“Rain has become much less acidic since the Clean Air Act was strengthened in the 1990s,” says U.S. Forest Service Southern Research Station (SRS) soil scientist Jennifer Knoepp. “However, some high elevation streams still have chronic or episodic acidity.”
Acid rain, as well as other forms of acidic deposition such as acid fog and acid mist, can still occur at high elevations in the southern U.S. Additionally, the acid rain of decades past has left a chemical legacy in high elevation soils.
Acids can be stored in soils for long periods of time before leaching out and making their way to rivers and streams. The acids do not move alone, either. Because of their powerful negative electronic charge, acids attract positively charged molecules. These positively charged molecules are often critical to plant growth, and their loss can weaken trees and slow growth.
Land managers need tools for identifying which watersheds are still affected by past acidic deposition, as well as strategies for restoring such watersheds. Knoepp and her colleagues addressed both of these issues in a recent study that was published in the Journal of Forest Ecology and Management.
The scientists studied high elevation watersheds in three national forests in the southern Appalachians, all in North Carolina: the North River in Cherokee National Forest; Santeeetlah Creek in Nantahala National Forest; and the North Fork of the French Broad in Pisgah National Forest.
High elevation watersheds are especially susceptible to acid deposition from rain, fog and mist. Knoepp and her colleagues studied individual catchments within each watershed to represent a gradient in elevation as well as a range of acidic deposition histories.
High elevation watersheds in the southern Appalachians tend to have deep, rocky soils with more organic matter than their lower elevation counterparts. They also have different vegetation communities and get far more acidic deposition.
As long as streams are not overwhelmed by acidic deposition, they can naturally neutralize some acids. Knoepp and her colleagues wanted to identify measurements that would help managers rank streams by their ability to resist acidification, or their acid-neutralizing capacity. The scientists also wanted to identify catchment and soil metrics that managers could use to rank streams by their sensitivity to acidity.
Knoepp and her colleagues found that the height of trees and the thickness of their trunks, or their basal area, were indicators of watershed acidity. Acid rain triggers a cascade of chemical reactions in soils that can last for years, and scientists identified a number of belowground indicators, such as soil mineralogy, total carbon content, and the depth and chemistry of specific soil layers.
The scientists also estimated the amount of lime it would take to restore acidic watersheds. “The lime requirements we identified were within the broad range found in other studies,” says Knoepp. “However, the amount required varied by watershed, and the effectiveness would likely vary by watershed as well.”
For more information, email Jennifer Knoepp at firstname.lastname@example.org.