URBANA Due to the introduction of exotic pests and pathogens, tree species are being eliminated one by one from forest ecosystems. In some cases, scientists can observe immediately how their loss affects the environment, whereas in other cases, creative puzzle solving and analysis reveal unexpected repercussions. In the case of the loss of the hemlock tree, University of Illinois landscape and ecosystem ecologist Jennifer Fraterrigo uncovered a surprising benefit to hardwood species.
Throughout much of the eastern United States, a pest called the hemlock woolly adelgid has decimated hemlock tree populations. While researching how hemlock mortality affects nitrogen retention in the soil and vegetation, Fraterrigo noticed that other components of the ecosystem were changing.
"Our findings were unexpected," Fraterrigo said. "We hypothesized that in this area of the southern Appalachians, where there is a lot of nitrogen available due to high rates of atmospheric nitrogen deposition, hemlock mortality would increase nitrogen leaching from the soil because the trees were no longer taking up that nitrogen, but we found the opposite. We found less nitrogen leaching from the soil because hardwood trees had compensated by increasing their productivity."
The hardwood trees were able to grow because, when the hemlock trees died, phosphorus was released and became available to the hardwood species in the area. The increase in available phosphorus stimulated the growth of existing hardwood trees, which then increased tree demand for nitrogen. As a result, we saw less nitrogen being leached from the soil. Without hemlock mortality, the hardwood trees could not take up the excess nitrogen in the soil because their growth was limited by a lack of phosphorus, Fraterrigo said.
"We believe chronically high nitrogen availability is actually driving the accumulation of phosphorus in vegetation and soil organic matter in this area. Without
|Contact: Debra Levey Larson|
University of Illinois College of Agricultural, Consumer and Environmental Sciences