Published on International Journal of Forestry & Plantation
Publication Date: December, 2019
Firaol Legesse Mereba & Meseret Chimdessa
College of Agricultural and Environmental Sciences, School of Animal and Range Sciences, Department of Range Ecology and Biodiversity, Haramaya University
P. O. Box. 138, Dire Dawa, Ethiopia
Oromia Agricultural Research Institute, Fadis Agricultural Research Center
Livestock Research Process, Animal Feed Resource and Rangeland Management Research Team
P. O. Box. 904, Harar, Ethiopia
An ecological study of the natural vegetation was carried out in Gobelle Valley of the BES, to identify of woody species, and to analyze diversity, richness, evenness, population structure, and regeneration status of woody species. Vegetation data were collected from 60 quadrats of 20 m x 20 m (400 m2) laid systematically at every 100 m along 3 transects. Diameter at breast height (DBH), richness, evenness and density of woody species were recorded. Shannon – Weiner Diversity Index was used to determine species diversity and evenness. A total of 58 plant species in 36 genera and 18 families were identified in the study area. The Shannon – Wiener Diversity index (H’) and evenness values of Gobelle valley vegetation were 3.05 and 0.75, respectively. The collected species were composed of 44.8 % trees, and 55.2 % shrubs. Fabaceae was represented by the highest number of species 15 (26 %). A total basal area of 22.38 m2/ha, and a density of 34687 individuals ha-1 were calculated for woody species (5159.6 trees and 29738.7 shrubs) individuals ha-1. The highest impact of invasive species was imposed from Lantana camara which had a density of 1,828 individual’s ha-1. Furthermore, 3 endemic species which are under the Red Data List of IUCN were also identified and requires close attention to conserve them in situ. The existence of strong anthropogenic disturbance in the sanctuary calls for an immediate intervention to conserve and ensure sustainable biodiversity of the study area.
Keywords: Endemic species, Invasive species, Species diversity, Population structure.
Babile Elephant Sanctuary (BES) is one of the protected areas in the semi-arid region of eastern Ethiopia. There are four drainage valleys (Fafem, Daketa, Erer and Gobelle) which extend southwards through the sanctuary to join Wabi Shebelle River Basin (Anteneh et al., 2011). The sanctuary was established to protect the only known population of the isolated and ecologically distinct subspecies Loxodonta africana orleansi (Barnest et al., 1999). The most recent molecular analysis indicated that this ecologically isolated relic elephant population is Loxodonta africana africana (Ishida et al., 2011; Anteneh et al., 2011). Study has shown that the home range of the elephants has shrunk by about 65.5% since 1976 (Yirmed et al., 2006). This attributed to the mass influx of a large number of farmers and their domestic stocks mainly from the eastern and northern part of the sanctuary which directly contributed to declined in size and quality.
Previous studies by Demel (1995) in Daketa and Erer valley were investigated ecologically by Anteneh et al. (2011), respectively. However, no ecological investigation has been conducted in Gobelle valley, which is the main part of Babile Elephant Sanctuary inhabited by elephants. Thus, this study was designed to carry out ecological investigation of Gobelle valley in BES to help design management strategy.
2. MATERIALS AND METHODS
2.1. Description of the Study area
The study was conducted in the natural vegetation of Gobelle valley in Babile elephant sanctuary (BES) of East Hararge zone, Oromia regional state, Ethiopia (Fig. 1). BES is found at about 560km away from Addis Ababa to the East. It is delineated with coordinates of latitudes 08022’30” – 09000’30” N and longitudes 42001’10” – 43005’50” E and its elevations ranges between 850 and 1,785 m a.s.l. (EHPEDO, 2004).
Figure 1. Location of Babile Elephant Sanctuary.
2.2. Climate of the study area
Gobelle valley is characterized by semi-arid climatic condition with mean annual temperature of 19.6oC. Rainfall is bimodal- occurring from March to May (short rain season) and July to October (long rain season). The mean annual rainfall 419.7 mm year-1, ranging from 343.4 mm to 505.1 mm year-1.
2.3. Reconnaissance Survey and Vegetation Sampling
Prior to actual vegetation sampling, reconnaissance survey was made in the study area to determine the position, number and length of transects. The three transects that cut across various vegetation segments were laid down at different sites across altitudinal gradient. A total of 60 sample plots (20 m x 20 m) were systematically laid along the 3 transects at every 100 m interval and in each transects 20 quadrats were employed. In each of the quadrats, the identity of all live woody species (WS), number of live individuals of all WS and diameter at breast height (DBH) of all WS (with DBH > 2 cm), except juveniles (seedling and coppices: height < 1.3 m) were recorded. In the case of seedling and coppices, the number of individuals of each species were counted and recorded in each quadrat. For this, 5 sub-plots of 5 m x 5 m four at each corner and one at the center of the big plot were laid. A caliper was used to measure diameter at breast height (DBH = 1.3 m). For WS that were branched at around the breast height, the DBH were measured separately and averaged. The woody species were preliminarily identified in the field by using the available literature (e.g. Flora of Ethiopia and Eritrea). Voucher specimens of each WS were collected, dried and pressed for further identification and confirmation of the authenticity in Haramaya University. Geographic location of each quadrat was measured using AndroiTS GPS Test. 2.4. Data Analysis Species richness was determined from the total number of woody species recorded in sample plots. The diversity of woody species was analyzed by using the Shannon-Weiner Diversity Index (Krebs, 1989; Magurran, 2004). Equitability or evenness, a measure of similarity of the abundances of the different woody species in the study site, was analyzed by using Shannon’s Evenness or Equitability Index (Krebs, 1989; Magurran, 2004). Density of the woody species were calculated by converting the total number of individuals of each woody species encountered in all the quadrats and all transects used in the site to equivalent number per hectare. The frequency was calculated as the proportion (%) of the number of quadrats in which each woody species were recorded from the total number of quadrats in the site. Dominance of the woody species, with diameter at breast height (DBH) of >2cm were determined from the space occupied by a species, usually its basal area. The total basal area of each woody species was converted to equivalent basal area per hectare (Kent and Coker, 1992).
Importance Value Index (IVI), which indicates the relative ecological importance of a given woody species at a particular site (Kent and Coker, 1992), was determined from the summation of the relative values of density, frequency and dominance of each woody species. Regeneration status of the forest was analyzed by comparing saplings and seedlings with the matured trees. Sorensen’s similarity index was computed to help comparison between species composition of other similar vegetation of the region.
3. RESULT AND DISCUSSION
3.1. Plant Diversity and Floristic Composition
A total of 58 woody plant species, representing 25 families and 36 genera, were recorded in the study sites. See table 1. below.
Table 1. Woody plant species abundance with their family distribution
Fabaceae had 15(26%) species, Tiliaceae had 7(12%) species, Anacardiaceae 4(7%) species, Burseraceae had 3(5.17%) species, Apocynaceae, Capparidaceae, Ebenaceae, Euphorbiaceae, Moraceae, Oleaceae, Rhamnacae and Sapindaceae each has 2(3.45%) species, Agavaceae, Balanitaceae, Cactaceae, Combretaceae, Flacourtiaceae, Guttiferae, Lamiaceae, Myrtaceae, Olacaceae, Salicaceae, Salvadoraceae, Sterculiaceae and Verbenaceae each has 1(1.7%) species. The most diverse family was Fabaceae, with 26% of the total number of species, followed by Tiliaceae and Anacardiaceae constituting 12% and 7% respectively (Table 1).
The family Fabaceae ranks first comprising because they are known for their drought tolerance, deciduous and well developed spiny species as defense mechanism for the environmental and human induced hazards and shows the same result for Erer valley (Anteneh et al., 2011). Out of the total plant species, 32 (55.2%) are shrubs where as trees accounted 26 (44.8%) of the total recorded species. Shrub that constituted 55.2% of the total recorded species was dominant life form as compared to tree, accounts only for about 44.8% of the total recorded woody species.
The overall plant diversity and evenness values of Gobelle valley vegetation were 3.05 and 0.75, respectively. The Shannon diversity index values observed in Gobelle valley vegetation fall within the range (0.70 – 3.57) and agrees with findings reported for other dry forests of the Sub-Saharan region (Shackleton, 1993; Obiri et al., 2002; Venter & Witkowski, 2010). The overall species richness of a given vegetation type can give a general impression of their diversity (Tadesse, 2003). Based on this assumption, the diversity index obtained for this forest shows that Gobelle valley vegetation has relatively high diversity with the different species having uniform abundance. The relatively high diversity of woody plants was probably a result of relatively high species richness and abundance of this study area. Therefore, the overall species diversity value shows relatively high value which indicates relatively high diversity and evenness in the study area. Higher evenness indicates that almost all species are equally important and there was no dominance of few species over the others (Feyera, 2006).
However, Gobelle vegetation is found to be less diverse than the value recorded in Erer valley vegetation for BES in which 67 woody species (H’ = 3.55) (Anteneh et al., 2011) and Vegetation of Sire Beggo in which 117 species with H’ ≥ 3.690 (Abyot et al., 2014). The high diversity may be a reflection that the area closure or protected area can restore the natural vegetation. In connection to this, it is also possible that environmental factors such as climate and edaphic factors govern richness and diversity of given vegetation.
Sorenson’s coefficient of similarity was computed to compare the similarity in family, genera and species composition of Gobelle valley vegetation with some other similar woodland forests of the country. Results showed that the highest similarity (74%) in family composition was observed between vegetation’s of Gobelle valley and Erer Valley, which is also found in the BES. The same similarity trend was also calculated in terms of species and genera composition. Least similarity in species composition was observed with vegetation of Erer Valley and ANP forest.
Even though Erer and Gobelle valley were found adjacent to each other, they have different diversity and evenness. The probable reasons for variation in floristic composition between Gobelle valley and Erer Valley vegetation’s could be variation between the sites in extent of anthropogenic disturbance, excessive exploitation of same species and variation in environmental conditions for regeneration. Anthropogenic disturbances, such as logging or cutting trees, charcoal production and farming practices usually, result in an immediate decline or complete destruction in species diversity (Noble and Dirzo, 1997). Out of the plants collected from the study area, 3 species viz. Acacia nigrii, Berchemia discolor and Euphorbia burgeri were endemic species. According to Vivero et al. (2005), these species are listed in IUCN Red Data List and requires close attention to conserve them in situ.
3.2. Density, frequency and dominance of woody plant species
The total density of all the woody species with DBH>2.0cm recorded was 34,897 per hectare out of which trees accounted for 5159.6 individuals per hectare and shrub accounted for 29738.7 individual ha−1.
Table 2. Lists of species with high densities from the valley
Species Individual ha−1 Percent
Acacia brevispica 2295 6.6
Jasminum floribundum 2017 6
Acacia melifera 1980 5.7
Ozoroa insignis 1919 5.5
Lantara camara 1828 5
Euclea schimperi 1824 5
Grewia tenax 1803 5
Euphorbia burger 1596 4.6
Acacia senegal 1416 4
Grewia flavescens 1295 3.6
Grewia ferruginea 1264 3.6
Grewia bicolor 1136 3
Cryptostegia grandiflora 1011 3
Capparis fasicularis 1011 3
The densities of 14 species viz. Acacia brevispica, Jasminum floribundum, Acacia melifera, Ozoroa insignis, Lantara camara, Euclea schimperi, Grewia tenax, Euphorbia burger, Acacia senegal, Grewia flavescens, Grewia ferruginea, Grewia bicolor, Cryptostegia grandiflora, and Capparis fasicularis, accounted for 64% of the total density of woody species(Table 2). The density of the rest 44 species constituted 36% of the total density.
The mean density of each recorded species was also seen against DBH class and result showed that number of individuals varied with DBH class. The least density of the most dominant woody species such as Balanites aegyptiaca, Berchemia discolor Acacia etabaica, Sterculia africana, Commiphora erythraea, and Tamarindus indica might be due to selective overexploitation for construction, firewood, charcoal and other multipurpose uses by local peoples. Similarly, the highest densities of few species like Lantana camara, and Acacia brevispica could be due to their unpalatable nature for both wild and domestic animals and wide range of dispersal mechanisms and rapid reproduction strategies (Feyera, 2006; Anteneh et al., 2011). The frequency of 86.67% of the species was between 20 and 60%, while 13.33% were below 20% frequency. The most frequent woody species in the sampled quadrats were Acacia robusta, Acokanthera schimperi, Boscia minimifolia, Grewia bicolor, Sterculia africana, and Dichrostachys cinerea, accounting for 57%, 53%, 52%, 51%, 51%, and 50% respectively. The species with the least percent frequency were Ziziphus spina-christi from Rhamnaceae family and Ficus vallis-choudae from Moraceae family which occurred in 11.67% and 1.67% of the sampled quadrats respectively.
Dominance of the woody species with diameter at breast height (DBH) of > 2 cm was determined from the space occupied by a species, usually its basal area. The normal value of basal area for virgin tropical forests in Africa is 23-37 m2 ha-1 (Lamprecht, 1989, cited by Anteneh et al., 2011). The total basal area of each woody species was converted to equivalent basal area per hectare. Basal area of all woody species in the study area added up to 22.38 m2/h-1. The most dominant woody species in Gobelle valley vegetation were Acacia robusta (2.41 m2/ha), followed by Commiphora erythraea (2.14 m2/ha), Ficus vallis-choudae (1.77 m2/ha), Boswellia neglecta (1.54 m2/ha), Tamarindus indica (1.33 m2/ha) and Ficus vasta (1.33m2/ha). The overall result from the calculation of basal area shows relatively low dominance of few individuals of woody species. Comparison of the total woody species dominance (basal area) of Gobelle valley vegetation with some other vegetation of the country showed that it is higher than that of Erer valley (17.8 m2/ha ), Sire Bego (19.3 m2/ha), ANP was 0.002787, 0.006613, 0.008613, and 0.004623 m2/ha for the control, protected, less human interference and high human interference areas of the park. This might be due to variation in edaphic, physical and anthropogenic factors between Gobelle and forests in comparison which resulted in the difference in mean density and DBH.
3.3. Importance Value Index (IVI)
IVI was calculated from the summation of the relative dominance, relative frequency and relative density values of each woody species (Kent & Coker 1992). Results show that Acacia robusta was found to have the highest IVI (16.44) followed by Commiphora erythraea (11.55), and Boswellia neglecta (11.2). This suggests that these species are dominant species of the area. They are species that are well adapted to the environmental factors of the area.
Many researchers (e.g., Zegeye et al., 2006; Feyera et al., 2003; Worku et al., 2012) explain that IVI is an important parameter that indicates the ecological significance of species in a given ecosystem. If a species has high IVI value, it will be regarded as more important than those with low IVI values (Zegeye et al., 2011). Out of the 58 woody species recorded in the study area about 57 species have IVI ˃ 1.0. The least (0.51) IVI ˂ 1.0 value was recorded for Ziziphus spina-christi, suggesting that this species needs. The Important Value Index (IVI) is useful to compare the ecological significance of species (Lamprecht, 1989). In the study area, most woody species resulted in IVI˃10 that could be considered as an important species with respect to ecological management and conservation plan for intervention. Only three woody species like Acacia robusta, Boswellia neglecta, and Commiphora erythraea resulted in an IVI value (IVI ˃10).
3.4. Population structure and Regeneration status of Gobelle Vegetation
Woody species of Gobelle forest were sub-divided into 10 DBH classes. Comparison between the DBH classes, about 27.5% of the total counted individuals of the entire species fall within DBH class of ˂ 2cm followed by 19.6% with DBH class of 2-5cm, 15.7% with DBH class of 5-10cm and 11.8% with DBH class of 10-15cm. The density of individuals of the entire species with DBH class >40cm accounted only for 1.2%.
The statistical result indicated that the numbers of individuals in the lower DBH classes are higher with subsequent decrement of densities of higher DBH classes. This tells us that seedlings and saplings are more a stable regeneration status to replace the older individuals.
This result shows the presence of more seedlings than saplings and matures individuals, which shows that the Gobelle forest is under regeneration. In this study, different species have different densities of seedlings and saplings. The highest density of seedlings and saplings were found in the species Euclea schimperi, Acacia brevispica, Acacia melifera, Acacia senegal, Jasminum floribundum, Lantana camara and Ozoroa insignis. Moreover, densities of seven densest species was assessed and graphed against DBH classes. The result showed that majority of individuals of each species was found to be in lower DBH class (s) with decreasing number of individuals in higher DBH class. According to Neelo et al. (2015) this population structure pattern is a stable one with good regeneration status.
4. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
Ecological assessments of this vegetation would serve as a basis for meaningful planning, sustainable utilization and conservation of biodiversity in this sanctuary. Woody Species Composition and Diversity Gobelle natural vegetation have been studied. The result obtained from the study area showed that a total of 58 plant species in 36 genera and 25 families were identified from the forest. Of these, trees and shrubs constituted 44.8% and 55.2%, respectively. The current finding indicated that shrubs were the most dominant species. Top four plant families with the highest proportion of the total recorded species were Fabaceae 15 (26%) species, Tiliaceae 7(12%) species, Anacardiaceae 4(7%) species, and Burseraceae 3(5.17% species. Fabaceae was represented by the highest number of species 15 (26%) species. Shannon-Wiener diversity index (H`) and evenness values (E’) were 3.05 and 0.75, respectively.
The total basal area of the recorded woody species was computed from DBH of all individuals in sampled quadrats. Therefore, the total basal area of the vegetation of the study area was added up to 22.38 m2/ha. In this study area, shrubs are found to be dominant than trees where most of the trees are found at lower DBH classes. The overall density of trees/shrubs species with DBH>2cm recorded was 34,897 individuals ha-1. The density of trees/shrubs decreased with increasing DBH indicating the dominance of small-sized individuals in the area. The importance value index (IVI) of the recorded woody species of Gobelle was also calculated and Acacia robusta was found to have the highest IVI (16.44) followed by Commiphora erythraea (11.55), and Boswellia neglecta (11.2).
Furthermore, from the total species recorded in the study area, 3 species viz. Acacia nigrii, Berchemia discolor and Euphorbia burgeri were endemic species. According to Vivero et al. (2005), these species are listed under the Red Data List of IUCN and requires close attention to conserve them in situ. The regeneration status of the woody species at Gobelle vegetation was determined and seven woody species were assessed through histogram. The highest density of seedlings and saplings were found in the species like Euclea schimperi, Acacia brevispica, Acacia melifera, Acacia senegal, Jasminum floribundum, Lantana camara and Ozoroa insignis.
In conclusion, the analysis of population structure showed that some tree species have normal population structures with many individuals at lower size classes. The assessment of regeneration status based on seedling and sapling count in comparison with mature individuals also showed that a significant proportion of woody species were regenerating. Overall vegetation of Gobelle showed stable population structure that suggests good regeneration status. It is therefore imperative to develop and implement effective conservation measures to ensure the sustainable use of this natural vegetation and save the biodiversity of this area.
Based on the results, the following recommendations have been given.
As it is not protected, Gobelle vegetation was exposed to human interference that may endanger some species of the area in the future. Therefore, creating awareness for communities on the importance of forest resources and ecological consequences of deforestation and device mechanisms by which human impacts can be minimized through discussion and consultation with the local communities as to be developing sense of ownership and responsibility for the management and conservation of the Forest.
Private and community plantation areas for particular uses such as fuel wood and charcoal production should be established to ease anthropogenic pressure.
The present study was limited to woody species composition and diversity in Vegetation of Gobelle valley hence; further studies on soil seed bank, seed physiology, and land use management system in the study area are recommended.
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