Mode of seed dispersal shapes placement of rainforest trees

The apple might not fall far from the tree, but new research shows that how it falls might be what is most important in determining tree distribution across a forest. A recent study of the seed dispersal methods of rainforest trees demonstrates that these methods play a primary role in the organization of plant species in tropical forests.

Joshua B. Plotkin, a junior fellow in the Faculty of Arts and Sciences at Harvard University, and co-author Tristram Seidler published the results of their study on seed dispersal methods in the journal Public Library of Science – Biology (PLoS Biology).

“Overall, there is a highly significant relationship between mode of seed dispersal and the clustering and arrangement of mature trees in the rainforest,” says Plotkin. “This strong correlation demonstrates the long-term impact that these dispersal methods have on the organization of the large-scale forest.”

In order to address the paradox of how so many rainforest species can coexist while competing for the same resources, Plotkin and Seidler studied a 50-hectare (500 meters by 1,000 meters) plot of lowland tropical forest at Pasoh Forest Reserve in peninsular Malaysia. They analyzed the dispersal mechanisms and spatial distributions of 561 tree species found in the plot. What they discovered was that species clustering was strongly correlated to the species’ mode of seed dispersal.

Each species was categorized by one of five dispersal methods: ballistic (where seeds are liberated explosively), gravity, gyration (where the progression of seeds to the ground is slowed by the shape of the seeds), wind, and animal, the last of which was subcategorized by fruit size. The animal subcategories were intended to distinguish among different sizes of animal that might disperse the fruit.

Plotkin and Seidler observed the distribution of individual tree species, determining an average spatial cluster size. Upon comparison across all species and seed dispersal categories, they determined that trees with ballistic dispersal methods tended to have the tightest cluster size, with trees dispersing their seeds via gravity, gyration, wind, and fruit of increasing size showing progressively more diffuse spatial distributions.

Plotkin will join the faculty of the University of Pennsylvania next year as an assistant professor of biology. Seidler, his co-author on the PLoS Biology paper, is a postdoctoral fellow at Imperial College, England. Their work was sponsored by the Conservation, Food and Health Foundation, the Burroughs Wellcome Fund, and the William F. Milton Fund.