The characteristics of diameter growth and increment of introduced North American ash species at the sites of common alder

Research was conducted into diameter growth and increment of introduced American ash species at the sites of common alder (Alnus glutinosa[L.] Gaertn.) in three mixed stands of introduced ash and common alder in the lowland part of Croatia. About 10 samples of increment cores of each species were taken from trees in the diameter class of the mean basal area. In the process, an attempt was made to reach the centre of the trees with the purpose of getting an insight into the development of the trees over a long period, i.e. from the moment when the trees reached breast height. The increment cores were analysed by measuring the width of each individual ring to the tenth of the millimetre. Bark thickness was also measured. The results obtained from the analysis of the increment cores yielded trends of ring widths for individual trees as well as trends of the average ring widths for each species within a locality. On the basis of the increment core analysis, parts of the diameter growth curve of the mean stand tree were obtained. The increment curve of mean stand diameter was obtained by deriving the growth curve of mean stand diameter. Introduced ash was found to have average ring widths from 0.92 to 4.21 mm. The measured minimal and maximal values of ring widths in total were 0.2 and 6.5 mm, respectively. Consequently, introduced ash may have significantly large ring widths in conditions of prolonged flooding as well. Therefore, its pioneering role is not irrelevant at poor sites naturally inhabited by common alder. The average ring width of introduced ash is the highest up to the age of 30 years, after which it retains the value of 2 mm or less. In common alder, the average ring width in the studied stand ranged from 1.40 to 4.59 mm. The measured minimal and maximal values of ring widths in total were 0.4 and 7.0 mm, respectively. A comparison of average ring widths of introduced ash and common alder revealed that in Draganić locality common alder had a statistically significantly larger average ring width than introduced ash. In the localities Karlovac and Đurđevac the difference was not statistically significant.