In order to understand whether and how population structure of different species varies among different fragments and within the same fragment, we studied the ontogenetic stage and size distributions of two common "cerrado" tree species, in four "cerrado" fragments with different physiognomies and in three sites with similar physiognomies within the same fragment in the Municipality of Itirapina (22°15' S, 47°48' W), São Paulo state, SE Brazil. We recorded the height and the stem basal diameter (DSH) of all individuals with DSH > 3 cm in the six samples of 200 contiguous 5 x 5 m plots. Individuals with DSH < 3cm were sampled in 1 m² sub-plots at one corner of the plots. Xylopia aromatica had greater proportions of initial ontogenetic stages in open physiognomies, whereas Roupala montana had high proportion of initial stages and greater young:adult ratio in the densest physiognomy. The ontogenetic stage structure had greater differences among samples within the same fragment than among fragments, i.e., it varied in small spatial scales. The spatial heterogeneity of environmental factors (differences in vegetation cover that varied from open to dense), different reproductive characteristics (R. montana can reproduce asexually, whereas X. aromatica reproduces only by seeds) and phenological patterns (annual in X. aromatica, and supra-annual in R. montana) could be the main variables associated to the ontogenetic structure variation, and they would act differently upon each species in each site. Thus, each site of "cerrado" constitutes a different environment, which can differently influence the success of a species.
cerrado; ontogenetic stages; Roupala montana; size structure; Xylopia aromatic
