooming unseasonably on an oak tree (»the green oak«) of uncertain hybrid origin prompted a detailed investigation into the biology of the flowering, and the morphology of various traits of the tree, which is growing in an urban area near Zadar, northern Dalmatia, Croatia. Hermaphroditic flowers were also detected on one holm oak tree from Weihenstephan, Germany. Since the first oak tree was described as a hybrid between Q. cerris f. austriaca Q. ilex, scanning electron microscope investigations of pollen and hairs on the leaves of three species were made, on: Q. ilex L., Q. cerris L. and the »green oak«. Oak pollen is of elliptical shape and tricolpate. The ornamentation of the pollen grains of Q. cerris and the »green oak« were similar concerning the ornamentation of the exine – warty and rough and with small rounded protrusions – as well as the size of the pollen grain. Q. ilex pollen was much smaller and the surface lacked the small irregularly shaped protrusions typical of the other oaks. Within samples of holm oak pollen, significant difference in the surface structures was observed. Concerning the different types and number of hairs on the leaf surfaces, Q. ilex revealed a much greater number of stellate hairs on the upper leaf side as compared to Q. cerris and the »green oak«. Simple uniseriate hairs and simple unicellular hairs were missing on the leaves of Q. ilex, but were present on both the other oaks. To sum up the results, the »green oak« and Q. cerris reveal some similarities whereas Q. ilex does not.
Quercus suber Distribution Revisited
The complex evolutionary history of Quercus suber is still under debate. Also, data and evidence at the eastern end of the species range are largely incomplete. In this study, historic floras, fossil data, and local toponyms were surveyed and genetic analyses and linguistic research used in order to point towards a previously neglected occurrence of Q. suber east of Italy. Such a multidisciplinary approach depicts a scenario in which cork oak survived in the Balkan Peninsula until recently and suggests how Q. suber might have been evicted and relegated westward, due to climate changes, ecologic competition and human impact. Our findings also suggest that the differentiation core of Q. suber was in a yet unidentified area corresponding to present-day central Europe to southwestern Asia. Radiation occurred during the Middle Miocene, with a later extension into southern Europe, Iberia and North Africa, as documented by samples collected from the late Miocene-early Pliocene. The causes of extinction in the east, however, should be investigated further and may lead to investigations about other species that might have experienced range shifts similar to that of cork oak.
CATKIN FREEZING AND ACORN PRODUCTION IN GAMBEL OAK IN UTAH, 1978
In May 1978, a general freeze of the male flowers of Gambel oak (Quercus gambelii Nutt.) was observed in northern Utah. In August, transect reconnaissance was conducted throughout Utah to assess acorn production and the incidence of freeze damage in Gambel oak. It was found that below ca. 1950 m elevation acorn production was relatively low while the incidence of freeze damage was high. Above ca. 1950 m elevation, where the oaks were still dormant at the time of the freeze, acorn production was relatively high while freeze damage was relatively low. Occasional acorn production observed at low elevations appeared to be associated with subterranean pooling of water and/or cold air drainage. Some biogeographic implications of freeze stress are discussed.