Commonness, rarity and oligarchies of trees in two tropical dry forests of Mexico
Neotropical dry forests are rich in plant species well-adapted to the seasonal drought that characterizes this biome. Despite such adaptations, most tree species in the dry forest are uncommon to rare, with a minority of species making up the majority of individuals in a stand. It remains to be seen to what extent the identity of these common species changes with increasing scale. Research suggests that the floristically diverse lowland humid forests of Amazonia are dominated by a relatively small number of widespread “oligarchic” species. This study examines whether the same may be true for large extents of the Mesoamerican dry forest. We measured tree species diversity and abundance in two widely separated forests on the Pacific coast of Mexico to evaluate whether these forests show evidence of (1) having oligarchic species and (2) similar patterns of species commonness and rarity.
Of the ~ 8200 stems sampled, 232 species were recorded at the northern site and 273 at the southern site, with 140 species shared by both sites. Abundance, frequency and basal area of shared species were greater on average than unshared species, and were positively correlated across sites. A subset of 63 shared taxa (17% of all species) accounted for more than 47% of all trees encountered at the two sites. The results have implications for predicting tree species composition and the role of oligarchic species in ecological processes across the Mexican TDF and potentially beyond, as well as for understanding the contribution of rarer and more geographically restricted species to this system.