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Why are there so many species of tropical trees?

In the 1970s ecologist Steve Hubbell, then a professor at Princeton University, and Robin Foster, a botanist at the University of Chicago, wondered if the forces structuring tropical forests might be different than those at work in the temperate zone. They had a hunch that chance and randomness played a big role in the tropics. Their ideas ran counter to prevailing scientific thought−that forest structure was predictable, the result of a succession of events that led to more or less the same end.

Imagine clearing a patch of forest, then fast-forwarding 100 years. How much would the forest that grows back look like the original forest? Equilibrium theory predicted that pioneer species would colonize the area first, followed by early-successional and then late successional species. The resulting mature forest would consist of nearly the same species that were in the original patch. Or so the theory goes.

Hubbell and Foster preferred a "non-equilibrium hypothesis." In this scenario the composition of the forest after 100 years would depend more on random immigration events−like a bird blown in by a storm carrying seeds from a distant site or an El Niño event that wipes out species that cannot survive drought.

To test their ideas, they would need an area with enough trees of each species to calculate averages−and because individual species are rare, that would mean a much bigger area than the plots used in temperate studies. A 50-hectare plot (1,000 meters by 500 meters) would do.

After convincing Ira Rubinoff, then the director of the Smithsonian Tropical Research Institute (STRI) in Panama, to set aside an area of prime real-estate on Barro Colorado Island’s central plateau, they set off with a team of 50 undergrads from the University of Panama to identify, map, mark and measure every tree.

They completed the first census in 1982, after measuring 240,000 trees of 303 different species more than 1cm in diameter. By 1995, after three more censuses of the plot, it was clear that this tropical forest, expected by biologists to be as stable as tropical temperatures, was remarkably dynamic and unstable. For example, changes in species distribution towards more drought tolerant species was a clear and rapid response of the forest to the acute El Niño event that happened during this time.

Soon forest biologists around the world took note of the surprising results coming from Panama. Were these patterns specific to this part of the world or could they be generalized? A group from Harvard established plots in Southeast Asia and STRI set up plots in South America, India and Africa. Staff Scientist Richard Condit headed up the Center for Tropical Forest Science, the group within STRI that partnered with researchers at the other plots and pooled all of the data for analysis.

Later Stuart Davies took over as director with the objective of adding plots in temperate areas as well. Now 51 plots using the same methodology comprise the Forest Global Earth Observatories, or ForestGEO. Detailed data from more than 5 million trees representing approximately 10,000 species give researchers an extraordinary finger on the pulse of the living world. The network makes it possible to look for very large-scale patterns and to ask new questions such as how warming temperatures affect forest health and if events like El Niño in one part of the world are felt in distant places.

Building on this work, plot researchers monitor carbon in soils, roots, and lianas, as well as in trees. Measurements of tree growth, death, litterfall, and soil respiration contribute to very detailed measurements of carbon stocks and fluxes. Their work provides high quality, on-the-ground data for calibration of carbon stock measurement based on satellite and LiDAR measurements.

Perhaps even Robin Foster and Steve Hubbell, when they scaled up sampling size in 1980 to a 50 hectare plot in order to tackle the megadiversity question, could not have imagined that 33 years later 51 plots would globally address such an array of fundamental biological questions.

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Asner Wright Hall Muller Winter Potvin
Turner Detto Hubell Rubinoff Davies