Why are trees and tree species abundant in tropical forests?

Tropical forests have always been characterized by more tree species than temperate forests. But why is it so? 

The answer came from a team of 50 researchers from 41 institutions in 12 countries who were working in 24 permanent forest dynamics plots (PFDPs) spread across the Americas, Asia, Africa, Europe and the Pacific. These PFDPs formed part of the network of permanent plots under the Center for Tropical Forest Science-Forest Global Earth Observatory of the Smithsonian Tropical Research Institute.

World map of stem-mapped forest plots (n = 24 forest plots) examined, which are part of the Smithsonian Center for Tropical Forest Science–Forest Global Earth Observatory network. Included in the analysis were 2.4 million trees from more than 3,000 species (Source: LaManna et al 2017).

They found that the diversity of species across tropical and temperate forests increased with conspecific negative density dependence (CNDD) or “the process by which population growth rates decline at high densities as a result of natural enemies (e.g., predators, pathogens or herbivores) and/or competition for space and resources.”

However, according to their results, only CNDD caused by species-specific mechanisms, such as intraspecific competition (that is, members of the same species compete for limited resources) and host-specific natural enemies (such as pests), helps sustain the diversity of species.

The researchers wrote in a paper published in Science that “these specialized interactions stabilize populations of individual species, causing population growth rates to decrease when a species is locally common and increase when a species is locally rare.” Such interactions, which brings down the risk of extinction, explain why many tropical communities have extremely large numbers of rare species. In contrast, CNDD in temperate forests preserves diversity “by limiting populations of common species but not by strongly stabilizing populations of rare species.”

The study cites four mechanisms that contribute to shifts in CNDD across species and latitudes.

First, species-specific mechanisms are stronger in the tropics.

Second, strong dispersal limitation for both trees and their specialized enemies can lead to more intense host-enemy interactions or intraspecific competition.

Third, rare tropical species may be more susceptible to specialized enemies.

Fourth, differences in biogeographic history, climate and speciation may also influence global patterns of species diversity indirectly, such as by changing the composition of enemy communities.

Few studies on the connection between CNDD and species diversity had been conducted before. This is also the first time that a study has looked into this relationship in temperate and tropical areas.

The results suggest that regional processes interface with local biotic interactions (for instance, how trees in forests interact with each other) to determine the strength of CNDD across species and the maintenance of biodiversity across tropical and temperate latitudes. For the researchers then, both local biotic interactions and regional processes must be considered to understand global patterns of biodiversity.

Also used in the study were recensus data generated from the Palanan PFDP in Isabela in 2004, 2010 and 2016. Located in the middle of the Northern Sierra Madre Natural Park, the Palanan PFDP was established in 1994 as an eight-hectare plot. In 1998, it was expanded to 16 hectares, making it the longest continuously monitored permanent forest plot in the country. The Palanan PFDP helps researchers understand how trees interact with each other and with their environment–which lives (survival), which dies (mortality) and which colonizes (recruitment) and how fast they grow. 

The 16 hectare Palanan permanent forest dynamics plot (PFDP) inside the Northern Sierra Madre Natural Park, Palanan, Isabela. More than 100,000 trees from 325 species had been tagged, measured and identified and its location recorded. The Palanan PFDP was set up to monitor how tropical lowland forests respond to the impact of catastrophic typhoons (Source: Co et al., 2006).

(Based partly on an article written by Dr. Perry Ong for the International Publication Award)