A Comparative Study of Indigenous Woody Species Diversity and Regeneration Status Among Plantations of Three Exotic Tree Species and Natural Forest, in Gomma District, South-western Ethiopia

Diversity, density and species composition of naturally regenerated indigenous woody species under three selected exotic tree plantations ( Eucalyptus camaldulensis, Grevillea robusta and Cupressus lusitanica ) and the adjacent natural forests were investigated in Gomma district. The objective of this study was to verify the hypothesis that the diversity and regeneration status of indigenous woody species differ among plantations of the three exotic tree species and the adjacent natural forest patches. Three sites (kebeles) were purposively selected from three altitudinal ranges (agroecological zones). Four quadrats of size 10 m × 10 m were established for each of stand at all sites, and generally 48 quadrats were laid down along transect line with 50m interval between plots. Within each main plot, five 1m x 1m subplots were also laid. All matured trees/shrubs of indigenous woody species were identified, measured and counted from each main plot, while saplings and seedlings were collected from subplots. A total of 49 woody plant species belonging to 41 genera and 27 families were identified from all plantations and natural forest stands. The highest number of species was recorded for natural forest patches followed by Eucalyptus camaldulensis, Grevillea robusta and Cupressus lusitanica plantation stands. Similarly, highest density was recorded for the Eucalyptus camaldulensis among plantation stands. The Eucalyptus camaldulensis stands showed the highest Shannon diversity (H’ = 2.67) and Cupressus lusitanica showed the least (H’ = 2.05) among plantation stands. Almost the population structure of all stands in the present study showed an inverse J-shape. This implies that the population is composed of numerous individuals at the seedling stage, and decreases continuously or gradually as height increases, and hence indicates healthy regeneration. The study revealed that regeneration and diversity of indigenous woody species were significantly different under the canopies of exotic plantations. E. camaldulensis and G. robusta plantation stands showed higher density and diversity than C. lusitanica stands, but for successful regeneration, there should be seeds sources and seed dispersal agents (e.g., animals) and knowledge of biodiversity management.