Brazil is a country of great forest vocation, with around 6.8 million hectares of planted forests and 385 million hectares of native forests. The diameter structure of a forest, from the point of view of production, enables to characterize and indicate the timber stock available prior to logging and provide information to assistin the decision-making on the need for reforestation. The purpose of this study was to analyze the distribution pattern of diameter class by empirical and statistical methods and the growth dynamics of two different forest fragments (native and planted) located in Rio Branco, State of Acre, Brazil. In the rubber tree plantation, forest measurements were made at 100% of the circumference and diameter of different tree species; the data were grouped in classes through both statistical and empirical methods. In the native forest, the trees from different species were measured in 10 plots of 500 m² (10 × 50 m) area, the data were grouped in classes through both statistical and empirical methods. There was higher concentration of individuals in the core classes of distribution, a very common factor in planted forests. Planted forests present a graphic layout that closely resembles anormal curve, which is a characteristic of this forest type. On the other hand, native forests are usually characterized by decreasing diameter distribution, in the form of an "inverted-J", that is, a greater amount of individuals in the classes of smaller sizes, which decreases with the increase of classes.
diameter; native forest; planted forest