Crown volume in forest stands of pedunculate oak and common hornbeam

Sa？etak Background and Purpose: The structure of the crown volume in a forest stand is one of the main factors that drive the growth and development of trees. It changes dynamically with the age of the forest stand and according to the management activities and natural disturbances that remove trees from the stand. The aim of this article is to analyse crown volume structure in relation to the stand age and diameter at breast height (DBH) of pedunculate oak and common hornbeam trees in one of the most important forest types in Croatia. Materials and Methods: Data for the research were collected from a set of 47 permanent sample plots established as a chronosequence over the distribution range of pedunculate oak and common hornbeam forests in Croatia. The combined area of all the plots amounted to 33.45 ha. Trees were measured for DBH, total height and height to crown base, and a detailed map of crown projection areas was made for each plot. In total, the crowns of 1,609 pedunculate oak trees and 1,979 common hornbeam trees were measured. Crown volumeswere calculated for each tree, the trees were pooled into age classes of 20 years, and analyses were carried out per tree species, per age class. A nonlinear regression with an exponential function of crown volume was performed to establish the relationship between the crown volume and DBH in each age class. Results and Discussion: Up to a stand age of 40 years, crowns of pedunculate oak dominate in the canopy layer (66%), after which crowns of common hornbeam trees assume dominance. Coefficients of determination for the regression lines for pedunculate oak crowns are higher than the coefficients for common hornbeam. In all age classes, they are higher than 0.50, except for the first age class for common hornbeam and the seventh age class for pedunculate oak. The shape of the regression lines of pedunculate oak crown volume shift more to the right side of the diameter range as the stand matures, while those of common hornbeam are more static and form a bundle of lines. Conclusions: Results indicate that the relationship between the crown volume of pedunculate oak and common hornbeam trees and DBH can be described by a nonlinear regression model with an exponential function. Further research is needed to assess the possibilities of integrating the obtained regression models into simulators of forest growth and development