Statement of problem: Tree size diversity is an indicator for biodiversity values of a forest. Microsite conditions of forest determine the survival and growth of tree. However, the contribution of variable habitats to tree size hierarchy and segregation is poorly understood. Variation in size of trees in a population is cause by different mechanisms. Therefore, size distribution and spatial pattern of trees can identify process govern growing resources utilization in the forest. The objective of the study is to determine the structural diversity of tree stems of Elephant Camp natural forest in Omo Forest Reserve.

Methodology: Three and four 0.09ha sample plots were established in Riparian (RF) and Old-growth forests (OF) of Elephant camp, respectively. The tree stems (Dbh≥5cm) were identified to species level and enumerated within each plot and stem density was computed. The diameter-at-breast height (Dbh) was measured with diameter tape. Species diversity was assessed using Shannon-Weiner (H´) and Simpson indices (1-D´) while size inequality was assessed using Gini coefficient (GC), Coefficient of Variation (CV), H´ and I-D´. The performance of single two- and three-parameter Weibull models were evaluated; Kolmogorov-Smirnov (K-S) Chi-Square (χ²), Root Mean Square Error (RMSE), Bias and Coefficient of determination (R²). Data were analysed using descriptive statistics at α_0.05.

Results and Conclusion: A total of 27 and 24 tree species were identified in RF and OF, respectively. Stem density of RF was significantly higher than OF. The value of species diversity (H´, 1-D) and Evenness (E´) were higher in OF than RF while richness (Margalef and number of species) was higher in RF than OF. The Dbh were 38.30±21.4 and 42.87±19.2cm in Riparian and Old-growth forests, respectively. Diameter distribution of Riparian and Old-growth forest were positively skewed and expressed exponential pattern. The Elephant Camp is rich in tree species and moderate in tree size inequality. It had good regeneration status with high density of small size and mid-size tree stems.