The majority of the landmass area of Ethiopia is covered by woodland vegetations dominated by the genera Acacia, Boswellia and Commiphora (ABC). These resources are of high economic importance because of their aromatic gum resins like frankincense and myrrh which are used as raw material in several industries (Lemenih et al., 2003; 2007). They also have local uses and ecological significances.
The annual production of aromatic gum resins is increasing over the last years and further increase is stimulated (PASDEP, 2005). However, these resources are facing severe threat of degradation from farmland expansion, fire, improper tapping, overgrazing and climate change (Ogbazghi et al., 2006; Rijkers et al., 2006; Lemenih et al., 2007). Studies demonstrated that younger trees for some of these species (e. g. B. papyrifera) are lacking (Lemenih et al., 2007; Nigussie et al., 2008). In this context it was observed that B. papyrifera shows a complex mechanism of regeneration with the seedlings exhibiting a kind of “waiting-under-the-ground strategy”: they re-sprout and subsequently die back after germination for many years before the sapling stage is achieved. This phenomenon illustrates that more knowledge on long-term population dynamics and its relation with environmental factors is required. Currently, the potential for sustainable use of these resources is being explored in the multidisciplinary research program FRAME (FRAnkincense, Myrrh and gum arabic: sustainable use of dry woodland resources in Ethiopia). Sustainable management of these resources entails, among other things, the understanding of growth behavior of these resources in relation to environmental changes.
Climate-related factors like soil and air temperatures, soil moisture, sunshine and wind can play an important role in plant growth. Plant carbon allocation strategy to different compartments (stem, root and leaf) depends on environmental conditions, among which climatic factors are very important. For instance, when water is the most limiting factor, carbon is preferentially allocated to roots to maximize water uptake (Scheiter and Higgins, 2009). The same study indicated that climate variations can induce changes in size distribution of tree populations.
Demographic studies are important to understand the way in which tree population structure changes in time (Sarukhán and Gadgil, 1974). Matrix models have been used as powerful tool to predict population structure (Caswell, 2001) and to describe the demography of plant populations (Lefkovitch, 1965; Harcombe, 1987; Valverde and Silvertown, 1998; Caswell, 2001; Zuidema and Boot 2002; Couralet et al., 2005).
For a better understanding of future trends in tree growth and population dynamics, studying the long-term responses of tree growth to climate variation is important. Investigation of growth rings is found to be important to understand past growth patterns and dynamics of tree resources in relation to ecological factors (Jacoby, 1989; Worbes et al., 1992; Worbes, 1995; Brienen and Zuidema 2005). The use of dendrochronology in combination with matrix modeling enhances understanding of the past behavior of trees, vegetation responses to climate and predictions of population dynamics (Couralet et al., 2005).
In Ethiopia, dendrochronology was successfully applied on Acacia and Juniper species (Eshete and Stahl, 1999; Couralet et al., 2005; Gebrekirstos, 2006; Wils, 2007; Wils and Eshetu, 2007; Wils et al., 2008; 2009; Sass-Klaassen et al., 2008). However, to our knowledge no dendrochronological studies have been conducted yet on Boswellia and Commiphora in Ethiopia. Based on the promising results for the above-mentioned species and the strong seasonality in climate in the study areas, species-specific procedures for growth-ring identification will be developed for Boswellia and Commiphora species. B. papyrifera is emphasized in this study because it is the chief resin producing tree species in Ethiopia (Tadesse et al., 2007) and it suffers from problems in regeneration with a lack of young trees in its population structure (Ogbazhgi, 2001; Lemenih et al., 2007; Nigussie et al., 2008; Abiyu et al. 2010). In this respect it is of high economical and ecological importance in Ethiopia.
Likely increase in precipitation is expected in east Africa (Hulme et al., 2000; IPCC, 2007) in which Ethiopia is also part. Blankespoor et al., 2009 showed that seasonal shift of precipitation in different parts of Ethiopia is anticipated as a result of climate change. Profound knowledge on the influence of climate on growth and population dynamics of ABC species is essential to come up with recommendations for sustainable management and mitigation of the impact of expected climate changes (IPCC, 2007) on the dry woodland resources of Ethiopia.
The aim of this research is to investigate the growth patterns and population dynamics of ABC species in relation to changing climate conditions. The following research questions and hypotheses will be addressed in this study:
- Do ABC species form annual growth rings, and if so which specific wood-anatomical features depict the growth rings?
- Which climate factors mainly influence the radial growth of the ABC species?
- Which climate factors drive the ontogeny (seedling -> sapling) and allocation to radial and axial growth in B. papyrifera?
- How is population structure of B. papyrifera influenced by climate and what will be the effect of the predicted climate-change scenarios on the future population structure?