Growth Characteristics of Introduced Green Ash (Fraxinus pennsylvanica Marshall) and Narrow-leaved Ash (F. angustifolia L.) in Lowland Forest Region in Croatia

American ash and green ash were used about 100 years ago in the lowland regions of Croatia to afforest and fill the depressions where narrow-leaved ash (Fraxinus angustifolia Vahl) could not survive. The growth of mean stand trees of green ash and narrow-leaved ash were studied in 14 sites (a systematic sample of 0.03 ha circular plots). A total of 212 trees were cored and 11 cross-section samples were taken for stem analysis. The Mann-Whitney U-test showed statistically significant differences in radial growth between narrow-leaved and green ash at all sites. In green ash, the test revealed a statistically significant difference in the radial growth between the last 20-year-period and the preceding 20-year-period. Green ash are characterised by the fact that annual radial growth in the first 20-30 years is very high (above average), while after that it is consistently suppressed below average growth and amounts to only about 1.5 mm. The obtained results of faster growth and a low final growth value suggest that green ash should be managed with short rotations, since this important pioneer species is intended to improve the conditions in those sites where forest vegetation has regressed.