Tropical trees are better neighbors than trees in temperate forests according to a new study published in the journal Nature by researchers from 29 different institutions including the Smithsonian Tropical Research Institute (STRI) and the ForestGEO global network of forest monitoring sites.
The study was led by Han Xu, Professor at the Chinese Academy of Forestry; Matteo Detto, Research Associate at STRI and Research Fellow at Princeton University; and Suqin Fang, Associate Professor at Sun Yat Sen University. The team's finding—trees growing closer to the equator have more positive interactions with their neighbors—may help explain why tropical forests are home to so many tree species, making them some of the most biodiverse ecosystems on the planet.
"Most research has focused on competition and other negative interactions among trees, but trees can also help their neighbors in many ways," Detto said. "We find that these positive interactions are more common in tropical forests, adding another piece to the puzzle of understanding their remarkable diversity."
These conclusions were based on a comparison of 17 forest study sites in North and South America, Asia, Africa and Oceania, and included almost 3 million trees representing more than 5000 species. Researchers from these forests, members of the ForestGEO network of forest research sites, all use the same methods to measure, map and mark every tree greater than a centimeter in diameter, making it possible for them to compare forests around the world.
It may look like trees just stand there, but they interact with their neighbors in both positive and negative ways. A tree species is considered to have positive (facilitative) relationships with its neighbors if it has more, and more diverse, neighbors than average for the whole forest at that site, and a negative relationship with its neighbors if it has lower species abundance and richness than average.
While the proportion of trees with positive and negative neighborhood interactions is roughly equal across all the tropical forests in this study, the proportion of tree species with more, and more diverse neighbors drops off in forests further from the equator.
The authors think there may be more positive (facilitative) interactions between trees in the tropics because:
- Further from the equator there are fewer trees in the bean family, the legumes. Legume trees capture nitrogen from the air and convert it into a form available in the soil for their neighbors to use. A previous study by researchers working at STRI in Panama showed that legume trees may speed up the recovery after a forest is logged or cleared for agriculture because legume trees capture nitrogen and carbon faster than other trees. Their ability to capture or "fix" nitrogen from the air and release it into the soil as the forest makes a comeback has far-reaching implications for forest restoration projects.
- Tropical trees are associated with non-arbuscular mycorrhizal fungi that spread out through the soil and help trees of different species capture scarce nutrients.
- The "canopy nursing effect" is lower further from the equator. Giant rainforest trees act as nursemaids for smaller species, protecting these neighbors from high temperatures and dryness.
Two previous studies of people in the U.S. and in China showed that people are more agreeable, emotionally stable and open to new experiences when they grow up in warmer climates closer to our optimal temperature of 72F or 22.2C, were published in the journal Nature Human Behavior.
As the planet warms will it become a friendlier place overall?
Funding for this research was from the National Natural Science Foundation of China, U22A20449 and U23A20156, and a National Non-profit Institute Research Grant from CAF, CAFYBB2017ZE001.
Reference: Han Xu et al. 2026. The importance of competition and facilitation for global tree diversity. Nature 10.1038/s41586-026-10349-2
