Diversity, Structure and Regeneration Status of the Woodland and Riverine Vegetation of Sire Beggo in Gololcha District, Eastern Ethiopia

This study was conducted on the woodland and riverine vegetation of Sire Beggo in Gololcha District, eastern Ethiopia with the aim of documenting the floristic composition, population structure and identifying major plant community types. Preferential sampling method was employed to collect vegetation data by focusing on vegetation homogeneity. From the study sites, 70 quadrats (each measuring 20 x 20 m = 400 m 2 ) were sampled. Regeneration status of woody plants was also assessed using the same quadrate size. The data on the herbaceous species were collected from five, 1 m x 1 m subplots laid at four corners each and one at the centre of the large plot. Vegetation classification was performed using TWINSPAN software package. A total of 185 plant species, representing 61 families were recorded. Fabaceae was the dominant family represented by 15 genera and 23 species, followed by Poaceae and Euphorbiaceae with 17 and 12 species each respectively. The output of TWINSPAN showed five plant community types. Structural analysis of the dominant species revealed different patterns of population structure. Some of the results of population structure and regeneration status indicated abnormal pattern which dictate the need for an urgent conservation of the study area.


INTRODUCTION
Loss of forest cover and biodiversity due to anthropogenic activities is a growing concern in many parts of the world (Singh et al., 1997;Hegde and Enters, 2000).This is because of the fact that declining vegetation cover and depletion of natural resources are closely associated with drought and food shortages that have become major threats affecting the life of millions of people (Pimm et al., 1995;Groombridge, 1992;Brook et al., 2006).
Ethiopia has a wide range of ecological conditions ranging from the arid low lands in the East to wet forests in the Southwest and high altitudes in the central high lands (EPA, 1997).This wide range of ecological conditions coupled with the corresponding diverse socio-culture has made the country to be one of the internationally recognized major centers for biodiversity (Edwards and Ensermu, 1999;Yonas, 2002;Tadesse, 2003;Alemayehu et al., 2005).The Ethiopian flora is very heterogeneous and has many endemic species.The country possesses about 6000 species of higher plants, of which about 10% are endemic (Hedberg et al., 2009).Ethiopia has also the richest avifauna in main land Africa (EWNHS, 1996).
The vegetation resources of Ethiopia, including forests, woodlands and bushlands have been studied by several scholars (Logan, 1946;Hedberg, 1957;Gilbert, 1970;Chaffey, 1979;Tewoldeberhan, 1986;Friis, 1992;EFAP, 1994, Tamrat, 1994).The results of these studies have categorized Ethiopian vegetation in to nine general categories for the purpose of developing the conservation strategy of Ethiopia.Among the 9 vegetation types of Ethiopia, four of them occur in the dryland regions of the country The four vegetation types that are found in the drylands of Ethiopia are: i) Combretum-Terminalia or broad-leaved deciduous woodland, (ii) Acacia-Commiphora or small-leaved deciduous wood land, (iii) Desert and semi-desert scrub land, and (iv) Riparian and swamp vegetation.
Ethiopia possesses diversified dryland vegetations which are collectively characterized as the Somalia -Masai vegetation formation (White, 1983).According to EFAP (1994), 2.5 million hectares of land was covered with woodland and bushlands out of the total 75 million ha dryland areas of the country.This figure indicates that the largest vegetation resources of Ethiopia are found in the drylands (Tefera et al., 2004;Abeje et al., 2005).However, this diverse valuable vegetation resource is under severe threat.Environmental degradation aggravated by poverty, which in turn accelerates the environmental degradation process is the biggest challenge that face the drylands of Ethiopia (Kindeya, 2004).According to EFAP (1994), 16% of the land areas of Ethiopia were covered with forests in the early 1950's.This number declined to 3.1% in 1982, 2.7% in 1989 and less than 2.3% in 1990.The rate of deforestation has long been estimated at 150,000 -200,000 ha per anum (EFAP, 1994) as a result of which many climatic hazards are emerging today.If this trend of forest devastation continues unabated, there is a great danger of serious decline or loss of the biodiversity resources.It is, thus, important to study and document the remaining vegetation resources upon which sound management plans of conservation and sustainable utilization can be based.The aim of this study was to describe the species composition, plant community types, population structure and regeneration status of the woodland and riverine vegetation of Sire Beggo in Gololcha District.

Study site
This study was conducted in Gololcha District, Arsi Zone of Oromia National Regional State, Eastern Ethiopia (

Methods
Following a reconnaissance survey, actual sampling of vegetation was done focusing on homogeneity via preferential sampling method.From the study sites, 70 quadrats (each measuring 20 x 20 m = 400 m 2 ) were sampled.All woody plant species were recorded in 20 m X Abyot, D., Teshome, S., Ensermu, K and Abiyou, T (MEJS) Volume 6( 1 TWINSPAN for windows version 2.3 (Hill and Šmilauer, 2005) was used to classify the vegetation data in to different communities.WCanoImp Program (Šmilauer, 2002) was used to convert cover/abundance data in to condensed format to be read by the TWINSPAN program.
The structure of the vegetation was described using frequency distribution of DBH, height and Importance Value Index (IVI).Tree or shrub density and basal area values were computed on hectare basis.Importance value indices were computed for all woody species based on their relative density (RD), relative dominance (RDO) and relative frequency (RF) to determine their dominance as recommended by Kent and Coker (1992).

Floristic composition
A total of 185 plant species, belonging to 61 families and 144 genera were recorded from the vegetation under study (Appendix 1).Fabaceae is the most dominant family with 15 genera and 23 species.Poaceae is the second dominant family with 15 genera and 17 species.Euphorbiaceae is the third dominant family with 8 genera and 12 species.The next species rich families were Lamiaceae, Rubiaceae, Tiliaceae, Asteraceae and Acanthaceae with 8, 7, 7, 7 and 6 species, and 6, 6, 2, 7, and 3 genera respectively.Twenty three families were represented by more than one species while 30 families were represented by a single species each.The genus Acacia was represented by 9 species, Grewia and Ficus by 5 species each, Cissus, Euphorbia, Maytenus, Barleria and Rhus by 3 species each and Justicia, Cordia, Capparis, Combretum, Bridelia, Croton, Strychnos, Ocimum, Plectranthus, Ochna, Olea, Eleusine, Panicum, Canthium and Triumfetta by 2 species each and the rest contained a single species each.
The dominance of Fabaceae was reported from other vegetation studies in the woodlands of Ethiopia (Gemedo, 2004;Getachew et al., 2004;Teshome et al., 2004;Motuma, 2007).This may imply that the environmental conditions in these areas are more favorable to this family.Of the total species recorded 42 (23%) were trees, 35 (19%) tree-shrubs, 40 (22%) shrubs, 22 (12%) climbers and 46 (24%) herbs.Out of the total species, Erythrina burana and Euphorbia nigrispiniodes have been included in the IUCN red data list of Ethiopia and Eritrea (Vivero, et al., 2005) qualifying for vulnerable category.

Vegetation Classification
Five plant community types were recognized from the output of TWINSPAN (Table 1).These community types were named after two of the characteristic species.A list of the community types along with the synoptic cover-abundance values of the species is given in Table 2.

Salvadora persica -Indigofera tanganyikensis community type
This community was distributed from 1344 to 1841 m a.s.l. and slope ranging from flat to 20%.
It comprises of 21 plots and 96 species.Except three plots the rest were distributed on flat terrain.Entada abyssinica, Ximenia americana and Kirkia tenuifolia are the characteristic species in the tree layer.Acokanthera schimperi is the dominant species in the tree layer.

Berchemia discolor -Clerodendrum myricoides community type
This community was found at the altitudinal range of 1407 to 1907 m a.s.l. and at slope ranging from flat to 70%.Seven plots and 62 species were associated to the community.Berchemia discolor, Gardenia ternifolia and Erythrina burana are the characteristic species in the tree layer.

Species Richness, Evenness and Diversity of the plant community types
From computation of vegetation data in the study area Shannon-Weiner diversity index showed the output in table 3.

Frequency
Frequency was the number of quadrats (expressed as a percentage) in which a given species

Basal Area (BA)
The total basal area of all tree species calculated from DBH data was 19.3 m 2 ha -1 .Mimusops kummel, Acokanthera schimperi, Ficus sycomorus, Diospyros abyssinica, Pappea capensis and Syzygium guineense accounted for 65.9% of the total basal area (Table 3).Ficus sycomorus had a low density but high BA due to its high DBH value.Twelve trees of this species contributed a total of 2.02 m 2 h -1 (10.57%).Basal area provides a better measure of the relative importance of the species than simple stem count (Cain andCastro, 1959 cited in Tamrat, 1994).Thus, species with the largest basal area can be considered the most important woody species in the study area.
Accordingly, the above species were the most important species in the study area.This pattern represents abnormal population dynamics.The underlying reason for such pattern is due to the nature of seeds of Ficus sycomorus in germination and recruitments of its seedling.
The seeds of this plant germinate on the stems of other plants and the seedlings also grow on stems of other plants as parasite until they become larger and replace the host plant by killing it (Ramirez, 1977;Putz and Holbrook, 1989).

Regeneration Status: Composition and Density of Seedling and Saplings
In the current study, out of the 60 tree species, 28 species were represented in the seedling class.
The total seedling density was 1108 per hectare.The sapling class had 39 species with a total sapling density of 396 individuals ha -1.Five species contributed 75.9% and 45% of the total seedling and sapling count respectively ( representative figures that show the seedling, sapling and tree/shrub status are given in figure 6 (a-h).Accordingly, four patterns are observed from the graph: i) Seedling > sapling > tree/shrub state, e.g.Acokanthera schimperi, Anogeissus leicocarpa, Acacia etbaica and Buxus hildebrandtii.This pattern represents good regeneration and recruitment.
ii) Seedling outnumbers sapling and tree/shrub state but sapling less than tree/shrub state, e.g.
Pappea capensis and Diospyros abyssinica.This pattern represents fair regeneration and recruitment of the species.
iii) Seedling < sapling < tree/shrub state, e.g.Mimusops kummel.This pattern shows poor reproduction and hampered regeneration either due to the fact that most trees are not producing seeds as a result of their old age or there has been loss of seeds by predators after reproduction.
In addition the fruits of Mimusops kummel were usually eaten as food by many animals including humans, which might be a reason for this pattern.
iv) With no individual in seedling and sapling stages but relatively many individuals in tree/shrub stage e.g.Ficus sycomorus.This pattern also shows poor reproduction and hampered regeneration (Bhuyan et al., 2003;Khumbongmayum et al., 2006).To use the regeneration analysis for priority setting of plant biodiversity conservation, the species considered in the study area were classified in to three groups based up on the density of the total regeneration (Table 6).
Accordingly, those species that were totally absent in regeneration were grouped under class 1; others whose density was greater than zero but less than 50 individuals ha -1 were grouped under class 2 and the remaining species were grouped under class 3.For the sake of conservation endeavors, those species under classes 1 and 2 should be given the highest conservation priority.6 (a-h).Seedlings (SE), saplings (SA) and tree/shrub (T/SH) distribution of some selected woody species occurring in woodland and riverine vegetation of Sire Beggo in Gololcha District.

Phytogeographical similarity
The vegetation of Sire Beggo in Gololcha is compared with other four woodlands in Ethiopia.
The vegetation in Gamo Gofa Zone is found in the Southern Nations, Nationalities and Peoples National Regional State, in Southern Ethiopia (Teshome et al., 2004).The woodland and riverine vegetation of Sire Beggo in Gololcha was compared to the above four woodlands to know the similarity of species in the woodlands and indicate to which woodland type it is related (Table 7).From the similarity index result, it is indicated that the species of Sire Beggo in Gololcha woodland and riverine vegetation shared the highest species similarity with the woodland of Dello Menna (0.33).The similarities observed could be due to their altitudinal ranges and climatic zones.Relatively low species were shared between Sire Beggo in Gololcha and the vegetation in Lake Abijata (0.16).These may arise from the different sample sizes and methods of the study, altitudinal differences, degree of anthropogenic impact, overgrazing and climatic conditions.

CONCLUSION
In the present study, species diversity, plant community types, population structure and regeneration status were determined for the woodland and riverine vegetation of Sire Beggo in Abyot, D., Teshome, S., Ensermu, K and Abiyou, T (MEJS) Volume 6( 1 The analysis of population structure shows that some tree species have abnormal population structures with no or few individuals at lower size classes.These species need urgent conservation measures that will facilitate healthy regeneration and guarantee sustainable use of these species.The assessment of regeneration status based on seedling and sapling count also showed that a significant proportion of woody species were not regenerating, implying that they are under threat.It is therefore imperative to develop and implement effective conservation measures to save the biodiversity of this area.

ACKNOWLEDGEMENTS
We would like to thank Mr. Husien Adem and Mr. Hamido Jilo, farmers in Sire Beggo PA, who had played a significant role during field data collection.Without their devoted assistance, the field data collection in such inaccessible and harsh environment would not have been possible.
We are also very much indebted to Mr. Mussa Ibrahim, an extension worker, in Sire Beggo PA for his provision of lodging service.

REFERENCE
Abeje Fig 1).Gololcha District is located 307 km East of Addis Ababa and 280 km North East of Asela town, the capital of Arsi Zone via Dera to Mechara road.It is situated between 8 0 -8 0 30' North and 40 0 -40 0 30' East.The study specifically looked at the vegetation of Sire Beggo Peasant Association of Gololcha District.Gololcha District covers an area of 1781 km 2 while the study area covered 5238 hectares (OBPED, 2000).

Figure 1 .
Figure 1.Location map of Gololcha District and study site.
while herbaceous were recorded in sub plots of 1 m X 1 m at the four corners and the center of the large plot.Height and Diameter at Breast Height (DBH) were measured for any woody plant species with height ≥ 2 m and DBH ≥ 10 cm thick.Height and DBH measurements were made using Clinometer and diameter tape respectively.Regeneration pattern of study species were assessed by counting of seedlings (woody species of height ≤ 50 cm and dbh ≤ 10cm) and saplings (woody species of height > 50cm and dbh ≤ 10 cm) within the main quadrats.Altitude and geographical coordinates were measured for each plot using Garmin GPS 72.Every plant species encountered in each plot was recorded.Plant specimen were collected, pressed, dried and brought to the National Herbarium (ETH), Addis Ababa University for taxonomic identification.The specimens were identified by comparing with authenticated specimens housed at ETH and by referring to published works on Flora of Ethiopia and Eritrea.
Acacia hockii are the characteristic species in the shrub layer.Barleria hildebrandtii and Acacia brevispica are the dominant species in the shrub layer.The lianas recorded in this community include Rhoicissus revoillii, Cissus quadrangularis, Toddalia asiatica, Secamone punctulata, Helinus mystacinus, Capparis tomentosa, Cissampelos mucronata, Caucanthus auriculatus, Jasminum grandiflorum and Asparagus falcatus.Indigofera tanganyikensis, Digitaria abyssinica, Eleusine floccifolia and Leucas martinicensis are the characteristic species in the field layer.Sporobolus pyramidalis and Heteropogon contortus are the dominant species in the field layer.

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Combretum molle and Acacia etbaica are the dominant species in the tree layer.Grewia bicolor, Commiphora africana and Bridelia scleroneura are the characteristic species in the tree-shrub layer.Dichrostachys cinerea and Rhus tenuinervis are the dominant species in the tree-shrub layer.Clerodendrum myricoides, Polygala obtusissima and Grewia flavescens are the characteristic species in the shrub layer.There was no prominent species in the shrub layer but few individuals of Croton dichogamus, Acacia brevispica, Carissa spinarum, Premna oligotricha, Ochna inermis and Acacia senegal have been encountered.The lianas recorded in this community include Cissus quadrangularis, Helinus mystacinus, Capparis tomentosa, Jasminum grandiflorum, Asparagus falcatus, Phaseolus lunatus and Jacquemontia ovalifolia.Endostemon tenuiflorus, Hibiscus micranthus and Aristida adscensionis are the character species in the field layer.Heteropogon contortus and Sporobolus pyramidalis are the dominant species in the field layer.
Abyot, D.,Teshome, S., Ensermu, K and Abiyou, T (MEJS)    Density was expressed as the number of individuals per hectare.Species were classified into 5 density classes, A -E as follows: A > 100; B = 50.1 -100; C = 10.1 -50; D= 1 -10 and E (<1).The density of woody species in the study area was 1845 individuals per hectare.Twelve species exhibited density class E, 23 species density class D, 16 species density class C, 5 species density class B and 4 species exhibited density class A. The four most abundant species in the density class A include Acokanthera schimperi, Anogeissus leicocarpa, Acacia etbaica and Buxus hildebrandtii.The distribution of species in the five density classes is shown in (Fig 2).
occurred in the study area.Species were grouped into five frequency classes: A = 81-100%; B = 61-80%; C = 41-60%; D = 21-40%; E = 0 -20%.No species were recorded in frequency class A. Only one species, Acokanthera schimperi, was recorded in frequency class B. Therefore, this species is the most frequent species recorded in the study area and it occurred in 43 quadrats.The next most frequent species belonging to frequency class C included Mimusops kummel, Strychnos henningsii, Maytenus undata, Anogeissus leicocarpa.The frequency gives an approximate indication of the homogeneity and heterogeneity of species.Lamprecht (1989) pointed out that high values in higher frequency classes (classes A and B) and low values in lower frequency classes (classes E and D) indicate constant or similar species composition.High values in lower frequency classes and low values in higher frequency classes on the other hand indicate a high degree of floristic heterogeneity.In this study, high values were obtained in lower frequency classes (Fig 3).This shows that high degree of floristic heterogeneity exists in the study area.
These species were abundant, frequent and dominant in the study area.The remaining percentages were shared among other 52 species.3.4.5.DBH and Height class distributionDBH and height class distribution of all individuals in different size class shows an inverted Jshape distribution (Figs 4a and 4b).This general pattern shows the majority of species had the highest number of individuals at relatively low DBH and height classes with gradual decrease towards both high DBH and height classes.This pattern shows healthier population dynamics of the vegetation under study.The pattern of diameter class distribution is an indication of the general trends of population dynamics and recruitment processes of a given species.Evaluation of DBH class distribution of top 8 selected species based on their IVI value demonstrated various patterns of population structure implying different population dynamics among species.The frequency distribution of DBH classes of the selected 8 species showed four types of population structure (Fig 5).

Figure 4 .
Figure 4. DBH and Height class distribution of all individuals.
, a = Common species to Sire Beggo and the woodland in comparison.b = Species only found in woodland and riverine vegetation of Beggo.c = Species only found in the woodland which is in comparison with Beggo.sc = Sorensen's Similarity Coefficient.
The results of this study indicate the presence of relatively high species diversity.Fabaceae was found to be the most dominant family followed by Poaceae, Euphorbiaceae, Lamiaceae and Rubiaceae.Herbs and trees were the dominant growth forms while climbers scored the least proportion.From the total species recorded, Erythrina burana and Euphorbia nigrispinoides are listed in the IUCN red data list under the vulnerable category.

Table 2 .
Synoptic cover abundance values for species reaching ≥ 1% in at least one community and some selected character species.

. Chlorophytum gallabatense -Barbeya oleoides community type
This community type was distributed from 1390-1669 m. a.s.l and at slope ranging from flat to 75%.Eighteen plots and 79 species were associated to this community.The characteristic species in the tree layer include Sterculia africana, Acacia tortilis, Croton macrostachyus and Cordia africana.Acokanthera schimperi, Anogeissus leicocarpa and Pappea capensis are the dominant species in the tree layer.Dodonaea angustifolia is the characteristic species in the shrub layer.Croton dichogamus is the dominant species in the shrub layer.The lianas recorded in this community type include Rhoicissus revoillii, Cissus petiolata, Toddalia asiatica, Secamone punctulata, Hippocratea pallens, Helinus mystacinus, Phaseolus lunatus, Capparis tomentosa, Cissampelos mucronata, Caucanthus auriculatus, macroptilium atropurpureum, Jasminum grandiflorum and Asparagus falcatus.Chlorophytum gallabatense and Selaginella yemensis are the characteristic species in the field layer.Leptochloa rupestris is the dominant species in the field layer.The associated species in the field layer include Panicum maximum, Sporobulus pyramidalis and Heteropogon contortus.

Table 3 .
Species richness, evenness and Shannon-Weiner diversity index of plant community types of the woodland and riverine vegetation of sire Beggo in Gololcha district.
Where I, II, III, IV and V refers to Lantana camara -Ficus vallis-choudae community type, Strychnos mitis -Ficus vasta community type, Chlorophytum gallabatense -Barbeya oleoides community type, Salvadora persica -Indigofera tanganyikensis community type and Berchemia discolor -Clerodendrum myricoides community type respectively.The results of Shannon-Weiner diversity index and evenness indicated more or less similar species diversity and evenness among the identified plant communities.Community IV is relatively the most diversified one attaining a diversity index of 3.963 but relatively least in evenness.Relatively highest value of species evenness index goes to community I.

Table 4 .
Curtis and Mcintosh (1951)percentage basal area of woodland and riverine vegetation of Sire Beggo in Gololcha District.Important value index combines data for three parameters (Relative frequency, Relative density and Relative abundance).Curtis and Mcintosh (1951)pointed out that Important Value Index gives a more realistic figure of dominance from the structural point of view.It is useful to compare the ecological significance of species(Lampriecht, 1989).About 59.6% of the IVI was contributed by Mimusops kummel, Pappea capensis, Acokanthera schimperi, Anogeissus leicocarpa, Diospyros abyssinica, Acacia etbaica, Buxus hildebrandtii and Ficus sycomorus.

Table 5
).These are Buxus hildebrandtii, Acokanthera schimperi, Anogeissus leicocarpa, Acacia etbaica and Diospyros abyssinica.Based on the regeneration status of the selected 60 woody species occurring in the study area, some

Table 5 .
List of top 5species in the regeneration.

Table 6 .
List of species under regeneration status group.

Table 7 .
Comparison of the woodland and riverine vegetation of Sire Beggo in Gololcha with other woodlands in Ethiopia.