Investigating Patterns of Commonness in Tropical Forest Tree Communities

Tropical forests play critical roles in the global carbon and water cycles, affect global climate, store the majority of the world's terrestrial biodiversity, and support the lives of millions. The vast number of tree species presents a formidable challenge to understanding these forests, as very little is known about most tropical tree species. However, if the majority of species are rare and a relatively small number of tree species are common, as seen in Amazonian forests, a focus on the common species may circumvent this challenge. To this end, I analyse species abundance data from networks of inventory plots from closed-canopy structurally intact old-growth tropical forests in Amazonia, Africa, and Southeast Asia, the combined dataset including 1,003,805, trees. I estimate that 50% of individual trees in the closed-canopy tropical forests of Africa, Amazonia, and southeast Asia comprise just 2.2%, 2.2%, and 2.3% of the total number of species on each continent respectively. Extrapolating the results across all closed-canopy tropical forests, I estimate that just 1,053 species comprise half of Earth's 800 billion tropical forest trees greater than or equal to 10 cm diameter. Overall, despite differing biogeographic, climatic, and anthropogenic histories, I find strikingly consistent species-abundance patterns across the continents. This consistency suggests that fundamental rules of tree community assembly apply to all tropical forests. Resampling analyses show that the most common species are reliably recognised, allowing targeted efforts to understand their ecology. These results open new opportunities to understand the world's most diverse forests by focusing on the common species that comprise the majority of their trees.