Negative density dependence explains tropical biodiversity
LOGAN, UTAH, USA-The diversity of woody plant species is one of the most notable aspects of forests worldwide. However, the mechanisms behind the high diversity in tropical forests and the lower diversity in temperate forests have been poorly understood. Over 50 years ago, Daniel Janzen and Joseph Connell proposed a theory that plant enemies – specialized insects, fungi, and bacteria – attack and kill seedlings near their parent trees, preventing common species from dominating a forest.
Recently, Utah State University scientists joined a global collaboration to reveal an important mechanism promoting species diversity from the tropical to temperate forests. A year ago, Jonathan Myers, an assistant professor of biology at Washington University in St. Louis, and Joe LaManna, a postdoctoral research associate at Washington University's Tyson Research Center, proposed a test of the Janzen-Connell hypothesis to the principal investigators of an international network of long-term forest dynamics research sites, called the Smithsonian Center for Tropical Forest Science-Forest Global Earth Observatory (CTFS-ForestGEO). "This is the first time we've had the data to do this kind of in-depth analysis and to look across temperate and tropical latitudes," said LaManna, who is the corresponding author on the paper. The CTFS-ForestGEO research sites involve large plots within which all woody stems larger than 1 cm in diameter are mapped using a common field methodology, so that spatial patterns of forests around the world can be directly compared.
The study found that suppression of trees near others of the same species is stronger in tropical forests than in temperate forests. Secondly, the study found that this self-suppression is stronger for rare species in the tropics compared to common species. The result helps explain why there are so many rare tree species in tropical forests. In temperate forests, self-suppression is relatively equal for all species, or slightly stronger for common species.
Utah State University Assistant Professor Jim Lutz contributed data from his three western plots – in California, Washington, and Utah. "The existence and maintenance of woody plant diversity is one of the most important features of landscapes," Lutz said. "How plant enemies control diversity can help us understand and manage western forests. The fact that plant enemies can help stabilize forests gives us new insights to promote forest resilience." Utah State University graduate student Tucker Furniss, who also participated in the study said, "I am glad that the Utah plot, at the elevational limit of closed-canopy forest, could help reveal this global pattern." Furniss' specific work on the Utah plot was recently published in the journal Ecosphere.
This work was carried out during the 2016 CTFS-ForestGEO Workshop in Hainan, China, supported by National Science Foundation (NSF) grant DEB-1545761. It was also supported by the Utah Agricultural Experiment Station as project 1153. The data used in the primary analyses will be available at the Smithsonian Institution's CTFS-ForestGEO database portal: [http://www.forestgeo.si.edu/group/Data/Access+the+data.]