Tree diversity at a global scale

diverse canopy of Amazonian forest at Los Amigos Biological Station, Peru. (Photo: Jonathan Myers)

Joe LaManna, a 2015 PhD graduate of the Wildlife Biology program, and professor Andrew Larson are co-authors on new research that provides insights into why tree diversity is higher in the tropics compared to more northern latitudes.

LaManna, a lead author of the study just published in the journal Science, analyzed data from 24 long-term forest dynamic research sites, including the Yosemite and Wind River Forest Dynamics Plots. Larson is a principal investigator at both sites – two of the largest permanent forest monitoring plots in North America.

LaManna and lead author Jonathan Myers from Washington University in St. Louis proposed a global-scale test of the Janzen-Connell hypothesis--the idea that host-specific plant enemies preferentially kill seedlings near parent trees, keeping tropical forests from being dominated by just a few species. The well-known idea had not been evaluated at a global network of sites using standardized methods until now.

Data from the 24 forest research plots spanning tropical and temperate forests, all member sites in the Smithsonian ForestGEO network, was key to the analysis. “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.

Thousands of hours are required to collect the necessary data at each site. “Our field protocols include individually measuring, mapping, and identifying to species every free-standing woody plant greater than about one half inch in diameter” said Larson.

The analysis also showed that plant enemies that kill rare species may also keep them from going extinct. “When species get too rare, their enemies also thin out, and they have what is known as a rare species advantage,” Myers said. So the specialized predators ultimately stabilize rare species instead of wiping them out.

“We were able to show for the first time that this stabilizing effect may be stronger for rare species in the tropics; this may explain why rainforests harbor so many rare trees,” LaManna said.

Larson noted the importance of long-term forest monitoring plots in California and Washington as key in the finding. “These northern-latitude sites provided an important comparison for the tropical forests” said Larson, “This new discovery was only possible because of the addition of temperate forest sites to the Smithsonian ForestGEO network over the last decade.”

Read the full paper

Plant diversity increases with the strength of negative density dependence at the global scale - Science, June 30, 2017

Other news about this research

Jungles beat forests for biodiversity, but why? UM researchers find unexpected answer - Missoulian, July 5, 2017

Global forest network cracks the case of tropical biodiversity - the Source, Washington University, June 29, 2017

Why do we see more species in tropical forests? The mystery may finally be solved - Smithsonian, July 6, 2017

Is this the long-sought answer to the question of tropical biodiversity? Smithsonian Insider, June 30, 2017

Negative density dependence explains tropical biodiversity - EurekAlert, June 30, 2017

Global forest network cracks the case of tropical biodiversity - Phys.org, June 29, 2017