Effects of Acid Rain on the Appalachian Trail

Results of the Appalachian Trail MegaTransect Study

Setting

The Appalachian National Scenic Trail (AT) is among the longest footpaths in the world, spanning ~2,180 miles (3,500 km) from Georgia to Maine. Over its length, the trail passes through a corridor with wide variations in climate, bedrock type, soils, and stream water quality. These factors create a diverse range of ecosystems.

A sugar maple forest at 4200 ft. elevation in Great Smoky Mountains National Park, TN (left), and a fully mature balsam fir forest on Mt Washington, NH, at the same elevation (right), illustrating the ecosystem diversity of the Appalachian Trail.

The health of these ecosystems is a cause for

concern because the AT passes through the heavily populated eastern U.S., which has many sources of sulfur (S) and nitrogen (N) air emissions. These emissions, produced by fossil fuel combustion, vehicles, and agricultural practices, produce acidic deposition, often called acid rain.

Haze produced by air pollution that causes acid rain. Viewed from Clingmans Dome, Great Smoky Mountains National Park, TN.

The AT is particularly vulnerable because it passes along ridgetops that commonly receive higher levels of acid rain than downslope terrain. Furthermore, these ridges typically have shallow, naturally acidic soils that are underlain by bedrock with low acid-buffering capacity. These characteristics limit the ability of the AT corridor to buffer acidic deposition.

Acid rain can acidify soils and water, leach nutrient base cations from soils, and mobilize potentially toxic inorganic aluminum (Ali) from soils to drainage water. Calcium (Ca) depletion from the soil is of particular concern because Ca is important as both an acid-neutralizer and an essential nutrient for plants. These chemical changes have caused adverse impacts on aquatic life, forest vegetation and soil fauna at numerous locations along the AT corridor.

A waterfall near Crawford Notch, NH, that exposes granitic bedrock with low acid-buffering capacity. This type of landscape is highly susceptible to acid rain.

Assessing Acid Rain Effects To address concerns about the health of the AT, a

study was designed to evaluate the condition and sensitivity of the AT corridor with respect to acidic deposition. A three-tiered sampling approach was designed to collect environmental data within a corridor 20 km-wide on either side of the AT.

Tier Sites Measurements Level 1 12 S and N deposition (4 sites)

Soil chemistry Stream chemistry Plant communities Tree biochemistry

Level 2 48 Soil chemistry Stream chemistry Plant communities (18 sites)

Level 3 205 Stream chemistry

Level 1 and 2 sample site locations.

To improve the coverage of existing acidic deposition monitoring along the AT, deposition monitoring was conducted at 4 sites during this project. These data strengthened models that quantified the spatial patterns of acidic deposition.

Acidic deposition was measured with throughfall collectors that capture pollutants in rain plus dry-deposited pollutants (including particles) that are washed off leaves.

This assessment also developed critical loads (CL)1, target loads (TL)2, and exceedances3 for the AT from relationships between site-specific model results and regional geomorphic features. This was accomplished using available soil, stream, climatic, hydrologic, vegetative, and topographic data.

The specific objectives of this study were to:

1. Model S and N deposition across the entire length of the AT, accounting for differences in topography and vegetation. Due to the terrain complexity and location of emissions sources, atmospheric deposition is spatially variable.

2. Measure soil and low-order (small) stream chemistry along the AT corridor. Soil chemistry is needed to inform understanding of plant responses to soil acidification and nutrient availability, and to aid in ecological effects modeling. Low-order streams were selected for study because they closely reflect the acid buffering capacity and nutrient status of soils.

3. Determine vegetation species composition and health at selected sites. The AT includes a vast diversity of plant species that are likely to differ widely in their sensitivities to acid and nutrient additions.

4. Map soil and water acid-base condition, which change across fine-scale gradients in elevation, soils, and geology and across the large-scale climatic gradients throughout the length of the AT.

5. Develop, analyze and map acidification critical loads, target loads, and exceedances. Relationships between regional and site-specific data were used to develop spatial models for the AT.

6. Use CLs, TLs and exceedances of S and N deposition to identify thresholds of air pollutant deposition to protect key ecosystem elements from harm. This information is useful for projecting future effects of acidic deposition.

1 The critical load (CL) is the level of acidic deposition below which damage to ecological resources is not expected to occur under long-term steady-state conditions. 2 The target load (TL) represents a CL calculated for a particular point in time, for example the year 2100. 3 Exceedance represents the extent to which the TL or CL is exceeded by existing levels of acidic deposition.

Findings Atmospheric Deposition of Sulfur and Nitrogen

Spatial modeling of S and N deposition revealed high variability over the length of the trail, with highest levels in the southernmost part of the AT and lowest levels in the northernmost part. Spatial patterns of S and N deposition were very similar.

Estimated atmospheric S deposition.

Soils

Concentrations of Ca in soils over the length of the AT ranged widely and did not show consistent patterns. Areas with high soil Ca often occurred in close proximity to areas with low soil Ca. However, soils with the lowest concentrations of Ca were generally located in areas having the highest levels of acidic deposition. This suggests that acidic deposition may have increased Ca leaching from soils in these areas. Soils with the highest concentrations of Ca were generally in areas with relatively low levels of acidic deposition. Approximately 70% of the Level 1 and Level 2 sites had values of base saturation4 under 20%, the threshold below which acid neutralization is insufficient to prevent the risk of Ali being released

4 Base saturation is an index of soil acidity determined by the sum of Ca and other base cations as a percentage of all acid and base cations.

within soil by acid inputs in forms that can be harmful to terrestrial and aquatic life.

Streams Mean and median acid-neutralizing capacity

(ANC) were lowest in the streams in the North section, intermediate in the Central section and highest the South section, despite the South having the highest acid rain levels. Chronic acidification, defined as ANC ≤ 0 µeq/L, was most common in the North section (16.3% of streams) less common in the Central section (11.7% of streams) and minimal in the South section (1.4% of streams). These results demonstrate the importance of acid-buffering in protecting against acidic deposition effects, and also indicate the greater vulnerability of the North section to acid rain impacts. Summary of stream ANC values for the North, Central, and South sections of the AT.

Description North Central South Sites 98 94 74 Mean (µeq/L) 50.5 64.2 71.2 Median (µeq/L) 19.1 33.3 49.3 Std. Dev. (µeq/L) 67.1 76.9 71.9 < 0 µeq/L (%) 16.3 11.7 1.4 0 – 50 µeq/L (%) 50.0 51.1 50.0 Results also showed that over the full length of the

trail, at least 40% of the study streams exhibited pH and/or Ali measurements that indicated potential harm to biota. Comparison of data from low and high flow sampling indicated that in nearly all streams, ANC decreased with increasing flow by an average of 30-50 µeq/L, a difference that is likely to have ecological significance for streams that have baseflow ANC values between about 25 and 75 µeq/L.

Potentially toxic levels of Ali in stream water were observed in watersheds where soil base saturation was

Stream in the Green Mountains, VT at high flow.

approximately 12% or less. This occurred at 60% of the Level 2 sampling sites.

Plant Communities

Tree mortality was weakly related (p < 0.10) to concentrations of potentially harmful Al in the soil. Mortality may have been increased by acidic deposition, but precipitation amount was more strongly related to mortality. Precipitation amount may be a factor that increases vulnerability to acid rain because it increases losses of soil Ca through leaching.

Tree mortality also decreased with increasing N deposition. This suggests that N may provide a fertilizing effect.

A strong positive relationship (p < 0.05) between atmospheric deposition and the compositional similarity of understory and canopy species was observed. This suggests that over the past several decades of acid rain, species that are poorly adapted to acid rain may have been eliminated. Analysis of plant metabolites in wood at Level 1 sites showed that sites with high availability of soil Ca and high levels of N deposition had the healthiest sugar maple, red spruce and oak trees.

Maiden hair fern (Adiantum spp.), a plant that grows only in high-calcium soils, was abundant at the Piney River, VA Level 1 site, which also had the healthiest oak trees.

Critical and Target Loads Modeling Target loads of S, based on thresholds that are

generally believed to put ecosystems at risk of harm (ANC = 50 µeq/L and base saturation = 12%), were projected to be exceeded in 2100 in some watersheds throughout the length of the AT. This occurred despite simulated 21st century decreases in acidic deposition. Watersheds projected to be in exceedance were often in close proximity to watersheds with S deposition levels below exceedance.

Model simulations also suggested that by 2100, reduced acidic deposition would result in higher ANC

Exceedance of sulfur target loads to achieve stream ANC = 50 µeq/L by the year 2100, extrapolated to all stream locations at which stream water chemistry data were available.

in northern streams but that sulfate adsorption that occurs in southern, unglaciated soils would delay ANC recovery in that region. In some southern streams, ANC was projected to remain stable or to continue to decrease from the present to 2100 despite lower levels of acidic deposition due to the dynamics of sulfate adsorption and base cation depletion.

Conclusion

Acidic deposition effects on soils and streams were documented in this study over the full length of the AT, despite high variability in acid rain levels. Areas well-buffered from acid rain effects were also spread throughout the length of the AT, but were less common than impacted areas. Vulnerability to further deleterious effects of acid rain is also distributed over the full length of the AT. It will be important to continue to track ecosystem health along the AT corridor as pollutant emissions and deposition continue to change.

More Information To learn more about the AT MegaTransect Study, see the full report on the National Park Service website: https://irma.nps.gov/App/Reference/Profile/2223220.