Abstract In this paper I examined the usefulness of tree population size distributions in evaluating the conservation status of populations of an endangered tree species. I set expectations derived from two complimentary views of the ecology of rainforest trees and examined whether they were met by size distributions of populations of the South American dominant conifer Araucaria angustifolia and its relationship with forest structural characteristics. Specifically, I evaluated the expectations that (i) A. angustifolia trees have larger diameter than average angiosperm trees and form a higher monospecific canopy layer above the shorter angiosperm canopies; (ii) A. angustifolia populations are characterized by size distributions with many large individuals and a long tail of relatively rare, small individuals (have symmetry coefficient <0); (iii) the symmetry of the size distribution of A. angustifolia populations is negatively related to the abundance of large (d.b.h. ≥ 10.0 cm) individuals in the population; and (iv) the abundance of A. angustifolia trees is negatively related to the abundance of angiosperm trees, as the successional accumulation of angiosperm stems would not be accompanied by the recruitment of new A. angustifolia. These expectations were evaluated using data on 25 populations of A. angustifolia sampled in the Rio Grande do Sul State in southern Brazil. The first, third and fourth expectations were met, while the second one was only partially met. This was because populations showed a bimodal distribution regarding symmetry in size distribution, with most populations showing normal or negative symmetry, and those logged, adult-depleted populations showing positive skewness.