Wetland vegetation composition and ecology of Lake Abaya in southern Ethiopia

Wetland vegetation and ecology of Lake Abaya in the southern Ethiopia was studied to determine floristic composition, plant community type and vegetation ecology. A total of 102 plots were laid along transects that were set up preferentially across areas where there were rapid changes in vegetation or marked environmental gradients to collect data on estimate of percentage aerial cover of plant species and environmental variables. Vegetation data was analyzed by agglomerative hierarchical cluster analysis using similarity ratio as a resemblance index and Ward’s linkage method. Multivariate data analysis was performed using appropriate packages in R version 2.14.0. Canonical Correspondence Analysis (CCA) was used to explore the relationship between the species composition and environmental variables. The environmental data included in the CCA were determined using stepwise backward and forward selection of variables by ANOVA test. Statistical measurement regarding species diversity, richness and evenness of the plant community types was carried out by using Shannon-Wiener diversity indices. A total of 92 plant species belonging to 66 genera and 34 families were identified. Families Poaceae, Asteraceae, Fabaceae, Cyperaceae, Solanaceae, Euphorbiaceae and Amaranthaceae account for about 56.99% of the total proportion. Based on the cluster analysis, five plant community types were identified. The most important factors influencing the plant species composition and pattern of wetland plant communities were water drainage, water depth, land use, slope, altitude, and hydrogeomorphology. Therefore, these factors should be considered in future management and protection under the circumstance of climate change and human activities.


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Enter: The author(s) received no specific funding for this work.

Introduction
Irrespective of the size and spatial distribution, wetlands provide a wide range of important values and functional attributes.Wetlands are among the most important productive ecosystems on earth because of the complex interactions between biotic (fauna, flora, microbes and unicellular organisms) and abiotic (soil, water and topography) components of wetland ecosystems (Mitsch and Gosselink 2002;Costanza et al. 1997).Wetland ecosystem is considered to be more productive than the adjacent area because of the periodic inflow of nutrients.
Their productivity is comparable to rainforests and coral reefs (Costanza et al. 1997).They constitute a resource of great economic, cultural, scientific and recreational values.Schuyt (2004) estimated that freshwater wetlands hold more than 40% of the entire world's plant species and 12% of all animal species.Finlayson and Moser (1991) estimated that more than 40% of fishes (of the 20,000 species in the world) live in fresh water wetlands.Individual wetlands can be important in supporting high numbers of endemic species; for example, Schuyt (2004) reported that Lake Tanganyika in Central Africa supports 632 endemic fishes and other animal species.
Wetlands produce an ecological equilibrium in the environment by maintaining the integrity of life support systems for sustainable socio-economic development.Wetlands are also important sources of water for agricultural, industrial and domestic uses.They play important roles in ecological functions in their natural states, which contribute to the wellbeing of human society.

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Despite the will in understanding of wetlands and their ecological importance, degradation of wetlands continues, mainly due to anthropogenic activities.About half the global wetland area has been lost as a result of human activities (OECD 1996).In tropical and subtropical areas conversions of wetlands to alternative land uses have accelerated wetland loss and agriculture is considered the principal cause for wetland loss.Wetlands were drained to control disease and for agricultural activities (Maltby 1986;Lean et al. 1990;Mitsch and Gossenlink 1993;Wetzel 1975).Other important reasons for their vulnerability are the fact that they are dynamic systems undergoing continual change (Barbier et al. 1996) and are the fact that they are often open-access resources with limited control over how they are used and what is harvested from them (Turner et al. 2000).
In Ethiopia, wetlands are distributed across all agro-ecological Zones from high altitude of 4000 m above sea level to 125 m below sea level (Afework Hailu et al. 2000).The different geological formation, ecological diversity and climatic conditions have endowed Ethiopia with all types of wetlands except coastal, marine-related wetlands and extensive swamp-forest complexes (EPA 2003).
Although wetlands in Ethiopia cover only a small area, they have immense economic, social, and environmental benefits.Wetlands are the main sources of valuable water resources in Ethiopia, where water resources are unevenly distributed and only a quarter of its population has access to safe water.In many parts of Illubabor and Wollega, many perennial and annual springs are associated with the existence of wetlands.Throughout the country, wetlands are important sites for livestock and wildlife grazing especially during the dry period.Floodplains of Borkena, Dabus and Fogera are vital sources of fodder, particularly during dry season, to both domestic and wild animals.
Wetlands are also vital sources of food, fuelwood, and raw materials for making household furniture.Growing number of people in our country, in both rural and urban areas depend on wetland resources for their survival.Poor rural households, particularly women depend on wetlands for additional income to their families.In the ethnic group known as the 'Agnuak' in the Gambella lowlands along banks of the Baro and Gello Rivers, women are heavily involved with fishing activities (Afework Hailu et al. 2000).Many peasant farmers in the western parts of the country make their living from wetlands.Sedges are one of the important wetland resources that local communities use in different parts of the country.For example, in western Oromia, sedges have great importance for thatching of houses.More than three fourth of the local households in Illubabor Zone (Afework Hailu et al. 2000), use sedges for roofing their houses.
Wetlands associated with lake Abaya provide range of ecological and economic importance to wildlife and people in the surrounding area.The wetland vegetation is an important nesting and feeding areas for hundreds of wetland birds and hippopotamus and protective shelter for spawning areas for crocodiles.Wetlands of Rift Valley Lakes are sites for tourist attraction.
Despite the global importance, wetlands in Ethiopia are still facing many problems.The major threat comes from the over harvesting of wetland resources, the expansion of human settlements in the main Ethiopian Rift Valley Lakes (Ziway-Shala, Hawassa, Abaya-Chamo and Chew Bahir basins), the construction of dams in Koka and Melka-Wakana, drainage for agriculture in southwest Ethiopia especially in Jimma and Wollega Zones (Afework Hailu et al. 2000).Wetland resources such as water, fishes and vegetation are subjected to over exploitation.Excessive exploitation of the resources from wetlands can lead to a direct collapse of the wetland and its resources.
Disappearance of Lake Haramaya wetlands through excessive use of water for irrigation can be evidence to demonstrate overuse of a resource from wetlands.
Threats to wetlands also originate from the catchments since wetlands are closely interacting with the catchment.
Wetlands in the lowland areas are threatened by encroachment due to shortage of cultivation and grazing areas that results from population pressure in the highlands.Overgrazing by livestock and wildlife leads to loss of biodiversity and compaction of the wetland soil during wet periods which can affect infiltration capacity of the wetland soil.This can in turn affect biodiversity of the wetland.In other instances, catchments are the source of agricultural discharges and will result in increased nutrient load to the wetlands (eutrophication) which leads to the colonization (homogenization) of the habitat by single species which are usually invasive (either exotic or indigenous species).
Invasive species are a major threat to global biodiversity and an important cause of biotic homogenization of ecosystems (Enserink 1999).Invasive species, either exotic or indigenous, are also threats to wetlands of Ethiopia.
Examples of invasive species threatening Ethiopia's wetlands include Prosopis juliflora in Awash River basin, Mimosa pigra in the Baro-Akobo basin, and water hyacinth, Eichhornia crassipes, in Koka reservoirs (Mitiku Tikssa et al. 2010;Bayafers Tamene et al. 2000).Increased accumulation and sedimentation will ultimately accelerate the rate of conversion of the wetland system to a terrestrial one (Wetzel 1975).The low-level awareness of communities regarding the benefits of wetlands, capacity limitations such as lack of skilled manpower, scarcity of wetland focused and coordinated institutions, lack of technical and financial support for wetlands conservation also accelerate loss of wetlands.
Although wetlands are among the most productive ecosystems on earth and protection of threatened natural wetlands and preservation of its biodiversity has received increasing attention globally, wetlands and their resources in Ethiopia are still facing many problems.High population densities within the catchments of the Ethiopian Rift Valley Lakes and in the highlands have been associated with a series of deleterious trends, in particular those arising from the clearance of vegetation for agriculture and overgrazing.Some of the key challenges to the wetlands of lake Chamo are intensive use of land in the buffer zones for crop production, land loss due to deforestation in the watershed, and eutrophication by nutrients from agricultural fields (Seleshi Bekele 2001).Overgrazing of lakeshore vegetation by livestock and overharvesting particular plant species such as Aeschynomene elaphroxylon for the construction of traditional boat, clearance of lakeshore vegetation and trampling due to fishing activities and deforestation in the watershed are also the main threats to the wetlands.The western parts of the two lakes are extensively used for big state farms, which were recently given for private investors.
Despite their importance to maintenance of biodiversity, wetlands and their resources (vegetation in particular) in Ethiopia have been barely investigated and their previous documentation was extremely limited.The vegetation ecology, species composition and diversity of wetlands in Ethiopia have not yet been in the detail it deserved.This is typically reflected in wetlands associated with Lake Chamo in south Ethiopia where research in vegetation ecology, species composition and diversity have not yet been conducted.Thus, this study was conducted with the purpose of determining the floristic composition, plant species diversity and richness, plant community types, trends in relationship between plant communities and ecological gradients and to recommend some corrective measures of sustainable management.It is also hoped that this research can provide baseline information for further studies and the issues raised and recommendations given in the study areas will apply to the other wetlands in Ethiopia and neighboring countries in the Horn of Africa.

Study area
Lake Abaya is part of the Ethiopian Rift Valley Lakes found in southern Ethiopia (Figure 1).Lake Abaya is the largest lake in the Ethiopian Rift, located between 5°3'19 ''N and 6°45'11''N latitude and 37°18'55''E and 38°7'55''E longitude (EMA, 1988).It has a maximum length of 79.2 km, maximum width of 27.1 km with a surface area of 2600 km2 (Seleshi Bekele, 2001).It has a maximum depth of 24.5 m and is located at an average altitude of 1,235 m.a.s.l.(Seleshi Bekele, 2001).

Vegetation Data Collection
Representative and relatively homogeneous vegetation units of sampling sites were purposively selected on the basis of physiography and physiognomy.Transects were set up preferentially across area where there were rapid changes in vegetation or marked environmental gradients following Muller-Dombois and Ellenberg (1974).
Transects were laid out to represent hydrological variations and habitat heterogeneity.Transects were started from the edge of the open water and extended to the ecotone along the edge on both sides of the wetland.Mueller-Dombois and Ellenberg (1974) and Kent and Coker (1992) suggested quadrat size for meadow and fen vegetation type is 4 m x 4 m (16 m 2 ) and a shrubby heath, tall herbs with grassland vegetation type are 10 m x 10 m (100 m 2 ).Thus, because of differences in vegetation zonation pattern and continuity of substrates, vegetation data were collected at each site using a plot size that meets the minimum area requirements for meadow, shrubby heaths, tall herbs with grassland vegetation and shrubby swamps (100m 2 ).
Sample plots were placed along transects in each of the purposively (preferentially) selected habitat units or strata based on moisture gradient and dominant vegetation types.Location of the plots was chosen visually and purposively to ensure that it gives a representative view of the plant species and abundance in that unit.The location of each plot was recorded using a GPS.Within each sample plot in all study areas, all plant species were recorded and the percentage aerial cover of each species was estimated.This was later converted to 1-9 scale following the Braun-Blanquette method modified by Van der Maarel (1979) Cover was estimated as the percentage of sampling area covered by the vertical projection of individuals of each species present (Mueller-Dombois & Ellenberg 1974).Plant species occurring outside the plots were also collected and identified to produce a comprehensive list of the plant species diversity of the study area.Voucher specimens were collected, coded, pressed and dried for subsequent identification and verification at the National Herbarium (ETH), Addis Ababa University, using Flora of Ethiopia and Eritrea and those of other neighboring countries.

Environmental data collection
The environmental variables recorded in each plot include: drainage, hydrogeomorphology, disturbance, slope and elevation.At each site or sample plot, estimate of disturbance intensity was recorded based on physical evidence of the site characteristics (e.g., soil irregularities, burning, defoliation and dung present).Plot level hydrogeomorphologic features were rated based on physical evidence of the site or sample plot (e.g., position of the land scape and additional water source) whereas drainage was recorded based on water holding capacity of the soil or level of saturation.Depth to water table was measured in each plot with a labeled PVC pipe.Environmental data on topographic parameters such as altitude and coordinates for each plot were determined with GPS.

Data Analysis
Multivariate data analysis methods were used to analyze the vegetation and environmental data.Statistical analysis was performed in the R version 2.14 statistical computing program (R Development Core Team 2009) using packages for classification and ordination.Both ordination and classification techniques were employed to study the ecology of wetland vegetation.
Vegetation data were analyzed using agglomerative hierarchical cluster analysis (Mueller-Dombois and Ellenberg 1974) using similarity ratio as a resemblance index and Ward's linkage method to identify vegetation assemblages.
Distinct clusters were identified at appropriate hierarchical levels and the quadrats of the data set were then arranged using the sequence of the quadrats in the dendrogram produced.The mean cover value of each species in each cluster identified was calculated and a synoptic table was produced.The species with highest mean cover value was used to determine dominance and sub-dominance of species in cluster groups.Based on the cluster analysis output and the resulting synoptic table and ecological evaluation in the field, the number of community types (clusters) was determined at appropriate dissimilarity levels (height of the dendrogram).The plant community types were named after one, two or three dominant species, using the highest mean cover values of plant species which occur in each group.Dominant species are those that are most conspicuous in the community and are high in one or more of the importance values (Whittaker 1975), mean cover value in this case.
The identified plant community types were tested for the null hypothesis of no significant difference between the groups using the Multi-response Permutation Procedures (MRPP) (ter Braak and Šmilauer 2002; Ludwig and Reynolds, 1988).A significance level of 0.05 was used to determine if the compositional differences between groups were statistically significant.The Plant community types were examined using non-metric multidimensional scaling (NMDS) ordination.NMDS was chosen because the tests can produce robust visualizations of data despite numerous zero-values and highly variable data with lack of normality (McCune and Grace 2002).Using the synoptic table and the habitat information gathered during the sampling period, the different plant communities were described.
Canonical Correspondence Analysis (CCA) (ter Braak 1987Braak , 1988Braak and 1990;;Hill 1979) was used for revealing patterns in the species composition data and relating the patterns to measured environmental variables.CCA technique assumes a unimodal distribution of species in relation to environmental variables (McCune and Grace, 2002;McCune and Mefford 1997).Species cover abundance data for each plot together with the corresponding plot versus environmental variables data matrix were subjected to Canonical Correspondence Analysis (CCA) to reveal the relations between the species composition and environmental variables.
The environmental data included in the CCA were determined using stepwise backward and forward selection of variables by ANOVA test.The CCA generated biplot scores (i.e., correlations between environmental variables and ordination axes) were used to infer the relative importance of each environmental variable for prediction of species composition and distribution (ter Braak 1995).

Diversity Analysis
Statistical measurement regarding species diversity, richness and evenness of the plant community types was carried out by using Shannon-Wiener (1949) diversity index.The Shannon diversity index (H') was computed using the mean cover values of the species in the plant community types as the input matrix (synopsis output).The Shannon diversity index is calculated by using the formula:

Shannon diversity index
Where s= the number of species; Pi = the proportion of the individuals of the i th species or the abundance of the i th species expressed as a proportion of total cover; Ln= log basen (natural logarithm).Equitability which determines the relative evenness of the species within the plant community was also calculated.Equitability (Shannon evenness, J) is calculated by the following formula: Evenness index Where, J = the equitability; LnS= log basen; S = the number of species Floristic similarities with regard to species composition among and between study sites were calculated by employing Sorenson's similarity coefficient (Kent and Coker 1992) by using the equation: SC = 2a/ (2a + b + c) Where SC = Sorenson's similarity coefficient; a = number of species common to both categories; b = number of species present in the first category and absent in the second and c = number of species present in the second category and absent in the firs

Floristic Composition
A total of ninety two plant species belonging to 66 genera and 34 families were recorded from the wetlands of Lake Abaya.Families Poaceae, Asteraceae, Fabaceae, Cyperaceae, Solanaceae, Euphorbiaceae and Amaranthaceae account for about 56.99% of the total proportion (Figure 3).Poaceae was represented by 14 (15.05%)species belonging to 11 genera; Cyperaceae by 13 species (13.98%) belonging to 2 genera; Fabaceae was represented by 7 species belonging to 4 genera while Asteraceae was represented by 6 species belonging to 6 genera.Euphorbiaceae was represented by 5 species belonging to 4 genera; Solanaceae was represented by 4 species belonging to 4 genera and Amaranthaceae was represented by 4 species belonging to 2 genera.
Figure 3: Families with the greatest representation in Lake Abaya wetland

Description of Plant Community Types of Lake Abaya Wetland
Based on the cluster analysis output and ecological evaluation in the field, five community types were identified between 2.0 to 2.5 heights (0.60 -0.75 dissimilarity levels) of the dendrogram (Figure 4).The plant community types were named by the dominant species, which occur in each group, using the highest synoptic values of plant species (Table 1).Communities with their dominant and sub-dominant species, the number of relevés they contained and diversity index are given in Table 2.
The most common herbaceous species based on mean cover values (Table 1)

Diversity of Plant Community Types
The Shannon-Wiener Diversity computed for five different plant communities (Table 2) showed that community type three (Cynodon aethiopicus) which was found in relatively dry terrestrial landscape and exposed for disturbance was the most diverse and has the highest species richness and eveness.Community type four (Typha angustifolia-Aeschynomene elaphroxylon) which occurred in deeper standing water around the open water zones and experiencing prolonged inundation had the least diversity index and had the least even distribution of species.
Community type one (Cyperus articulatus) was the least in species richness and occurred in deeper water next to community four.

Canonical Correspondence Analysis (CCA)
Species and plots ordination in the space defined by the first two CCA axes are shown in Figures 6a and 6b.CCA ordination illustrates the relationship of the four environmental variables (water depth, drainage, slope and hydrogeomorphology) to five community types and to species distribution pattern.The first two axes together accounted for 63% (Table 4.8) of the variation explained.The proportion of variances explained by these axes was 36% and 27%, respectively (Table 4).
Results from ANOVA test showed that water depth, slope, drainage and hydrogeomorphology were significantly related to the floristic composition of the plant community types.The first axis extracted by the analysis was closely related to the drainage and slope, as indicated by the biplot scores of -0.43 and -0.78 (Table 4).The second axis extracted by the analysis was more closely related to water depth and hydrogeomorphology, as indicated by the biplot scores of -0.71 and 0.54, respectively (Table 4).The second axis was associated negatively with water depth and positively with hydrogeomorphology.(a) Analysis of variance showed that there was a strong relationship between species and environmental factors such as water depth (p<0.01),drainage (p<0.01),hydrogeomorphology (p<0.05) and slope (p<0.01).The species and environment correlations with the first axis was 0.85 and the second axis was 0.74 (Table 4).Canonical correspondence analysis (CCA) (Figure 6b) for the species and explanatory variables revealed essentially the same pattern as ordination of plots and explanatory variables.
The analysis of variance also showed that there was a marked relationship between the environment and the scores of many species.The plant species Pharagmatis karka, Cordia africana, Ficus sycomorus and Eragrostis japonica had strong relationships with the first axis (more closely related to slope).Euphorbia tirucalli, Hippocratea africana and Leucas deflexa were more closely related to areas with decreasing water depth and drainage), whereas Phalaris arundinacea, Eichhornia crassipes, Pistia stratiotes, Nymphaea lotus and Potamogeton pucillus had significant relashinship with increasing water depth and drainage. (b)

Discussion
The ordination diagram displayed a measure of overlap among vegetation types and also indicated various groups (plant community types) that uniquely occupy ordination space.One of the insights obtained from the ordination process was that the dominant graminoids (with several exceptions) were rather broadly distributed and overlapping in ordination space.Furthermore, the study on the wetlands resulted in plant communities of emergent species, aggregating free floating, floating leaf and submerged plants as minor members of associations dominated by the more obvious emergent species and did not indicate the differences that may exist between emergent and submerged plants as distinct components of wetland communities.This could be due to shared affinities for environmental conditions.
Disturbance in the form of burning, land use types, drainage, pollution, eutrophication or overgrazing can reduce plant species diversity by eliminating disturbance sensitive species (Connell 1978).Drainage for cultivation can affect ground water table and in turn the organic matter content of the soil.Species losses occur as a result of habitat loss and fragmentation of ecosystem because of direct human activities, including trampling and collecting.
Trampling associated to fishing could be responsible for the lower number of plant species (diversities) encountered in the wetland.
Sileshi Bekele ( 2001) pointed out that Lake Abaya and Lake Chamo wetlands which receive agricultural inputs and waste water predominantly from human settlements in the catchment and surrounding highlands were found to experience eutrophication with higher levels of nutrients.Eutrophication is a gradual increase in the concentration of phosphorus, nitrogen and other plant nutrients in aquatic systems, which results in the system becoming more productive.Wetlands fed by surface water from agricultural and urbanized watersheds tend to have invasive species (Galatowitsch et al. 1999).Manure transported from the surrounding areas under cultivation has an additional effect that contributed to eutrophication.Therefore, eutrophication may have caused the Lake to be more productive through nutrient enrichment and hence causing the wetlands to be invaded by invasive species and agricultural weeds.Similar situation has been reported in other wetlands with high eutrophication level in Africa by Gichuki et al. (2001), Abila et al. (2008).
The presence of introduced or invasive species and their high frequency around the wetlands might indicate a strong anthropogenic disturbance effect of land use on these wetlands.Human related disturbances are well known to facilitate successful establishment of non-native (including invasive species) and weedy plant species in many vegetation types (Hobbs 1991;Hobbs and Huenneke 1992).It is therefore, possible that an expansion of cultivated land and associated eutrophication process and a change in grazing regime, due to increase in the number of cattle in the wetlands had contributed to the current spatial heterogeneity in species composition of weedy and invasive plant species.Most of the invasive and weedy plant species take advantage of the changes in site conditions, especially increase in nutrients following a significant disturbance (Milbau and Andi 2004).Large herbivores influence species composition by removing herbage, trampling and through dung and urine depositions (Hobbs 1991).Dung deposition modifies site conditions by supplying nutrients and facilitates seed dispersal (Hobbs 1991).Rea andStorrs (1999) andDiTomaso (2000), reported that increase in cattle grazing effects such as trampling and nutrient inputs may accelerate the spread of invasive which consequently changes plant species composition and functional structure.

Conclusion and Recommendations
The study has resulted in the documentation of 92 plant species representing 66 genera and 34 families.Most of the plants in the wetlands under investigation were emergent macrophytes except a few of them that were free-floating, floating-leaf and submerged species.The presence of weedy, introduced or invasive species and plant community types dominated by these species indicate a relatively more anthropogenic disturbance effect of land use on wetlands of Lake Abaya.
The distribution and composition of plant species and vegetation assembly at study site were influenced significantly by a combination of different environmental factors.In the wetland, the dominant grasses and sedges were widely distributed resulting in an overlap among plant community types.Furthermore, the study on the wetlands resulted in plant communities of mainly emergent species, aggregating free floating, floating-leaf and submerged plants as minor members of associations dominated by the more obvious emergent species and did not indicate the differences that may exist between emergent and submerged plants as distinct components of wetland communities.
Thus, the presence of non-wetland shrub and forb species and their high frequency in the wetlands indicated that the wetland was experiencing disturbances from both natural environment and anthropogenic factors.
Results of the study showed that there were differences in the spatial and temporal patterns of plant species composition and diversity due to variations in environmental gradients (hydrologic factors in particular) and anthropogenic factors.Therefore, this should be considered in future management and protection under the circumstance of climate change and human activities.Overall, results support the prediction that different environmental variables had varying influence on the overall plant species composition, diversity and distribution.
Long term monitoring of the wetland plant communities is recommended as the environment changes with increased human activities that are likely to increase the abundance of invasive species.The number and cattle movements should be controlled in order to maintain the integrity of the wetland.
Protective measures should be developed to safeguard the study areas from destructive anthropogenic impacts; otherwise the damage can become irreversible.The effective and continuous protection measures should be identified and local community should be informed about the importance of the conservation of these fragile ecosystems.Wetland managers and decisions makers at all levels should consider this baseline data on the species composition and vegetation ecology to guide management decisions and to detect changes over time and space as a result of management and impacts of anthropogenic alteration.
Further study of soil nutrients and water chemistry data is recommended to better understand the compositional variation found between and among the wetlands.In addition, an assessment of the vegetation in terms of its variability and reaction to various practices such as burning and grazing should be done.
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Figure 1 :
Figure 1: Map showing location of Lake Abaya.The Abaya-Chamo lakes basin experiences bimodal rainfall pattern (Figure 2).Lake Abaya-Chamo basin is covered by different vegetation types and experiences different land use practices.The land use of Lake Abaya and Chamo wetlands, especially western side of the two lakes, has been changed rapidly due to extensive deforestation as a result of increased number of populations in the area (Seleshi Bekele 2001).The extensive deforestation resulted in the replacement of vegetation cover by cultivated lands.Farming activities like livestock rearing and crop production are the main land use practices in the catchment surrounding the lakes.

Figure 2 :
Figure 2: Climate diagram for the study area.

Figure 4 :
Figure 4: Dendrogram showing plant community types of Lake Abaya Wetland vegetation. 1. Cyperus articulatus community type: This community type was dominanted by Cyperus articulatus.Cyperus laevigatus, Typha angustifolia, Cynodon aethiopicus, Aeschynomene elaphroxylon, Leersia hexandra and Eichhornia crassipes were also important species with high mean cover values in the community type.This group included plant species that form wet meadow vegetation.Most of the plants in this community type were emergent macrophytes except few of them were floating plants such as Nymphaea nouchali and Eichhornia crassipes.Thus, in this community type, emergent plant species were more prominent than free floating, floating-leaf and submersed plants.This community type consists 13.7% of the stands most of which were from north and northwest of the lakeshore.
in waterlogged areas characterized by presence of standing or permanently flowing water for most of the year.This wetland dependent community type also occurred in permanently flooded areas as well as in and around the edge of open water bodies.Alternatively, Cynodon aethiopicus (community type 3) with large number of plots (in the upper part of the ordination space (Figure6b) was found in excessively drained areas with high disturbance and low water level.These groups also commonly occurred in areas characterized by low lying dry terrestrial landscape with well drained soils.Water depth and hydrogeomorphology were strong factors differentiating these groups from other community types.Slope was a strong factor differentiating community type five (Aeschynomene elaphroxylon) from others.It was relatively intact and pristine or semi-natural vegetation found in areas occasionally used for grazing activities.

Figure 6 .
Figure 6.Canonical correspondence analysis (CCA) of wetlands of Lake Abaya vegetation.(a): shows the relationships between the plant community types and environmental variables, (b): shows the relationships between plant species distribution and environmental variables.Only higher priority species with high variances are visible and all other less dominant species are indicated by plus sign (+).Species were abbreviated by combining the first four letters from generic names and specific epithets.Environmental attributes represented by the vectors included water depth, slope, drainage and hydrogeomorphology.The vectors representing environmental gradients point in the direction of the most rapid change in each variable.

Table 3 .
Pearson (r 2 ) correlations of Nonmetric Multidimensional Scaling ordination axes with environmental

Table 4 .
Results of CCA analysis of Lake Abaya wetland vegetation data.Canonical coefficients from the best-fit multiple regression models (ANOVA test), biplot scores for the constraining variables, eigenvalues and proportion of variances explained by the first two axes were indicated.Factor values that differed significantly between the groups according to an ANOVA are shown in bold.Figure6a) for plots and environmental factors showed that Eichhornia crassipes-Pistia stratiotes (community two) and Typha angustifolia-Aeschynomene elaphroxylon (community four) were found in areas with high water level and poor drainage.Therefore, distribution of these community types was significantly affected by hydrologic factors such as water depth, drainage and hydrogeomorphology.These community types were consisted of wetland dependent plant species that are tolerant to high water table which most commonly occur