Growth trajectories and ages of main tree species in dry Afromontane forest fragments of northern Ethiopia

Abstract

Information on long-term growth rates, ages, and survival of dominant tree species is vital to understand forest dynamics. In this study, tree-ring analysis was used to reconstruct lifetime growth patterns and to examine age–diameter relationships of selected tree species from the dry Afromontane forest fragments in northern Ethiopia. Ring width measurements were based on increment core samples and stem discs collected from three tree species (Juniperus procera, Olea europaea subsp. cuspidata, and Podocarpus falcatus). Standard dendrochronological methods were used for data collection and analysis. Overall, across species and sites, significant mean radial growth differences were found. P. falcatus trees showed the highest mean annual radial growth (2.45 mm/year), while O. europaea trees showed slow growth rates, with average annual growth ranging between 1.6 mm in Desa’a and 2.0 mm in Hugumburda. The mean ring width of an individual J. procera tree ranged between 1.8 mm/year (in Desa’a site) and 2.3 mm/year (in Hugumburda site). Many trees of the sampled species exhibited more or less sigmoid growth curves. Age variation is mainly determined by the variation in passage time through the smaller diameter classes (juvenile classes). In general, the results obtained in this study have important implications for understanding growth dynamics of tropical dry Afromontane forests and for planning conservation and restoration activities. This study has to be supported by eco-physiological studies to further understand the responses of these and related main tree species to the varied environmental gradients to better explain the dynamics in relation to emerging environmental changes.