ort new growth,” Fraterrigo said.
Fraterrigo explained how the balance of nutrients operates in the environment. “Nitrogen and phosphorus are among the most important elements for growth and carbon storage,” she said. “Plants fix carbon in the atmosphere, but if they don’t have enough of either of these elements, they’re limited as to how much carbon they can actually fix. It is the relative, not the absolute, amounts of nitrogen and phosphorus that limit growth and carbon storage.”
Although this would seem to be beneficial, at least for the hardwood industry, Fraterrigo said it’s important to look at the entire ecosystem and the ramifications of losing a species such as hemlock. Fraterrigo said hemlock is significant ecologically. “It’s a foundation species in this ecosystem. It provides structure because it’s an evergreen so wildlife depends on it year round for shelter. It also influences many biophysical processes, including those that affect ecosystem hydrology. Losing a species such as hemlock that is biologically active all year can alter stream flow, which could affect aquatic organisms,” she said.
Fraterrigo said that disturbances created by exotic pathogens and pests such as the hemlock woolly adelgid are increasing. “An introduced fungus is decimating oak populations in the West, and there’s the emerald ash borer in the Midwest. We need to study how the loss of tree species is affecting forest ecosystems,” she said.
The only places that hemlock stands can still be found in the Southeast are where an insecticide called Imidacloprid has been sprayed, Fraterrigo said. “But that’s just a temporary solution. You’d have to continue to apply it again and again in order to deter the hemlock woolly adelgid.”
The hemlock woolly adelgid is host specific, meaning it only infests hemlock trees. The aphid-like insect attaches itself to a needle, sucks the sap from it, and the tree dies.
“Although the hemlock woolly adelgid doesn’t do well in cooler climates, it is clearly affecting hemlock populations in the Northeast as well. It’s just taking longer to see the impact,” Fraterrigo said. “We’re seeing warmer temperatures at night across the nation and warmer winter temperatures in some places. Those two factors together could allow the insect to move slowly farther north.
“It’s difficult to anticipate how species loss will affect forest ecosystems,” she said. “Our research demonstrates that it is important to consider other drivers of global change, such as air pollution, to reveal ecosystem-level changes.”
Fraterrigo said she’d like to continue the work in the Great Smoky Mountain National Park, which has also lost hemlock and has even higher rates of atmospheric nitrogen deposition. “We’re curious if we’ll see similar changes in hardwood productivity and ecosystem nitrogen retention,” she said.
Interactive effects of disturbance and nitrogen availability on phosphorus dynamics of southern Appalachian forests was published in a 2012 issue of Biogeochemistry. Impacts of hemlock loss on nitrogen retention vary with soil nitrogen availability in the southern Appalachian Mountains was published in a 2012 issue of Ecosystems. Other authors were Corinne Block, Jennifer Knoepp, and Katherine Elliott.
Partial funding was provided by federal Hatch, the USDA Forest Service, and the National Science Foundation Long-Term Ecological Research Program.
View the entire article at the ACES News site.