2 ilus. 4 tab. Sum. (En, Pt)

The objectives of this research were to test several mathematical models which express the diameter distribution, as well as to verify the influence of the diameter class interval in the fitness of the tested models. The models were applied individually for all species in the area, for the most frequent species and for the commercial species. The data for this research came from the National Forest of Tapajós, located in the county of Santarem, state of Para, Brazil. A total number of 11.173 trees with diameter breast height above 15 centimeters, and totalling 167 species were measured. The following mathematical models were tested to describe the diameter distribution of the area: Two negative exponential equations having the diameter and the square of the diameter as the independent variable, constituting respectively the models 1 and 2; other tested models were: Potential of Mervart, hyperbolic of Pierlot, Polymonial of Golff West, Weibull function and Beta function. The sum of squares of residual was used as a criterium to select the equations by group of species, and by class intervals. The probability associated with the number of runs of residuals, which gives the probability that the observed number of signs changes in the plot of residuals ocurred randomly, was also used. Following these criteria, among the seven tested models, to represent the relationship of the number of trees by 100 ha on diameter classes, the Beta function, the polymonial and the exponential models presented the best results in fitting such relationship. The Weibull function also seems to estimate very well the percentages of the number of trees smaller or equal to a specified diameter of the stand. In comparing class intervals the models in general presented the best results for the 10 cm class interval. The precision of the equations decreased with the decreasing of the class intervals.