Siyum et al. Forest Ecosystems (2019) 6:15 https://doi.org/10.1186/s40663-019-0178-y RESEARCH Open Access Climate forcing of tree growth in dry Afromontane forest fragments of Northern Ethiopia: evidence from multi-species responses Zenebe Girmay Siyum1,2* , J. O. Ayoade3, M. A. Onilude4 and Motuma Tolera Feyissa2 Abstract Background: Climate-induced challenge remains a growing concern in the dry tropics, threatening carbon sink potential of tropical dry forests. Hence, understanding their responses to the changing climate is of high priority to facilitate sustainable management of the remnant dry forests. In this study, we examined the long-term climate- growth relations of main tree species in the remnant dry Afromontane forests in northern Ethiopia. The aim of this study was to assess the dendrochronological potential of selected dry Afromontane tree species and to study the influence of climatic variables (temperature and rainfall) on radial growth. It was hypothesized that there are potential tree species with discernible annual growth rings owing to the uni-modality of rainfall in the region. Ring width measurements were based on increment core samples and stem discs collected from a total of 106 trees belonging to three tree species (Juniperus procera, Olea europaea subsp. cuspidate and Podocarpus falcatus). The collected samples were prepared, crossdated, and analyzed using standard dendrochronological methods. The formation of annual growth rings of the study species was verified based on successful crossdatability and by correlating tree-ring widths with rainfall. Results: The results showed that all the sampled tree species form distinct growth boundaries though differences in the distinctiveness were observed among the species. Positive and significant correlations were found between the tree-ring widths and rainfall, implying that rainfall plays a vital role in determining tree growth in the region. The study confirmed the formation of annual growth rings through successful crossdating, thus highlighted the potential applicability of dendroclimatic studies in the region. Conclusions: Overall, the results proved the strong linkage between tree-ring chronologies and climate variability in the study region, which further strengthens the potential of dendrochronological studies developing in Ethiopia, and also has great implications for further paleo-climatic reconstructions and in the restoration of degraded lands. Further knowledge on the growth characteristics of tree species from the region is required to improve the network of tree-ring data and quality of the chronology so as to successfully reconstruct historic environmental changes. Keywords: Climate-growth relationship, Climate change, Dry Afromontane forest, Restoration, Tree-ring width * Correspondence: [email protected] 1Pan African University, Life and Earth Sciences (including Health and Agriculture) Institute, University of Ibadan, Ibadan, Nigeria 2Wondo Genet College of Forestry and Natural Resource, Hawassa University, P.O. Box 128, Shashemene, Ethiopia Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Siyum et al. Forest Ecosystems (2019) 6:15 Page 2 of 17 Background forests and their sensitivity to the changing climate through Accounting for about 25% of the global terrestrial carbon the provision of reliable results (Brienen et al. 2016). (Bonan 2008), tropical forests play a crucial role in regulating Despite the continued success stories on the wide ap- regional and global climate dynamics (Lewis et al. 2009; plicability of tree-ring research in the tropical regions, Zhou et al. 2013). The dry forests and woodlands in particu- the formation of annual growth rings in tropical forest lar harbor diverse and multifunctional landscapes, thus play trees was once doubtful (Lieberman et al. 1985) and still vital roles in supporting vulnerable households at times of remains controversial (Worbes 2002). There is a notable hardships (including those increasingly affected by climate lack of intensive tree-ring study in the tropics and thus change and variability) (Blackie et al. 2014). However, the relatively fewer tree species have been documented com- climate-induced challenge remains a growing concern in the pared to the temperate and boreal regions where a dense dry tropics and is threatening the counterbalancing poten- network of tree-ring data exists (Wils et al. 2011a). Hence, tials of these ecosystems (IPCC 2007, 2013). Studies docu- more widespread application of rigorous tree-ring studies mented the dynamics of these ecosystems along with the will yield important insights into the mechanisms that mechanisms by which the diverse species and their ecology govern tree growth and forest dynamics in these regions. coexist in the face of global changes. But, most studies Such knowledge is highly valuable to understand past undertaken on tropical forest dynamics paid much attention growth patterns and dynamics of tree resources in relation to inventories from permanent plots and plant physiological to ecological factors (Brienen and Zuidema 2005), and in experiments(Lewisetal.2009). Even though such monitor- assessing how tropical forests will respond to the pre- ing plots provided much of the evidence for understanding dicted climate changes. tropical forest dynamics, they have shortfalls in that they In regions with distinct climate seasonality, tree species are accounted only for short time horizons and limited areas forced to enter into periodic dormancy, thus tend to form (Lewis et al. 2009;Zhouetal.2013). This hindered under- annual growth rings (Fritts 1976, pp. 1–25). But, their radial standing of the long-term responses and feedback of tropical growth (i.e., the activity of vascular cambium) is influenced forests to global changes (including climate change). not only by climate, but also by a range of environmental Tree species in the dry tropics are predicted to be particu- and genetic factors (Cook 1987), and indeed their interaction larly sensitive to the unprecedented climatic changes. Due to effects. This implies that the formation of different anatom- the expected heightened warming and drying, tropical trees ical wood features is both species-and site-specific. In the will likely experience slow growth and high mortality rates tropics, the formation of annual tree-rings was reported in (Nath et al. 2006;IPCC2007). So, in the absence of a thor- association to various factors; for instance, seasonality in ough understanding of the climate-growth relations of these rainfall (Worbes 1999), day length (Borchert and Rivera tree species, it is difficult to make inferences on the re- 2001), temperature (Fichtler et al. 2004) and even flooding sponses of dry forests to the changing climate. This entails (Schongart et al. 2002). The different dendrochronological that there is a dire need to intensify such studies in the dry studies conducted for various tropical tree species (e.g. Stahle tropics, and this, in turn, requires long-term datasets on et al. 1999; Worbes 1999; Fichtler et al. 2004; Brienen and growth and climate relations (Clark 2007). In view of this, Zuidema 2005; Schongart et al. 2006; Brienen et al. 2010)re- shifts in research approaches were observed over the past vealed that rainfall is the main determinant factor for tree few decades, targeted at addressing the gaps of forest moni- growth although the effect of temperature variability was also toring studies which used permanent sample plots. Of the reported in some regions. However, growth response to the various approaches, tree-ring study has been identified as a climate signal is modified by tree species, provenance, com- powerful tool to study climate-growth relationships and petition, and site conditions, among others (Fritts 1976, pp. thereby rectifying the scarcity of long-term growth data for 14–23). Therefore, such studies need to be intensified in dif- tropical forest trees (Rozendaal and Zuidema 2011;Zuidema ferent regions considering various tree species, and this is et al. 2012, 2013). Tree-ring research is generally envisaged important for planning and implementing restoration and to provide the much-needed long-term data on tree growth sustainable management endeavors in tropical forests, par- and tree recruitment. Tree-ring research in the tropics is in- ticularly that of the tropical dry forests (TDFs). creasingly being employed in a range of applications, for in- In Ethiopia, successful dendrochronological studies were stance, to understand forest dynamics, determine forest age previously reported for different tree species (e.g. Gebre- structure, construct lifetime growth trajectories, predict tim- kiristos et al. 2008; Sass-Klaassen et al. 2008; Couralet et ber yield, estimate minimum age for use and understand al. 2010;Toleraetal.2013; Mokria et al. 2017). But, it was climate-growth relationships (e.g. Fichtler et al. 2004; Brienen also revealed that the formation of annual rings is and Zuidema 2005;Trouetetal.2006; Gebrekiristos et al. species-specific and the formation and distinctiveness of 2008; Rozendaal