Tropical Trees Use Social Distancing to Maintain Biodiversity

Tropical forests often harbor hundreds of species of trees in a square mile, and scientists struggle to understand how such a diversity of species can coexist together. In their work published in the August 4 issue of Science, researchers at The University of Texas at Austin have provided new insights into the answer by uncovering a key characteristic of the spatial distribution of adult trees.

Combining computational modeling with data collected over a 30-year period, the researchers discovered that adult trees in a Panamanian forest are three times more distant from other adults of the same species than what the proverbial ‘the apple doesn’t fall far from the tree’ would suggest. Annette Ostling, an associate professor with the University’s Oden Institute of Computational Engineering and Sciences and the Department of Integrative Biology, and postdoctoral researcher Michael Kalyuzhny, utilized data collected from a forest research plot the size of 100 football fields located on Barro Colorado Island in the Panama Canal which has been studied for the last 100 years.  

“What we discovered is trees’ distances from each other is much greater than what seeds typically travel,” according to information from Smithsonian Tropical Research Institute’s S. Joseph Wright, a senior scientist and collaborator on this research. The researchers wondered why would there be so much repulsion (repelling) of the juvenile from its parent tree, and the only theoretical explanation is something that would prevent them from establishing near their parents.

Using models, the team learned that each tree species is much more negatively affected by its own kind than by other species, likely because species suffer from species-specific enemies:  pathogens such as fungi, or herbivores such as insects. These enemies “make room” for other species to establish around every tree, leading to a more diverse forest and keeping any one species from dominating.

The long-term study of tree species provides critical tools to learn how tropical forests and their inhabitants change through time. Kalyuzhny adds “In this forest, we know the exact location of every tree, and also how far seeds travel. Using computational models, we asked: how should the forest look like if trees just established where the seeds fell? It turned out that the real forest does not look like this at all – the real trees are much more far apart!" This novel application of computational modeling led to the discovery, according to Ostling.

In a time of an ongoing mass extinction, scientists have been working to better understand what determines species diversity. “Trees are the ecosystem engineers that provide resources for the entire ecosystem, and since most of the species in the world reside in the tropics, we are looking into what maintains the biodiversity of planet earth. Many medications are sourced from the tropics, including thousands of substances with anti-cancer activity. Our research digs into this very fundamental question about the natural world,” said Kalyuzhny, whose love of nature began when he was young with a desire to grasp a deeper understanding of the environment around him.

The new research bridges the gap between contrasting results on how forests are shaped. Researchers have found numerous evidence for low survival of juveniles under adults of the same species for decades, but until now there was no evidence this has any impact on the adults in the next generation and their distribution in space. 

To understand and preserve an ecosystem and its diversity, one must first understand its main features and what drives those features, according to Ostling. “This research is a steppingstone to understanding the dynamics of things like carbon storage that matter in relation to climate change. It's important to identify the key mechanisms maintaining diversity to understand what could pose a risk to it. Climate change models predict some of the tropics would become dryer, potentially making things more difficult for juvenile trees but also for their enemies, with the overall effect unknown. It’s such a fundamental question that, even if the applications are not yet known, there’s still a lot to learn and this is one ingredient in understanding,” Ostling added.

According to Ostling, other important next steps include showing if this pattern exists in other forests and how it varies, such as between tropical and temperate forests which have fewer species. The existing global network of forest sites will make this possible as more information on dispersal of seeds is learned.

In addition to Ostling, Kalyuzhny and Wright, Jeffrey K. Lake from The University of Michigan co-authored the research. The research was funded by the Michigan Life Sciences Fellowship, Zuckerman STEM Leadership Program, Sabbatical support from the University of Michigan and Adrian College, Mcubed, The University of Michigan, Associate Professor Support Fund, and the Ostling Lab at The University of Texas at Austin.

For more information, contact: Joanne Foote, Oden Institute for Computational Engineering and Sciences, or Christine Sinatra, College of Natural Sciences, at The University of Texas at Austin.