This study was carried out to determine radial variation of wood density, modulus of elasticity (MOE), modulus of rupture (MOR) and stress at elastic limit in plantation and natural alder forests in the north of Iran. Testing samples were taken at breast height of the stem and in three radial positions (10, 50 and 90% of radius) from natural and plantation forests to determine wood mechanical strength properties, according to the ASTM standard. The analysis of variance indicated that planting conditions (natural and plantation forests), radial position and their interaction had no significant effects on the modulus of elasticity (MOE), modulus of rupture (MOR) and stress at elastic limit in alder wood, while only radial position had a significant effect on wood density. Wood density was increased along radial direction from the pith to the periphery for both planting conditions. Overall, the mechanical strength properties in the plantation forest were slightly higher compared to the natural forest. The relationship between wood density and mechanical properties were analyzed by linear regression. A positive relationship was found between wood density and mechanical properties for both planting conditions. This relationship was stronger in plantation grown, compared to naturally grown trees.
