Floristic composition and plant community types of Agama Forest , an “ Afromontane Forest ” in Southwest Ethiopia

Tropical Afromontane forests are among the most species-rich ecosystems on earth and comprise exceptional species richness and high concentrations of endemic species. The natural forest of Agama, an Afromontane forest, was studied with the objectives of determining its species composition, diversity and community types. Systematic sampling design was used to collect vegetation data. Soil samples were taken from each relevé at a depth of 0 to 30 cm and soil pH, sand, clay and silt were analyzed. The plant communities’ classification was performed using the hierarchical cluster analysis. We evaluated species richness, eveness (Pielou J’ index) and diversity (Shanon-Wiener index). Sorensens’s similarity ratio was used to compare Agama forest with other similar forest in Ethiopia. A total of 162 plant species, 130 genera and 70 families were recorded from which Acanthaceae and Rubiaceae were the richest families. Furthermore nine endemic plant species were identified. In this study, four plant community types were identified and described. Post-hoc comparison of means among the community types showed that altitude was differed significantly between community types, indicating altitude is the most important factor in determining community type. Phytogeographical comparison of Agama Forest with other vegetation using Sorensens’s similarity ratio revealed the highest similarity with Masha and Godre forest. In conclusion Agama forest presents high richness, diversity and endemism, with different plant communities according to altitude. Thus conservation of plant biodiversity is highly recommended.

this region was designated as the "Eastern Afromontane hotspot," which is one of the 34 regions globally important for biodiversity conservation (Conservation International, 2005).The Ethiopian highlands are the largest mountain complex in Africa and comprise over 50% of the African land area covered by Afromontane vegetation (Tamrat Bekele, 1994;Demel and Tamrat 1995).The Afromontane vegetation of Ethiopian highlands comprises a center of plant biodiversity and endemism (Vivero et al., 2004) due to variation in climate and altitude.Altitudes of the country range from 125 m below sea level in the Dallol depression to 4,620 m above sea level at Ras Dejen.As a result, the country possesses rich biodiversity that occur from the highest mountain to the lowlands.Accordingly, the flora of Ethiopia is estimated to contain between 6,500-7,000 species of higher plants, out of which about 12% are endemic (Tewolde, 1991).
These floral resources found in different vegetation types comprising in forests, woodlands and bush lands and others.The flora of Ethiopia have been investigated by several scholars since the beginning of the 19 th C to the mid of the 20 th C. Some studies have provided substantial contribution to describe vegetation types of Ethiopia.Pichi-Sermolli (1957); Chaffey (1979); Friis (1992); Sebsebe et al. (1996); Zerihun (1999); Friis and Sebsebe (2001) and Sebsebe et al. (2011).These studies were carried out in different parts of the Ethiopia and contributed in generating valuable botanical information for the Ethiopian flora.
The vegetation composition and structure of forests in southern and southwestern part of the country was also studied Lisane work Negatu L., (1987); Kumlachew and Tamrat (2002); Tadesse (2003); Feyera (2006); Ensermu and Teshome (2008).These studies have described floristic composition and analyzed plant communities and their relation with environmental factors.According to above studies several plant communities were identified, with characteristic plant species of Ilex mitis, Syzygium guineense, Pouteria adolfi-friederici, Olea welwitschii, Psychotria orophila, and Schefflera abyssinica in southwest Ethiopia.These species are also reported by (Friis, 1992) as the characteristic species of Afromontane rainforest in southwest Ethiopia.The vegetation of southwest Ethiopia varies with altitude and affecting the diversity of plant species.The study of variation of modern pollen rain (Bonnefille et al., 1993) along the ecological gradients containing range of vegetation types in southwest Ethiopia.This indicated that the vegetation types vary with altitude and altitudinal variation is an important environmental factor contributing for diversity of vegetation communities.Afromontane forests are the place of origin of the Coffea arabica and encompass a variety of commercially valuable spices and honey from wild bee.Furthermore the forests also play a pivotal role in providing water resources for the flow of the Baro-Akobo river system which is an important tributary of the Nile and it accounts for 42% of the water in the White Nile (NTFP, 2006).It is also important for carbon sequestration which has implications for climate change management.
In spite of the ecological and economic role of Ethiopian forests, the forest cover of Ethiopia has declined by human impact.About 35%, of the country's area was once covered by natural high forests, (EFAP, 1994).By the early 1950s, high forests were reduced to 16% and the country's forests have declined at fast rate and reached 3.6% by 1980, 2.6% by 1987(IUCN, 1990), 2.4% in 1992, (Sayer et al., 1992), and were finally reduced to 2.3% in 2003(Shibru, 2003)).Because of this shrinkage of the forest resources, most of the remaining forests of Ethiopia are restricted to the south and southwest parts of Ethiopia, which are less accessible, and less populated (Kumelachew and Tamrat, 2002).These forests are continuously threatened by human activities such as clearing forest for coffee and tea plantation, subsistence farming and periodic movements of immigrants from northern and southern parts of Ethiopia looking for fertile land, resulting in the loss of forest cover in the region.
Recognizing the above mentioned threats to forest biodiversity of afromontane rain forests, the government designed different strategies to conserve the remaining forest resources in the region.Participatory forest management (PFM) was one of the solutions to solve the problem of open access to forest resources and to promote sustainable forest management.For instance farm-Africa introduced the Participatory Forest Management approach in southwest Ethiopia particularly in Bonga and the implementation process has been developed since 1996.Agama forest is part of Bonga forest delineated for PFM since 1996 (Aklilu et al., 2014).Though the forest has been under protection since its demarcation, still it has been continuously exploited for agricultural land expansion, timber harvesting, firewood collection and charcoal production due to lack of awareness on principles of PFM.Botanical assessment such as inventory of floristic composition, providing information on species diversity and community structure is necessary for the forest management and sustainable resource utilization by the community members.Therefore the current study aimed to assess the floristic composition and diversity of an Afromontane forests (Agama forest), to analyze community types of the forest and to evaluate the ecological relationships between plant communities and environmental parameters and to analyze phytogeographical relationship of Agama forests with other similar Afromontane forests types in Ethiopia and finally to recommend conservation action for protecting the forest biodiversity of the region.

Descriptions of the area
The Agama forest is part of eastern African Biodiversity hot spot and located in east Africa, in southwest Ethiopia (Figure 1) and it has a total area of 1868.5 ha.The area lies at 7°16'N and 36°11' E, and altitude between 1700 m to 2370 m.a.s.l.(Aklilu et al., 2014).The Geology of area belongs to the Precambrian basement complex, the Tertiary Volcanic Rocks from the trap series, and Quaternary Sediments consists of a variety of sedimentary, volcanic and intrusive rocks (Tafesse, 1996).The major soil groups, according to the FAO/UNESCO legend of soil classification, are Nitisols, Acrisols and Vertisols (Anon., 1988).
The vegetation of the area belongs to Afromonatin rain forest and transitional rain forests (Friis et al., 2011).The forest was stratified into four different layers, namely, upper canopy, sub-canopy, shrub layer and the ground layer and Pouteria adolfi-friederici occupied emergent trees of the upper canopy.The area receives very high annual rainfall reaching up to 1830 mm in some peak years.The rainfall pattern shows low rainfall in January and February, gradually increasing to the peak period in July and then decreasing in November and December.Maximum and minimum monthly mean temperatures of the area are 26.6 and 9.5°C respectively.

Data collection
A systematic sampling design was used to collect vegetation and environmental data, following Kent and Cooker (1992).Vegetation data were collected using quadrates of 25 m x 25 m (625 m 2 ), for woody plant species within which a 2 m x 2 m sub-plot was used for recording herbaceous species and soil sample collection.Ten line transect were established and the distance between each transect was 1km.The vegetation was sampled following the transect and located at 1 km apart and were laid systematically at every 500 m.The number of plots per transect is vary depending on length of the transect and accessibility of the sample plots.
All level of altitudinal ranges starting from 1700 m up to 2300 m were covered during the sampling.A total of 60 releves were sampled and all vascular plant species found in each relevé were recorded and identified.The cover of all the vascular plant species found in each relevé was estimated and rated according to modified 1-9 Braun Blanquet approach (Vander, 1979).The specimens were collected and identified at the National Herbarium (ETH), Addis Ababa University using the published volumes of Flora Ethiopia and Eritrea by comparing with the authentic specimens in the National Herbarium.

Environmental data and soil analysis
The environmental parameters recorded in this study were altitude, slope, aspect, disturbance, soil pH and soil texture (sand, silt, clay), Altitude was measured by Garmin GPS and slope and aspect were measured using Silva Clinometer and 15T Silva Ranger Compass respectively.Soil analysis was performed in the soil laboratories of Addis Ababa University, following the procedures, (Sahlemedhin and Taye, 2000).The soil samples were analyzed for texture, using Hydrometer method of mechanical analysis and Sodium hexametaphosphate were used as dispersing agent.The pH measure was taken using Digital pH meter and it was standardized using buffer solutions of pH 4.0 and 9.2.Disturbance was determined on the basis a five point scale following Anderson and Currier (1973).Disturbance scores were based on visible signs of, tree cutting, grazing and presence of beehives.The points of scale were 0-5, with 0= (No disturbance), 1= (0-20% of the quadrat disturbed), 2= (21-40% of the quadrat disturbed), 3= (41-60% of the quadrat disturbed), 4= (61-80% of the quadrat disturbed), 5 = (81-100% of the quadrat disturbed).

Phytogeographical comparison
Agama forest was compared with five other afromontane rain forests in Ethiopia for the purpose of investigating forest similarity and differences among the afromontane rainforests of Ethiopia.These included Godre,Gera, Masha, Harenna, Jibbat and Yayu.Godre and Masha forests are close to Agama forest found in Bench Maji and Sheka zones in Southern Ethiopia Nations and Nationalities People's Regional State.Jibbat forest is a transitional forest between dry evergreen afromontane and moist evergreen afromontane forest and found in western Shewa in Oromia region.Harenna Forest is located in the Bale Mountain National Park in south eastern Ethiopia.

Data analysis
An agglomerative Hierarchical Cluster Analysis was performed using Similarity Ratio (S.R), using R software version 2.1.5.2 (Venables et al., 2012).The cut point of the Clusters was decided after visual inspection of the level of aggregation/homogeneity of relevé.Plant community types were further refined in a Synoptic table.The resulting groups were recognized as community types and the species occurrences were summarized by synoptic cover abundance values.The community types were named based on the tree and shrub with high synoptic value.
Ordination was computed using Detrended Correspondence Analysis (DCA) is a technique that is used to display sample plots (sites).In DCA similar samples are plotted close together and dissimilar samples are placed far apart (Hill et al., 1980) (Figure 4).Richness was calculated and Shannon and Wiener (1949) index was applied to quantify species diversity.Shannon diversity index (H') was based on frequency of species ∑ Where, "H" = Shannon and Wiener diversity index, S= number of the species Pi=the proportion of individuals or the abundance of species i th = the proportion of total cover in the sample and ln = natural logarithms.The Pielou's (1966) J' evenness index (J) was calculated using the formula: where J = evenness H' = Shannon-Wiener Diversity Index S= total number of species in the sample and ln = natural logarithms.
A similarity analysis was carried out to evaluate the relationship between forests based on presence of trees, shrubs and herbs.Evaluation was conducted using Sorensen's similarity index.It is described using the following formula (Kent and Coker, 1992).Ss = 2a/ (2a+b+c), Where, Ss = Sorensen's similarity coefficient a = Number of species common to both samples; b= Number of species in sample 1; c = Number of species in sample 2. All the environmental parameters such as altitude, slope, aspect, disturbance and soil data were analyzed as follows: aspect was codified according to Zerihun Woldu et al. (1989): N = 0; NE = 1; E = 2; SE = 3; S = 4; SW =3.3, W= 2.5; NW = 1.3;Ridge top = 4.In order to examine the significant differences and similarities between the community types identified, Tukey's multiple tests were performed to detect significant differences between the community types for different environmental parameters (altitude, slope, aspect, and pH and soil texture).Pearson's correlation was calculated to evaluate the relationship between the environmental parameters, anthropogenic disturbances (timber, charcoal and encroachment and grazing).

Floristic composition and endemic plants
A total of 162 plant species (Annex I) belonging to 130 genera and 70 families were recorded and identified in the sample plots in Agama forest (Table 1).Herbs, trees, shrubs, and liana, constituted 50.95, 24.34, 17.19 and 7% of species abundance respectively (Figure 2).Acanthaceae was the richest family representing 8.44% of total floristic composition, followed by Rubiaceae and Asteraceae with 6.49% of species.Euphorbiaceae (4.01%), Roseaceae (3.06%), and Poaceae (3.08%) were also important families in terms of species richness.The remaining families represented less than 3% of species each.Based on the information available on the published Floras of Ethiopia and in Vivero et al. (2005) a total of 9 endemic plant species were recorded (Table 2), comprising more than 5.73% of the recorded species.

Plant community types
A total of 162 clusters were derived from the output at dissimilarity level above 0.80 (Figure 3).These clusters were designated as local plant community types and given names after two dominating woody species, usually a tree and a shrub with higher synoptic value (Table 3).The cluster numbers in the dendrogram correspond to the community types.The description of the plant community types is based on the dominant and characteristic species.

Species diversity, richness and equitability
From analysis of vegetation data using the Shannon-Wiener diversity index, community II had the highest species diversity (4.18), followed by communities IV (4.08) and III (3.96) and I (3.94).Community II had the highest number of species (110) followed by communities III (103) and IV (103), and I (79).Community I had thehighest evenness value (0.902) followed by communities II (0.89), Community IV (0.88) and III (0.85).

Plant community-environment relationship
The community types identified from cluster analysis showed significant difference with respect to altitude, slope, soil texture and number of species (Table 4).Community I was significantly different from the rest of the communities in terms of altitude and slope.All the communities identified in this study occur in slightly acidic soils (pH 4 and 5).There is no significant difference between plant communities for soil pH and sand content.Community types II differed in clay and silt content from community type 1, 3 and 4.
The result of Pearson's correlation of the environmental parameters shows that some of the environmental parameters were correlated (Table 5).Altitude is positively correlated with sand with R² = 0.64 and negatively correlated with clay and silt (-0.732 and -0.47)

Phytogeographical comparison
The Agama forests in southwestern Ethiopia are floristically related more to the Masha and Godre forest since they are situated in the same climatic region and geographical location.Accordingly Masha and Godere forests share similar Sorensen similarity Index of (0.59) and (0.46) respectively (Table 6).Furthermore Harenna and Jibbat forests are less similar with lower similarity index (0.38 and 0.32) respectively.repeated coexistence of species over all plots in a community (Vepris dainellii).The highest species richness and diversity were observed in community II.

DISCUSSIONS
The possible reason may be the altitude since it is found at mid altitudes which provides with optimal conditions of environmental factors that favor vegetation growth (Rosenzwieg, 1995).Some species may exclusive to live in mid, low and high altitudes depending on their physiological need and adaption (Austrheim, 2002).Community type I showed the lowest species richness and diversity.This difference is the result of differences in site productivity, habitat heterogeneity and anthropogenic influences such as selective removal of economically important trees and grazing by livestock.Local people reported that this community was used as settlement area for indigenous people before introduction of PFM.
The altitude was relatively the important environmental factor that separates the four plant communities studied.Community I was significantly different from the rest of the communities since it is found at higher altitude (2343 m) and the communities II, III and IV are found at intermediate altitudes.This study is in agreement with Bonnefille et al. (1993) reported that the presence of altitudinal Zonation is delimiting vegetation types in southwestern Ethiopia and affecting atmospheric pressure, moisture and temperature which have again a strong influence on the growth and development of plants and the distribution of vegetation.Studies by Herdberg in 1951 also confirmed the altitude effect on vegetation in eastern African mountain.
The acidity of the soil in southwest Ethiopia is relatively higher as compare to other parts of the country due to intense breakdown of organic matter and leaching of the soil by heavy rainfall.This results in leaching of appreciable amounts of exchangeable basic ions like calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K) from the surface of soil (Achalu et al., 2012).The soil pH decreases with increasing altitude and this could affect the chemical reaction between plant roots and nutrients, the availability of nutrients in the soil for plant use and microbial activity (Donahue et al., 1983).These could be the possible reasons for the decline of species richness and diversity with altitude.The differences in soil texture among the communities are not strongly significant due to limited size of the study area and the altitude ranges are not strongly significant to show variation among the plant communities.
The Agama forest was more floristically related to the Masha and Godre forests due to geographical proximity and located in similar climatic zone.Proximity of the areas favors seed dispersal and migration which result in a high floristic similarity.On the other Jibbat and Harenna forests are found in south eastern parts of the country and it has low floristic similarity due to variation in altitude soil, and climatic factors (rainfall, temperature).On top of this human influence on the forest resource also causes variation in floristic diversity.Bonnefille and Hamilton (1986), reported that the destruction of montane forest in southeastern Ethiopia as far back as ca.2000 years and these historical factors may have led to the variation in floristic diversity between the southwest (Agama) and southeastern (Harenna).

CONCLUSION AND RECOMMENDATION
The Agama forest, in southwest Ethiopia, has high floristic richness and diversity.Four community types were identified at an altitude between 1800 and 2371 m.The communities at the bottom and middle of the altitudinal gradient were richest while the community restricted to the top had less species.The Soil of Agama forest is acidic (with low pH) caused by excessive rainfall.The Agama forest is rich in plant biodiversity as compare to its limited size thus conservation of forest through, strengthening of existing PFM and provision of environmental education for forest user groups are highly recommended.

Figure 1 .
Figure 1.Location Map of Agama forest Community I: Macaranga capensis-Sapium ellipticum community-This community type is distributed between the altitudinal ranges of 1945-2343 m a.s.l. and the slope gradient vary 30 to 65%.It is dominated by the upper canopy of, Macaranga capensis,Sapium ellipticum, Allophylus abyssinica, Apodytes dimidiata, Ficus sur and Croton macrostachyus.The shrubs and herbs include Galiniera saxifragarepandum, Piper capense and Aspilinum anisophylum are the major plant species found in this community.The climbers/lianas of this community are Tilicora rouplii,Culcasia falcifolia and Vernonia wollastonii.Some of the plant species in this community like Cyathea manniana and Phonix relinata are found along the small streams.Community II: Millitia ferruginia-Olea capensis community-This community is found between 1781-2085

Figure 2 .
Figure 2. Growth habit of plant species in Agama Forest.

Figure 4 .
Figure 4. Distribution of relevés on the first and second axis of Detrended correspondence analysis.

Table 1 .
Plant Families with their number of genera and species occurred in Agama forest.Polyscias fulva.The shrubs are Bersama abyssinica Cantium oligocarpm, Clausena anisata and Oxyanthus speciosus.The herbs are Alecmella abyssinica, Achryanthes aspera, Hypoestes triflora, Piper capense and Oplismenus hirtellus This community is also dominated with Piper capense, one of the spice plant commonly collected by women for income generation.Community IV: Vepris dainellii-Schefflera abyssinica-The community is distributed in the altitude range of 1798-2115m a.s.l. and the slope gradient varies 10 to 50%.The emerging dominant tree species in the community are Elaeodendron buchananii, Syzygium guineense, Schefflera abyssinica and Vepris dainellii.The shrubs in this community are Dalbergia lactea,Maytenus gracilipes and Dracaena afromontana.The field layer is dominated by Desmodium repandum, Pteris pteridioides and Asplenium anisophyllum and the climbers include Clematis hirsuta, Landolphia buchananii, Hippocratea

Table 2 .
Endemic species and their habit in Agama Forest, southwest Ethiopia.Schefflera abyssinica) known for white honey and contributing for the income of the local communities in the area.which needs conservation attention.

Table 3 .
Synoptic cover value of plant for species reaching ≥ 1% in at least one community.
Abraham (2009)s one of the moist Afromontane rainforest and rich in plant biodiversity.It comprises an economically important plants used for coffee, spices and honey.These plant species are Coffee arabica, Aframomum corrorima, Piper capense and Schefflera abyssinica are the most frequent species in almost in all sample plots.Even though the size of the study area was limited; it had high number of plant species which is more or less comparable with that reported for the Schefflera abyssinica (Hochst.ex A. Rich.)Harms,Vepris dainellii (Pichi-serm.)Kokwaro,Dracaenaafromontana Mildbr, and Acanthopale aethiogermanica Ensermu in the community and similar observation was reported byAbraham (2009)for Masha forest.On the other hand, high evenness in community I, II and IV indicates little dominance by any single species but

Table 4 .
Post-hoc comparisonof means between environmental variables and plant community.

Table 5 .
Pearson's correlation between environmental variables measured in Agama forest.

Table 6 .
Phytogeographical Comparison of Agama forest with other forests in Ethiopia, according to Sorensen similarity Index.
Values in a row with different letters are significantly different (P<0.05).