Ecological Effects of Plantations of Exotic tree Species on the Understory of Native Vegetation in Yerer Forest Oromia Regional State Central Ethiopia

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Addis Ababa University


A study on the effects of plantations of exotic tree species (Eucalyptus globulus, Cupressus lusitanica, Grevillea robusta, and Pinus patula) on native species both in the above and below ground with compared adjacent natural forest was conducted in the central high land of Ethiopia, in Yerer forest. The aim of the study was to investigate and document the effects of plantations of exotic tree species on the above and belowground native vegetation composition, diversity and density, and soil properties in Yerer forest of central Ethiopia. A systematic sampling design was employed to collect vegetation and soil data in all land-use systems. The vegetation and soil data were collected from a total of 20 transect lines and 60 quadrats, each with a 20 m x 20 m (400m2) area established, that distributed equal 12 quadrats for each five forest land-use systems. Within each quadrat, five sub-quadrats with 2m x 2m (for seedling and sapling), and 1m x 1m (for herbaceous, and soil samples) were used for data collection. In the study, there are three parts, the first part focussing on one of the land uses, the natural forest. In which, a total of 93 woody species in 46 families were identified from 122 quadrats along 22 transect lines in the natural forest only. In the natural forest, the distribution of the density of seedlings > saplings > mature trees, which indicates woody species had s healthy regeneration profiles. In the natural forest, five plant community types were identified with varying degrees of species diversity, richness, and evenness. The species composition similarity coefficients among five plant communities ranged from 36-78%. In the second part, the five forest land-use systems with four plantations of exotic tree species (Eucalyptus globulus, Cuppressus lustanica, Grevillea robusta, and Pinus patula) and adjacent natural forest. From 60 quadrats, a total of 211 native species in 59 families, represented by 134 herbs and 77 woody species were recorded. Among the 211 identified native species, 32 species (15.17%) were commonly observed in all five forest land-use systems (LUS) 66 species (36.49%) only in a single forest (36 species only in a natural forest, and 30 species only in exotic tree plantations), while the rest species were recorded in between two to four forest LUS. The overall native vegetation composition in different forest land-use systems (LUS) ranged from 71 to 181 species. The overall aboveground vegetation species composition in similarity indices among five forest land-use systems (LUSs) ranged from 0.26 – 0.49. The third part focusing on eight land-use systems (natural forest, Eucalyptus globulus, Cuppressus lustanica, Grevillea robusta, Pinus patula, grassland, grazing land, and cropland) to study the soil seed bank composition and density, and soil properties. From a total of 96 quadrats, along 32 transect lines, the soil seed bank results showed, a total of 98 plant species belonging to 42 families and 84 genera were identified. In all the different land-use systems a total seed bank composition, and seed density /m2 ranging between 29 to 57, and 1625 to 3250, respectively. General, The seed bank species composition, and density revealed a significant variation as a function of land-use systems. The analysis of variance (ANOVA) for the majority of soil physical and chemical properties variables of sand particle size, MC, BD, PD, SP, OC, TN, Avial. P, soil pH, EC, CEC, and exchangeable bases (Ca, Mg, K, Na) were showed that a significant variation among land-use systems at (P<0.0001). The study results confirmed that plantations with different exotic tree species showed variability in their effect on the understory native species composition, diversity, density, and natural regeneration status. The changes in land use systems under similar climatic conditions significantly influenced the belowground native species composition and density, and soil property.



Exotic Tree Plantations, Land Use Systems, Soil Properties, Soil Seed Bank, Yerer Forest, Native Species