The effect of tree diameter class on cambium phenology and radial growth of hornbeam (Carpinus betulus L.) in Kheiroud forest, Nowshahr

According to the importance of hornbeam (Carpinus betulus L.) in Hyrcanian forests, studying cambium activity and tree-ring growth of this species with respect to different tree diameter classes can present information about the growth of these trees. Therefore, the beginning, the end, and the duration of cambium activity (growth period) as well as growth rate were studied in three different breath height diameter classes i.e. 25-35, 36-50 and 60-90 cm in 2016 growing season in Kheiroud forest. The study method was consecutive extracting of 1080 micro-cores from trees and further microscopic investigations. Results showed that the cambium activity of trees belonging to the first and second diameter classes started in late April and, respectively, continued until early and mid-October. In the third diameter class, cambium activity began with a significant delay compared to the first and second ones (late May) and ended earlier (late September). In addition, this class had the shortest duration of growth among classes resulting in much narrower tree rings. Younger trees (class one) showed a higher synchronization in the terms of daily growth rate while with increasing age and reaching the longevity, homogeneity in growth decreased. Overall, it seems that trees in the first and second diameter class were in the phase of maturity and thus the timing of their growth were similar, but old trees of the third diameter class, reaching the longevity of hornbeam, were very different from two other ones.