Competition for growth resources contributes to size hierarchy in tree populations. Competition hierarchy of trees is dependent on rate of growth and stages of stand development. Competition hierarchy may not cause size symmetry in tree populations. Size structure of even-aged stand can identify tree competition mechanisms for growth resources. The study investigated tree size structure of Teak stand in Valley-Bottom and Hilltop of Omo Forest Reserve.

Ten (10) years old Teak plantation was divided into Valley-Bottom and Hilltop stands base on topography. Five (30m x 30m) sample plots were systematically demarcated in each of Valley-Bottom and Hilltop stands. Tree stems were enumerated and stem densities of both stands were estimated. Diameter at -breast height and total height were measured using Girth tape and Spiegel Relaskop. Stem size inequality, diversity and evenness of both stands were evaluated. Data collected were analyzed using descriptive, correlation, regression analysis and t-test at α_0.05.

Mean diameter and height of Valley-Bottom (11.42±4.83cm dbh and 3.46±1.35m) were not significantly different from Hilltop stands (10.29±4.59 cm dbh and 3.41±1.55m). Stem density of Hilltop (1431.0 stems/ha) was higher than Valley-Bottom stands (1248.0stems/ha). Coefficient of determination (R²) of Height-Diameter allometry for Valley-Bottom (0.59) was higher than Hilltop stands (0.45). Diameter distribution of Valley-Bottom and Hilltop expressed bimodality and unimodality, respectively. Height distribution of Valley-Bottom and Hilltop expressed positive skewed unimodality. Inequality was higher in Hilltop than Valley-Bottom for height and diameter. Elevation affected the stem form and size hierarchy of Teak stems in Hilltop habitat than Valley-Bottom habitat.