1996 National Technical
Report on Forest Health

Forest Health Issues

Air Pollution

Since the industrial revolution, air pollutants have shown the potential to cause minor to severe impacts on forest ecosystems (Innes 1993, Olson and others 1992, Smith 1981). Before the advent of pollution controls, uncontrolled smelters, power plants, and other pollutant-producing sources caused significant damage to portions of forest ecosystems. The extent of the damage was related to the size of the source. The damage severity was often very high near the source; when all vegetation was removed and soil was eroded down to parent material, that portion of the forest ecosystem essentially reverted back to a stage of primary succession. Today, the most significant threats to forest ecosystems come from the regional deposition of ozone, nitrogen, sulfur, and hydrogen ion.

Plant species (vascular and nonvascular) and soil analysis can be used to estimate the percentage of forest ecosystems affected by air pollutants. For example, ozone has caused visible foliar injury to vegetation (herbs, shrubs, and/or trees) in many forests in the West, Great Lakes States, and East. In California, ozone has reduced needle retention, tree growth, and resistance to bark beetles and increased susceptibility to drought (Miller 1992). The relationships between visible foliar injury and reductions in growth, increased susceptibility to pathogens, and reduced progeny are still being discovered.

The deposition of atmospheric nitrogen in a forest ecosystem, acting as a fertilizer, might be considered a positive impact if the purpose of that ecosystem is to produce wood volume. Nitrogen fertilization can also affect the abundance and distribution of species and reduce biodiversity by favoring nitrophilus species. In addition, nitrogen saturation of the soil can occur, with subsequent acidification of the soil and loss of important cations, and nitrogen compounds can cause acidification of precipitation with subsequent effects on foliar and soil chemistry.

Sulfur, like nitrogen, can cause acidification of precipitation and soil and loss of important cations. Sulfur compounds are also known to be highly and directly phytotoxic to vascular plants and, especially, to some lichen species. Excess hydrogen ions, or lowering of pH in precipitation and in soils, generally leach important cations from foliage and soils and mobilize other undesirable cations like aluminum.

The effects of air pollution on forest ecosystems are evaluated regionally in the FHM program in three ways: ozone bioindicator plant evaluation;5 lichen community analysis focusing on sulfur and nitrogen sensitive species;6 and soil chemistry analysis evaluating soil pH and exchangeable Ca, Mg, and K.7 Lewis and Conkling8 provide details of each method. Currently in the FHM program, the implementation of the ozone bioindicators in the East and Great Lakes States is well developed; implementation of lichen and soil chemistry indicators has been done only in limited areas in the Southeast and West.

Ozone bioindicator plants are vascular plant species (herbs, shrubs, or trees) that are highly sensitive to ozone air pollution and respond with distinct foliar injury symptoms that can be diagnosed under field conditions. Ozone bioindicator plants are evaluated on or near the FHM plots where the appropriate ozone-sensitive plant species are found. Plots are scored as positive or negative for ozone injury. The severity of ozone injury at each plot is also recorded (see footnote 5).

Lichen community field procedures include collecting macrophytic lichens from woody substrates on the FHM plots. The FHM crews are trained to collect a sample of each species found and record the relative abundance of each species. The actual species identification is done by lichen experts. In this report, the air quality scores from plots in the Southeast United States are discussed. The air quality scores are based on models developed from analysis of lichen communities along established sulfur and nitrogen air quality gradients (see footnote 6).

Soil chemistry focuses on key components of the soil that air pollutant deposition will probably affect first. As with lichens, trained FHM crews collect samples and make basic horizon depth measurements. The chemical analyses are performed by laboratories specializing in this field. This report presents the spatial patterns of mineral soil pH and total soil carbon from plots in the Southern Appalachian area of the Southeastern United States.

The spatial pattern of ozone injury in the Northeast, mid-Atlantic, and Great Lakes States indicates distinct areas where tropospheric ozone injures forest plant species (fig. 22). Plots in southern Vermont, Maryland, Pennsylvania, and parts of the Great Lakes States were positive for injury, whereas most of Maine, West Virginia, and Minnesota were negative for injury.

The spatial pattern of lichen air quality scores at plots in the Southeastern United States indicates air quality is generally better along the coastal areas of Georgia, South Carolina, and North Carolina than the mountainous areas of these States, as well as Tennessee and Virginia (fig.23). Air quality at most plots in Virginia was relatively poor throughout the State.

The spatial pattern of mineral soil pH in the Southern Appalachian region indicates no distinct patterns but does suggest that many plots have relatively low pH values (fig. 24). These values may be representative of normal, expected ranges dependent on local parent material, type of vegetation growing on the site, and other site factors.

Air pollution appears to affect forest vegetation to varying degrees, depending on local conditions. The spatial patterns of injury to forest vegetation from air pollutants generally reflect the distribution of deposition of relevant air pollutants in these regions (Shadwick and Smith 1994, Southern Appalachian Man and the Biosphere 1996a). Even though elevated ozone concentrations might occur over large regions in the Eastern United States, injury to vegetation appears to be more localized in areas where higher ozone concentrations occur at sites with environmental factors, such as soil moisture and relative humidity, conducive to the uptake of ozone.

5 Smith, G.C.; Brantley, E. Bioindicator plants. In: Lewis, T.E.; Conkling, B.L., eds. Forest health monitoring indicators of forest health: indicator development research. Chapter 5. Manuscript in preparation.

6 McCune, B.M.; Dey, J.; Peck, J. [and others]. Lichen communities. In: Lewis, T.E.; Conkling, B.L., eds. Forest health monitoring indicators of forest health: indicator development research. Chapter 11. Manuscript in preparation.

7 Hudson, B.D.; Van Remortel, R.D. Soil morphology and chemistry. In: Lewis, T.E.; Conkling, B.L., eds. Forest health monitoring indicators of forest health: indicator development research. Chapter 15. Manuscript in preparation.

8 Lewis, T.E.; Conkling, B.L., eds. Forest health monitoring indicators of forest health: indicator development research. Manuscript in preparation.


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