Sentinel last week. We also have met Scott. He showed us around the greenhouse where he is doing some of his research. The photo shows Scott describing his current research to us in the STRI greenhouses at Gamboa.
UWM tree study
sheds light on forest diversity
Researchers draw parallels between communities and growth
Trees do not make good neighbors, at least to their own kind.
That so-called negative feedback, where young seedlings tend to die if an adult tree of the same species is nearby, is one way forests maintain diversity. Now, a group of researchers, including two scientists from the University of Wisconsin-Milwaukee, say they know how it happens.
Their results, published online Friday in the journal Nature, show that abundant tree species are less susceptible to disease inflicted by soil-borne organisms.
"People have approached this question and assumed that the major enemies are above ground, such as rodents or insects," said Scott Mangan, a post-doctoral researcher at UWM and the Smithsonian Tropical Research Institute and the lead author of the study. "Our paper suggests we have to consider what is below ground as well."
Working on Barro Colorado Island in the Panama Canal, the researchers selected six tree species found in forests there for a greenhouse experiment. They planted seedlings of each species in pots containing soil collected near adult trees of those same species - each species was planted in the six different soil types. They then measured the effects the soil had on the growth of seedlings from the various species.
"We wanted to test how seedlings perform in the presence of their own soil communities as opposed to other species' soil communities," Mangan said. "The worse a species does when grown with their own soil, the stronger the negative feedback."
Previous studies have suggested that predators of a specific tree grow in numbers as that species becomes more abundant in a forest. If that were true, the researchers would expect that seedlings of more abundant species show more negative feedback.
Instead, they found the opposite was true: More abundant species were less harmed by the soil communities. In other words, the soil communities are the cause of the diversity, not an effect.
"The species that are hit hardest by these (soil) organisms are always going to be lowest in abundance," said Stefan Schnitzer, a UWM professor of biology and co-author of the study.
The protective environment of a greenhouse does not perfectly mimic a forest, however, so the researchers needed to determine if the soil near adult trees in a forest has the same effect on seedling growth. They grew seedlings much like they had done in the greenhouse, only now they planted seeds in soil near adult trees in an established forest at Gigante Peninsula in mainland Panama, where rodents and insects could get to the seedlings.
"If it were insects driving this pattern, we'd expect leaf damage to be higher in offspring of the same species," Mangan said. Instead, they saw no pattern of leaf damage between the species, which meant the negative feedback they were seeing in the forest was also due to organisms in the soil.
Implicating soil-borne organisms in plant disease is not a new idea. Farmers, for example, rotate what they grow in a particular field from season to season to prevent crop-specific soil pests.
This study shows for the first time that the soil organisms are targeting seedlings in tropical forests in a way that helps to maintain the diversity of the forests.
"One of the great mysteries of lowland tropical rainforests is their extraordinary richness in tree species," said Stephen Hubbell, a professor of ecology and evolutionary biology at UCLA who was not involved in the research. "This is a marvelous, well-designed study that makes a major contribution to our understanding of how tropical forests work."
Mangan said the next step will be to identify the soil organisms that specifically affect each species.
"If we can identify the mechanisms and important organisms that are needed to maintain a diverse forest, that goes a long way when we are trying to, say, reforest a cleared area," he said.