Cooper, Declan LM; (2023) Investigating Patterns of Commonness in Tropical Forest Tree Communities. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Trees structure Earth’s most biodiverse terrestrial ecosystem, tropical forests. The diversity of tropical tree communities presents a formidable challenge to understanding these forests, as very little is known about most tropical tree species. However, if a small number of tree species are common while most are rare, focusing on the common species may circumvent this challenge. The primacy of a modest number of common species has been observed in Amazonia but remains unexplored elsewhere in the tropics. This thesis analyses species abundance data on 1.13 million trees ≥ 10 cm diameter from 2,130 locations from the largest networks of forest inventory plots across Africa, Amazonia, and Southeast Asia to characterise tropical tree species composition and commonness. Using established methods, I estimate that 50% of trees in the tropical forests of Africa, Amazonia, and Southeast Asia comprise just 2.2%, 2.2%, and 2.3% of each continent’s tree species, implying that only 1,053 species comprise half of Earth’s 800 billion tropical forest trees. I then develop, from first principles, a new area-adjusted zero-inflated Poisson lognormal (ZIPLN) model for the number of stems per area of each species, designed to capture how tropical trees are sampled by inventory plots and enable rigorous estimation of commonness patterns. Employing a Bayesian hierarchical implementation of the ZIPLN that comprehensively accounts for uncertainty, including from unsampled species, again strikingly consistent cross-continental commonness patterns emerge. Specifically, I estimate that 1.4%, 1.5%, and 1.6% of species comprise 50% of overall stems per unit area in Africa, Amazonia, and Southeast Asia, respectively. The ZIPLN also provides ecological characterisation of all sampled species and identifies which of them are likely to be among the most common in the underlying assemblages. These findings open new avenues to understand the world’s most diverse forests by focusing on the common species that dominate their trees.

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