Aiming to study the alterations in the diametric structure of an Ombrophyllous Mixed Forest located in São João do Triunfo (Paraná State - Brazil), nine plots of 1 ha, first evaluated in 1979, were recovered and measured in 2000. In 1979, all trees with dbh > 20 cm were identified, tagged and had the dbh measured. In 2000, based on the same criteria described before, the trees which had not been counted in 1979 were considered as ingrowth and the missing ones were considered mortality. In 1979, the sample (9 ha) had 2.133 trees of 51 species. In 2000, the number of trees increased to 2.202 and the species to 55. During this period, the mortality amounted to 513 trees (24.05%) and the ingrowth to 591 trees (27.71%) in the studied area, giving an increase of 3.66% in trees' density. In both evaluations, the diametric distribution of the forest (all species included) presented a progressive decrease, like a J-shaped curve, although the amount of trees in the diametric classes above 50 cm increased significantly in 2000. Only Matayba elaeagnoides and Ocotea porosa did not present a "J-shaped" diameter distribution. In 1979, Araucaria angustifolia had 11.66% of trees in diametric classes above 50 cm, the broadleaf (non-conifers) 6.37%. In 2000, 25.42% of the A. angustifolia trees were in classes above 50 cm, while 8.99% of the broadleaf trees were in the same classes. Despite the increase in the forest density, some plots presented a decrease in the number of trees during the analyzed period. This study showed that is not the high frequency in smaller diameters classes that guarantees the presence of a species in the future forest structure, but its capacity to compete.
Diameter distribution; forest dynamics; araucaria forest
