Dendroclimatic investigations on Quercus rubra and Quercus robur in north-western Poland

The study was aimed at investigations of the relationships growth-climate at two oak species growing in forests of north-western Poland; the red oak, the most abundant deciduous tree species of foreign origin, in terms of the appearance sites and surface, and the common oak, the most abundant domestic oak species. The research material were samples taken with a Pressler increment borer from 51 oak trees from two research plots. Within the surfaces selected, trees grow in a fresh forest habitat, in the floral association of the lowland fertile beech forest, also called the Pomerania beech forest, with plants characteristic for the association of the lowland humid beech forest with mercuries, encountered only in the Puszcza Bukowa (Beech Forest). The trees investigated were in good and very good health conditions, only scarce branch and bough deadwood could be observed in the canopies. On the basis of measurements of the annual growth widths, three local chronologies were constructed; two for the red oak and one for the common oak. After the indexation, they were used for dendroclimatical analyses; signature years, correlation and response function. The red oaks forming the shortest chronology (79-year-long pattern representing ca. 90-year-old trees) exhibited the narrowest mean annual increments (1.27 mm), in comparison to the longer chronologies (156 and 151 years in length, the increment widths 1.55 and 1.59 mm, respectively). This could be due to relatively high density of the tree stand, formation of the second stage, beneath the main canopies, as well as lack sufficient amount of the sunlight for the red oaks from that research plot. At both the species analysed, the predominating factor affecting the annual growth widths proved to be the amount of precipitation in the growth season, particularly in June and July, as well and thermal conditions of October of the year preceding the vegetation season in question. In the time of present climatic changes, as well as shifting ranges of the species of plants and animals (including insect pests of the forest trees), knowledge of the relation growth-climate for one of the foreign trees, most often introduced in Poland, characterized by fast growth, resistance on insect pests, and high quality of wood, becomes highly important for the forest management, particularly at planning seedlings, of which effects would be possible to evaluate in over 100 years.