Woody Species Encroachment and Related Ecological Conditions in the Grass Plain of Nechsar National Park
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Date
2008-06
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Addis Ababa University
Abstract
Encroachment of woody species has been reported as the major threats to the Grassland of
Nechsar National Park of Ethiopia. The objectives of the study were to determine the
invasive bush species, woody species density and woody species cover; to determine plant
diversity and community; to asses the relationships between woody species cover,
herbaceous cover, grass cover, forbs cover, and bare land cover in the Grass plain of the
Park. Vegetation survey methodology was employed. Floristic composition, species relative
frequency and abundancy, plant community, species diversity, species richness, species
evenneness, density and cover of woody species, grass cover, forbs cover, total herbaceous
cover were determined in 75 relev`es. One hundred and thirty one plant species belonging
to 45 different families were recorded. Dicrostachys cinerea, Acacia nilotica, Acacia
mellifera, and Acacia Oerfota were the major woody species encroacher in the area.
Abutilon anglosomaliae (RF = 0.95), Bothrichloa inssculpata (RF = 0.81), Rhynchosia
malacophylla (RF = 0.79), Endostemon tereticaulis (RF = 0.72), Commelina diffusa (RF =
0.70), Digitaria abyssinica (RF = 0.67) were the most frequent species in the area.
Abutilon anglosomaliae (6.5%), Dicrostachys cinerea (6%), Chrysopogon plumulosus
(5.3%), Bothrichloa inssculpata (4.5%), Pupalia lappacea (4%), Loudetia flavida (4%) are
species with higher cover abundance in the Grass plain of Nechsar National Park. The
hierarchical agglomerate cluster analysis using average linkage strategy provides six
major plant communities and one solitary releve. The low average specie richness (23spp.
Plot-1) resulted in low over all average species diversity (2.92). ANOVA showed there is no
variability among plant communities with respect to species diversity, richness, and
evenneness at p≤0.05. Results also showed that woody species density was 1995 woody
plants ha-1. Mean cover of woody, grass, unpalatable forbs, and total herbaceous species
was 31%, 58%, 68% and 121% respectively. The bare land cover was 12%. The high
woody and unpalatable forbs cover indicated the progressively increasing grass species
diversity deterioration in the grass plain of the Park. Relatively high bare land cover
observed resulted in high incidence of gully erosion in the area. Analysis of variance
showed significant variability among the plant community at P≤0.05 for woody, grasses,
forbs, and bare land cover. Pearson correlation coefficient matrix indicated that woody
species cover was negatively correlated with total herbaceous cover (grass and forbs) and
bare land cover. Grass cover was negatively correlated with forbs, bare land cover, and
woody species density. Total herbaceous cover was negatively correlated with bare land
cover, bush density. The relationship between bush cover, woody species density, grass
cover, herbaceous cover, bare land cover and related soil erosion is complex and related
to climate, land escape geology and anthropogenic disturbances. Main threats to the
grassland ecology of Nechsar National Park are progressive increase in bush cover, loss of
grass cover, and increase in unpalatable forbs. Currently greater than 50% of the
landscape of the grass plain is in a poor to fair grassland conditions. Decline in the
grassland condition, unless reversed, will jeopardize the biological diversity as well as
aesthetic value of the Nechsar National Park.
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Biology