UW Madison: Diversity of tropical trees

MADISON - On small Barro Colorado Island, in the middle of the Panama Canal, hundreds of tree species thrive. Jacob Usinowicz, an ecologist, recalls visiting there one field season and being astounded by the number of mahogany seeds littering the tropical forest floor.

"They produce seeds like maple trees, all these little helicopters, and the trees produce thousands of them," says Usinowicz, a former graduate student in the Department of Integrative Biology at the University of Wisconsin-Madison. "If you go when they are reproducing, the seeds are just everywhere."

It's these seeds, and the full-grown trees they ultimately produce, that took Usinowicz there in the first place. Tropical forests boast a diversity of tree species - Barro Colorado Island has roughly as many tree species as all of Europe - and as part of his Ph.D. research, Usinowicz wanted to understand why and how they all manage to coexist. He recently published his findings in the journal Nature with a research team that included UW-Madison professor of integrative biology Tony Ives and Joe Wright from the Smithsonian Tropical Research Institute.

Using vast quantities of long-term data from 10 forests around the globe - from the Appalachian foothills to a mountain range spanning China and North Korea - Usinowicz built reliable mathematical models that show trees in the tropics can better coexist because the climate allows for longer growing seasons, producing reproductive asynchrony that alleviates competition between species.

"The reason you have so many species is because when a tree does really well in the tropics, it's mostly competing only with other individuals of its own species," says Usinowicz, who is currently a postdoctoral researcher at the Swiss Federal Institute of Technology in Zurich.

In temperate and boreal forests, the growing season for trees is often shorter, driven by cold temperatures or excessive moisture. This constrains all species to compete with one another for limited nutrients and space over a very short period of time.

"Things get pushed on top of each other and the thought is that, as a result, species reproduce at the same time of year and are exposed to the same conditions in that year," says Usinowicz. "If everyone reproduces in June and June happens to be rainy, it's a bad year for everyone. In the tropics, however, you essentially have all year."

Long growing seasons give each species the opportunity to take advantage of the right conditions over a greater span of time, allowing them to better time their recruitment, the process of successfully producing seedlings that survive to become trees.

"The goal of the tree is to grow big enough to be in the overstory canopy," says Usinowicz. "Most of the competition happens in the push to establish new seedlings, and the seedlings eventually become saplings. A lot of these species are very shade tolerant and will hang around for a really long time - decades or longer - in the understory, waiting for the death of an adult tree."

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