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Mixed Forest May Be Result of Disease

Mixed forests where there are numerous tree species in the same area are usually thought of as being a healthier situation than a forest consisting almost entirely of one tree species. The theory is that a mixed forest is better able to withstand attacks by insects or disease that may target one particular species.

A recent study by two researchers from Indiana University suggests that while a mixed forest may be less vulnerable to a major, widespread attack by disease or insects, it is in fact organisms that attack specific types of trees that play a significant role in creating mixed forests. Their study suggests that a tree will attract diseases that attack that species, making the area immediately around it less desirable for seeds that they produce.

"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 Keith Clay, a professor of biology. The researchers found that the mature tree that attracts the disease can usually withstand the attack, but not so any young trees of the same species trying to grow nearby. Since the soil pathogen is species specific, however, other types of trees will find the area more hospitable.

Black cherry trees produce large numbers of bird-distributed fruits throughout the forests of eastern North America. Preliminary studies showed that black cherry seedlings placed in soil collected beneath black cherry trees experience high mortality, but low mortality when planted in soil collected beneath adults of other species.

Distance from adult trees had a greater effect on black cherry seedling survival than density of seedlings, a factor frequently looked at when studying tree distribution. 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.

When the researchers 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, they found 41 saplings of other species within 10 meters, including multiple individuals of beech, sugar maple, dogwood and ash. The total number of seedlings of these other species in the area was small relative to the number of black cherry seedlings, suggesting that the survival rate of these species' seedlings is high beneath black cherry adults compared to the survival rate of young black cherry trees.

To test the effect of the soil pathogen, the scientists sterilized soil they had collected beneath mature black cherry trees, and then planted seedlings from those trees in the sterilized soil which then placed 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 collected at a distance from the parent tree did not affect seedling survival, demonstrating that a biological agent is killing black cherry seedlings close to adult black cherry trees.