BLOOMINGTON, Ind. -- Microbes in the soil beneath a parent tree may kill most of the tree's seedlings in that particular area. This would clear the way for unaffected seeds of other species to take root and flourish near that tree, thereby promoting diversity of trees in forests, according to a study published in the March 16 issue of the journal Nature by Indiana University biologists Alissa Packer and Keith Clay.
The study is described in an article on Nature's Web site at http://helix.nature.com/nsu/000316/000316-12.html
"We showed that a soil pathogen causes the patterns of seedling mortality that we observed in a temperate tree, the black cherry -- high mortality close to the parent tree and low mortality farther away," said Clay, professor of biology. "Animal predators and herbivores may be less important than microbial pathogens in the soil in creating the diversity of tree species in temperate forests. Our results provide the most complete evidence that native pathogens influence tree distributions."
This work has both national and international significance because it extends to temperate forests one of the best-known ideas in ecology, the Janzen-Connell hypothesis. According to this hypothesis, the diversity of tropical forests is enhanced by the presence of host-specific natural enemies that kill offspring around parental trees, creating opportunities for other species to become established.
"While this hypothesis has often been tested in tropical forests, with some supporting results, our study demonstrates for the first time that the same process can occur in temperate forests," Clay said.
Black cherry (Prunus serotina) produces large numbers of bird-distributed fruits throughout the forests of eastern North America. Preliminary studies showed that black cherry seedlings experience high mortality in soil collected beneath black cherry adults, but low mortality in soil collected beneath adults of other species.
Distance from adult trees had a greater effect on seedling survival than density of seedlings, the more traditional factor. Survival of black cherry seedlings beneath adult trees is 35 percent less than survival farther away.
Soil pathogens can increase rapidly in the presence of their host, causing conditions unfavorable for nearby growth of the host's seeds. So interaction between plant and soil communities could be an effective way to promote species coexistence.
The microbe Pythium spp., which Packer and Clay isolated from roots of dying black cherry seedlings and injected into healthy seedlings, decreased the seedling survival rate by 65 percent. This is the soil organism that makes it so difficult for black cherry seedlings to survive within a few meters of their parent tree, but it has no effect on most other common tree species.
For example, when the IU scientists counted the saplings beneath three black cherry trees that had high densities of black cherry seedlings, they found only four black cherry saplings over half a meter high within 10 meters.
"In contrast, we found 41 saplings of other species within 10 meters, including multiple individuals of beech, sugar maple, dogwood and ash. Seedlings of these species were sparse relative to the black cherry seedlings, suggesting that the survival rate of these species' seedlings is high beneath black cherry adults, but not the survival rate of black cherry seedlings," said Packer, a doctoral student.
To test the effect of the soil pathogen, the IU scientists sterilized soil they had collected beneath parent black cherry trees, and then planted seedlings from those trees in the sterilized soil and placed them in a greenhouse.
The seedlings' survival rates improved greatly in the sterilized soil compared with seedlings planted in unsterilized soil from the same locations. Sterilization of soil distant from the parent tree did not affect seedling survival in the same conditions, demonstrating that a biological agent is killing black cherry seedlings close to adult black cherry trees, Packer said.
An important goal for future research is to determine how frequently this mechanism operates in temperate compared with tropical forests, and the relative importance of animal predators and herbivores versus microbial pathogens for maintaining diversity in forests, Clay said.
Packer can be reached at 812-855-1674 or apacker@indiana.edu and Clay can be reached at 812-855-8158 or clay@indiana.edu
Journal
Nature