Research Article: Woody species composition and diversity of riparian vegetation along the Walga River, Southwestern Ethiopia

Date Published: October 17, 2018

Publisher: Public Library of Science

Author(s): Misganaw Meragiaw, Zerihun Woldu, Vegard Martinsen, Bal Ram Singh, Suzannah Rutherford.


The primary objective of this study was to examine the status of woody species composition and diversity along the Walga River of Wonchi, Southwestern Ethiopia. Fifty quadrats of 10 m x 50 m were laid at 500 m interval through systematic sampling method along the river line. Vegetation height (≥2.5 m) and DBH (≥2.5 cm) of only tree species were measured and altitude, ecological disturbances such as, grazing intensity and human impacts were included as main environmental variables at each of the sampled plots. The data was analyzed using different R statistical packages. Ninety-nine woody vascular plant species belonged to 81 genera and 45 familieswere recorded in Walga riparian vegetation. Only 10% of specieswere endemic to the Flora area. Asteraceae and Fabaceae had the highest number of species. Majority of the species (52.5%) were shrubs. Four major plant community types were identified: Euclea divinorum-Maytenus arbutifolia (1), Pterolobium stellatum- Calpurnia aurea (2), Brucea antidysenterica-Prunus africana (3), Erica arborea-Hagenia abyssinica (4). Species richness, true diversity and importance values were highestin community type 2(the lowest altitude ranges between 1976–2212 m a.s.l.) while evenness was highestin community type 3(mid altitude ranges between 2359–2676 m a.s.l.). Both community typeswere comprised of 56% of all recorded species and all endemic taxa except two. The highest percentage of species in lower frequency classes indicates a higher degree of floristic heterogeneity. There was a strong negative correlation (r = -0.65, p<0.001) between species richness and altitude with 42% of the variation in species richness per plot being explained by altitude. Our findings suggest that human disturbances and excessive livestock grazing are the main threats in community types1 and 2. We conclude that identifying major plant community types and underlying environmental conditions may help to manage and conserve forest resources in the area.

Partial Text

Plant communities grown along the river edges are known as the riparian vegetation, which is one of the fifteen terrestrial biomes of the earth [1,2]. Riparian vegetation is broadly defined as a terrestrial land with visible vegetation that interacts to permanent or temporary aquatic systems such as rivers, streams, lakes and wetlands [3–7]. Healthy riparian vegetation provide home for aquatic and terrestrial wild animals, water quality protection, stream bank stabilization, water capture and storage, as buffer zone to reduce flood and soil erosion, maintaining soil fertility and moisture for fast growth of plants [4; 8,9]. Because of such huge advantages, scientists and land managers are interested in understanding the status of riparian vegetation as key indicators to measure management practices [5,7–11]. Riparian vegetation isnaturally heterogeneous in the flora area with diverse species composition due to the change in microclimate, soil and land use types [6,12–14]. The variations of riparian vegetation are not only from region to region, but also shown along a single river line with altitude gradients and/or level of anthropogenic effects [15–17]. There are numerous perennial and flashing rivers and streams in Ethiopia, which support rich assemblages of plant species which respond differently to altitudinal variation [16,18–22]. Some of these are highlythreatened.

The present study describes the riparian floristic communities along an altitude gradient on the Walga River in the southwestern Ethiopia. These floristic composition groups are characterized by species richness, diversity, and evenness matrices and correlated with livestock grazing and browsing, human disturbance, and altitude. Additionally, forest structure measurements were collected on a subset of riparian trees and analyzed for population structure. The result shows that about 42% of the variation in species richness per plot being explained by altitudegradient. Despite theirsmaller numbers of sampled quadrat, the upper stream riparian vegetation of Walga(2359–3078 m a.s.l.) had higher number of species per ha (89) than the downstream riparian vegetation (76). Human and livestock activities are correlated both with specific community typesand altitude because altitude is correlated with all community types while it is more strongly correlated with community types 3 and 4. Thus, considering multiple environmental gradients would give a better picture of richness and the potential mechanisms responsible for distributing plant diversity in the riparian vegetation.Although it is widely interrupted by human and livestock disturbances, Walga riparian vegetation is remain rich in plant diversity (99 species) with more than 10% of them are endemic to the Flora area. Based on our finding, the highest species diversity, richness, endemism and even distribution of individuals occurred in community types2 (Pterolobium stellatum—Calpurnia aurea in the lowest altitude ranges between 1976–2212 m a.s.l.) and 3 (Brucea antidysenterica-Prunus africana in the mid altitude ranges between 2359–2676 m a.s.l.). Therefore, these sites need special priority to conserve majority of species which are ecologically very important and harbor nine endemic taxa. In conclusion, the finding of this paper provides information that is necessary for making conservation and management practices.




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