Scientists have long struggled to explain how rainforests can maintain their impressive diversity of trees without a handful of species taking over – or that many other species disappear.
The answer, researchers say, lies in the soil found near individual trees, home to the natural "enemies" of tree species. These enemies, including fungi and arthropods, attack and kill many of the seeds and seedlings near the host tree, preventing local recruitment of trees of the same species.
Also playing a key role in the dynamics of the rainforest are seed dispersers. Seeds of individual trees that are transported at a distance – often by rodents, mammals or birds – have a chance to establish themselves because fungi and arthropods in the new region target different species. This restriction on recruitment of trees near adult trees creates a long-term stabilizing effect that favors rare species and hampers common ones, say researchers.
By nullifying the previous theory, researchers demonstrate that these interactions with enemies are important enough to maintain the incredible diversity of tropical forests. The results of the study are being published this week in Annals of the National Academy of Sciences (PNAS).
"In many North American forests, trees compete for space and some have a niche that allows them to outgrow others," said Taal Levi, an ecologist at Oregon State University and lead author of the study. "Douglas-firs are the species that grow best after a fire." Hemlock thrives in the shade and grows well under a canopy.
"But in the tropics, all species of trees seem to have a similar competitive advantage." There are an abundance of species, but few individuals of each species, and the chances of disappearing must be high. become common, becoming dominant, and it is these natural enemies that have a high host specificity. "
Egbert Leigh of the Smithsonian Tropical Research Institute best described the diversity of rainforests in a statement: "How can a half square kilometer (of forest) in Borneo or Amazon contain as many tree species as 4.2 million kilometers of temperate forest "in Europe, North America and Asia combined?"
Levi said that some rainforests have about a thousand species of different trees living in the same general area. The idea of natural enemies restricting the recruitment of juvenile trees is not new, he said, and was actually postulated almost half a century ago by two scientists in what became known as the Janzen-Connell hypothesis.
Although the effects of Janzen-Connell should prevent a species from taking control, they do not explain or predict how a thousand species of trees can be held together. In fact, previous researchers have suggested that the Janzen-Connell effects could only maintain very few species and therefore were relatively unimportant for the overall maintenance of tropical forest diversity.
Instead, Levi and his colleagues at the University of Florida, Oregon State and James Cook University in Australia say that this close relationship between trees and their natural enemies is the key to the diversity of rainforests. They found that if fungi, arthropods and other natural enemies produce even small areas around trees where a new tree of the same species can not establish, then the very high levels of tree diversity observed in rainforests can be maintained almost indefinitely.
"There is a seed shade around adult trees and some escape the curve and leave, allowing recruitment into other areas until the host's specific enemies settle in the new location," Levi said. "That's why it's extremely important to keep the biodiversity of birds and mammals in these forests, or recruitment will eventually decline – especially in overcrowded areas."
Levi is at Oregon State Department of Fisheries and Wildlife at the Faculty of Agrarian Sciences.
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