Secondary subtropical Atlantic forests shelter a surprising number of rare tree species: outcomes of an assessment using spatially unbiased data

There is an increasing interest in secondary forests due to their potential to shelter a substantial amount of tree species. Hence, this study sought to assess the rarity form of 646 arborescent species occurring in secondary stands of three forest types within the Brazilian subtropical Atlantic Forest biodiversity hotspot. The species were classified into one of seven rarity forms encompassing population size, habitat preference, and geographical distribution. Data gathered within 418 systematically distributed 0.4 ha sample plots were used. Among all recorded species (dbh ≥ 10 cm), 67% were classified into one of the seven rarity forms at least in one forest type. Approximately 50% of the species found in each forest type were rare. Myrtaceae was the family with the greatest number of rare species, followed by Fabaceae, Lauraceae, and Melastomataceae. Yet, rare species represented a diminished portion of the ~ 90,000 sampled individuals: less than 5% of the trees in each forest type. Rare species with scarce populations—i.e., forms 1, 3 and 7—represented more than 40% of the total species richness recorded in each forest type. The sample plots located in old-growth forests, ecotone areas, or areas with specific environmental conditions (e.g., hydromorphic soils), often showed larger proportions of rare species. Public protected areas appeared to be important for conservation as some species were observed exclusively in these areas and were singletons. Quadratic plateau regression models brought up evidence that the mean proportion of recorded rare species did not increase substantially in sample size classes greater than 70–80% of the reference samples.