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Western-montane forest lands have some of the cleanest air in North America, yet there are concerns about increasing levels of regionally dispersed ozone and acid deposition in alpine zones. Although SO
2 emissions are declining in much of the West, NO emissions are stable or increasing.
Historically, the
air pollution damage has been caused by point sources associated with mining, smelting, and power generation. Such sources have caused extensive damage to local forests and forest soils.
The western-montane forest region is defined here as forested land in the Western United States located east of the Cascade and Sierra Crests, and west of the Continental Divide, but including the Lewis and Gallatin
Ranges in Montana and the Absaroka Range in Montana and Wyoming. This region is characterized as having some of the cleanest air in North America, although there is a history oflocalized air pollution damage to western forest stands resulting from metal smelting, refining, and electric power generation. The region is also periodically subject to pollution from large air masses from areas such as the greater Los Angeles Basin. Pollutants of concern include regionally dispersed ozone (03) and acid precursors (oxides of sulfur and nitrogen), and local sources of acid precursors, fluoride (F), and heavy metals arising from metal smelting and fossil fuel burning or processing. Some of these pollutants directly affect forest vegetation (03' F, acids). Metal pollutants are stored in soil and damage is mediated by soil biological and chemical processes. Acid deposition that reaches the forest floor is also processed by a variety of soil reactions.
Ozone is the only regionally dispersed pollutant that has been demonstrated to injure foliage and cause forest decline or mortality in the West (Bohm 1989; Heck and others 1986), although I found no published reports of injuries to plants in the montane west. There is a perception by "experts" (18 to 35 North American scientists active in the field of forestry and air pollution) that volume yields of western conifers outside the Los Angeles airshed
Paper presented at the Symposium on Management and Productivity of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
James L. Clayton is Principal Research Soil Scientist, Intermountain
Research Station, Forest Service, U.S. Department of Agriculture, Boise, ID
83702. have probably declined slightly in the 1980's, and will continue to do so should current levels remain unchanged
(de Steiguer and others 1990).
Adverse effects due to 03 can start at ambient concentrations of 60 ppb. Bountiful, UT, has experienced average hourly 03 concentrations in excess of 120 ppb during at least 6 of the 12 months (summer concentrations are usually higher), and the Lake Tahoe Basin has 12-month average concentrations exceeding 80 ppb (Bohm 1989). Ozone is at near-damaging levels virtually everywhere in the
United States, but probably less so in the montane west
(Fox 1990). Ponderosa and Jeffrey pines (Pinus ponderosa;
Pinus jeffreyi) exhibit visual symptoms, reduced growth, and general decline in the San Bernardino Mountains, CA.
Ozone damages chloroplasts, destroys chlorophyll, and reduces tree resistance to pests and pathogens.
Acid precursors (NQI[ and SQ.) have not been detected in damaging concentrations in regional air masses in the montane west, although there are demonstrated vegetation declines downwind of point sources originating from ore smelting operations (Carlson 1990) and pulp and paper mill operations (Carlson and others 1974). Chemical concentrations of SOx and NOx in the west show a spatial pattern that is highly correlated with proximity to pollutant sources (fossil fuel refineries, pulp and paper mills, power plants for S, urban and industrial centers for N).
S02 emissions in the West for 1985 were estimated at
10 percent of the total U.S. emissions (Placet and Streets
1987). Major point sources are located in most westernmontane States; the metal processing industry and electricity generation are the major polluters (Bohm in press).
S02 emissions have generally decreased in the montane west over the last decade, although they increased by 10 percent in Wyoming and 25 percent in Idaho over this period (Bohm 1989).
NOx emissions result primarily from the combustion offossil fuels; consequently, emissions are largest around large cities and metropolitan areas. Emissions of NO x remained fairly constant in the West in the 1980's, although
States with new power plants (Arizona, Colorado, Montana,
Nevada, and Wyoming) showed increases (Bohm in press).
A gradual increase after 1990 is projected for the West as a whole (Young and others 1988).
There are numerous hypotheses to explain the role of acid deposition in forest decline in North America and Europe.
These hypotheses include nutrient loss from soil (especially
Mg, K, and Ca), mobilization of toxic elements (Al), and increased susceptibility to disease or insect attacks. The concept of "forest decline" is a category of diseases for which no
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single cause can be assigned. It is probable that forest damage from acid deposition generally results from multiple causes including changes in soil chemistry.
Air pollution resulting from prescribed and wild fires is often discussed and publicized in local news media, particularly in the last few years because of the long fire seasons and large fires. Smoke from fires reduces visibility, increases complaints of respiratory ailments, and smells bad. The pollutant of concern is particulates emitted during the combustion process, and there is no evidence (or reason to expect) that particulates harm the forest ecosystem.
Fire effects are direct, on-site effects associated with damage to vegetation, loss of soil cover, and accelerated erosion, and are discussed elsewhere in these proceedings
(see W. F. Megahan; L. F. DeBano). Clayton (1976) documented measurable down-wind transport of nutrients during a large wildfire in central Idaho, but this off-site effect, although measurable, was a small percentage of the annual nutrient requirement of the site. He concluded that redistribution of nutrients by smoke is ecologically unimportant.
Toxic metal air pollution is a common, local problem associated with smelting operations. With the exception of lead pollution from gasoline-powered vehicles, metal pollution is from stationary sources, and direct effects are typically within 10 km of the source. Lead is the only metal that currently has a national ambient air quality standard.
There has been a dramatic nationwide improvement in ambient lead concentration from >lllg/ma in the mid-
1970's to <0.21lg/ma in the mid-1980's (Council on Environmental Quality 1989). This improvement is a direct result of increasing use of unleaded gasoline and control equi pment installed in stationary source stacks. Studies from the northeastern United States have not found that concentrations of metals in forest soils are correlated with regional patterns of deposition except near point sources
(Friedland and others 1986); however, lead and cadmium concentrations in north-central U.S. forest floors are related to regional deposition patterns (Grigal and Ohmann
1989). Similar studies have not been conducted in the montane west, but the generally cleaner regional air masses and strong topographic effects on air movement make such associations unlikely. Litter decomposition as measured by microbial CO2 evolution is repressed by some heavy metals
(notably Pb and Zn), but not Cu, according to Moloney and others (1983). High levels of heavy metals in soils around smelters have been shown to decrease populations of soil microorganisms. Friedland and others (1986) suggested that metal concentrations ranging from 100 to 1,000 mglkg are required to measurably decrease decomposition rates in the northeastern United States.
Mobility and bio-availability of metal cations following deposition generally decrease with time, increasing soil pH, and increasing clay content. Time allows for oxidation of metals, which decreases solubility, and time also allows for ion diffusion to strong sorptive sites or formation of relatively insoluble secondary mineral phases (Bohn and others 1979). The type of sorptive site controls how energetically, and, therefore, how persistently immobile an adsorbed cation will be. On inorganic exchange sites, competitive adsorption indicates that the relative replacing power corresponds to the order of increasing pH of the first hydrolysis product of the metal (for example, Pb2+ +
H20
=
PbOH+ + H+) under acid conditions, but this is not the case on organic exchange sites. Elliot and others (1986) found that competitive adsorption followed the sequence
Pb>Cu>Zn>Cd for mineral soils where organic carbon was
<2 glkg, but Pb>Cu>Cd>Zn for organic soils where organic carbon was >20 glkg. Their results suggest that increasing soil organic matter will restrict the mobility of Cd and Cu.
Metal smelting usually results in increased air pollutant loads of nonmetals such as S02' acids, and fluoride (as HF or F- salts). These accompanying pollutants are less persistent in soils, but often more directly damaging to vegetation on the site.
Northern Idaho and western Montana have a long and rich history of mining and smelting. For example, the
Coeur d'Alene region of northern Idaho, which includes
10 mining districts, has yielded roughly 80 percent of the total value of all metals produced in Idaho since mining began in the State (about 1852). Exploitation of the great lead and silver lodes began in the 1880's, and the area is now as well known for the extensive pollution problems associated with the smelting as for the tremendous value of the processed ore. For example, the Bunker Hill lead and zinc smelters near Kellogg, ID, closed in 1982 after
70 years of continuous operation, are now a 54-km2 Superfund site. The EPA has estimated that more than $100 million, expended over the next 50 years, will be required for cleanup. High emissions of S02 eradicated vegetation in the vicinity of smelters, and soils contain high levels of Pb, As, Cd, and Zn. Soil pH in the nonvegetated areas averages 3.1.
In 1955 an Al reduction plant was opened by Anaconda in Columbia Falls, MT. The process involves a high temperature, electrolytic reduction of Al
2
0 a to elemental Al, and results in particulate NaF and AlFa waste, plus gaseous HF. In 1970, daily emissions exceeded 3,500 kg ofF, but these were reduced to less than 1,000 kg/day by mid-
1971. Carlson and Dewey (1971) estimated that elevated
F levels in vegetation could be found on over 80,000 ha of forested lands downwind of the plant. They also documented over 28,000 ha of forested land with visible injury to plants. Most F damage to vegetation is direct, and visual symptoms on conifers include needle necrosis and terminal bud dieback. Carlson and Dewey (1971) found a high incidence of injury on plants with >100 ppm F in needle tissue, and most plants with >30 ppm showed some degree of needle burn. They presented no data on soil F content. Normal concentrations ofF in soils range up to a few hundred ppm. However, plant uptake of F is not usually related to F concentration in soil, but to other factors that affect availability, such as soil.pH and calcium and phosphorus content of soil (Adriano and Doner 1982).
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The damage resulting from copper smelting near
Anaconda, MT, is even more extensive. The first copper smelter at Anaconda was built in 1884, and by 1914 was processing nearly 10,000 metric tons of ore daily (Carlson
1974). Accompanying this ore reduction were dailyemissions of S02 estimated at greater than 2,500 tons! This caused extensive damage to the surrounding forest. Carlson
(1974) cited historical evidence suggesting that nearly all conifers within 15 km of the smelter were dead by 1915.
Since 1900 several abatement devices have been installed to reduce emissions, but the smelter was still emitting more than 1,000 metric tons of S02 per day in the early 1970's.
The smelting operations were shut down in 1982. In 1989 the EPA claimed that Anaconda was its largest and most perplexing Superfund site.
In 1972 Carlson (1974) measured S02 and heavy metal deposition at soil plots located 7 to 18 km from the smelter.
Plots located 7 km downwind of the smelter averaged 7 Jlg/ cm2/day ofS02 deposition. Soil pH in the top 10 cm of mineral soil from these plots averaged 4.4 compared to a normal pH of 5.3 to 6.5. Lead content ranged from 50 to 60 ppm, and Zn content ranged from 130 to 200 ppm in the forest floor and top 10 cm of mineral soil. Carlson (1974) described "moderate" damage to live limber pine (Pinus
flexilis), and he reported that prior vegetation damage resulted in sufficient mortality and ground cover loss to cause excessive erosion.
Walsh and Bissel (1979) investigated the effects of S02 and heavy metal emissions from the Anaconda Copper
Smelter on two subalpine plant communities (Abies
lasiocarpalVaccinium scoparium and Abies lasiocarpa-
Pinus albicaulis/Vaccinium scoparium habitat types).
They found dramatic changes in plant community composition and reduction in cover, which they attributed to air pollution. Overs tory crown cover decreased from a mean value of 66 percent on distant control sites to less than 20 percent within 13 km of the smelter; shrub crown cover dropped from 50 to 66 percent at remote sites to 3 to 6 percent at 13 km. On the basis of pedestaled bunch grass, Walsh and Bissel (1979) estimated that accelerated erosion due to loss of ground cover had removed 15 to 30 cm of soil from one site located 11.5 km from the smelter.
At this same site the authors found an absence of decomposed litter (Oa horizon), but 8- to 12-cm-deep layers of undecomposed needles (Oi horizon). They hypothesized that decomposer organisms may not be functioning properly due to air poll utants.
Alpine and subalpine areas of the Western United States exhibit many characteristics that suggest that their aquatic ecosystems are vulnerable to acid deposition. These characteristics include: (1) shallow, coarse-textured soils, (2) a high percentage of the land area in rock outcrop, (3) soils with a low base saturation, (4) soils formed from rocks that contain minerals that weather slowly, and (5) rapid delivery of water to streams and lakes during summer snowmelt.
The higher elevation ecosystems of the Wind River Mountains in western Wyoming have all of these characteristics.
The Wind River Mountains are also located 50 to 200 km downwind ofS02 and H2S0
4 sources that originate from coal-fueled electric power generation and natural gas fields that contain considerable H2S. Clayton and others (in press a, b) described the soil distribution and characterized soil properties related to acid neutralization in the southern end of the range. Soils are formed from Precambrian acid igneous rocks, and are coarse-textured and not well developed (Inceptisols). Two soils, a Humic Cryaquept and a Dystric Cryochrept, plus large areas of rock outcrop, talus, and scree comprise the surface deposits in the basin.
The Aquept is located along stream courses and lake margins. This soil contains 10 to 20 percent (by weight) organic matter in the A horizon, and has a relatively high cation exchange capacity (12 to 25 cmol/kg) and base saturation (0.4 to 0.98). The Ochrept is an upland soil, found in patches under a sparse canopy of white bark pine (Pinus
albicaulis), Engelmann spruce (Picea engelmannii), and subalpine fir (Abies lasiocarpa). This soil has a lower exchange capacity and base saturation than the Aquept.
Both soils are capable of adsorbing SO
4 at ambient soil pH.
This is an important mechanism for neutralizing H
2
S0
4 deposition in older, well-developed soils such as Ultisols in southeastern United States.
Large, single-horizon soil columns were leached with
H2S0
4 in the laboratory to assess the capability of these soils to neutralize acid deposition (Clayton and others, in press b). The treatments were equivalent to 3 to 6 years of runoff and 50 to 100 times annual S deposition rates
(estimated at 3 to 6 kg/ha/yr). A complete proton budget was computed for each leaching test, and the relative H+ consumed by base exchange and mineral weathering, SO
4 adsorption, and Al(OH)a dissolution calculated.
Base cation exchange for protons was very effective at buffering all horizons during application of 12 to 16 pore volumes of pH 4 H
2
S0
,
Mter 3 to 5 pore volumes of pH 3
4 acid, leachate pH dropped below 5 in the Band C horizons of the Ochrept. Sulfate adsorption and Al(OH)a dissolution became important neutralizing processes in those two horizons when pH dropped, and dissolved Al reached a concentration of 130 JlmollL in the C horizon. Over the entire experiment, cation exchange and weathering were the dominant neutralization processes, accounting for 56 to
96 percent of total proton consumption. Sulfate adsorption was important in the Ochrept horizons, accounting for 20 to
35 percent of total proton consumption. Sulfate adsorption was not important in the A and C horizons of the Aquept.
This could be attributed to the abundance of organic matter, which inhibited SO
4 adsorption, or to the fact that pH did not decrease due to effective buffering by cation exchange.
Postleaching analysis of the soils indicated that adsorbed
Ca and K were approximately at the same value as unleached soil, although large amounts of Ca (up to 30 percent of exchangeable Ca) were detected in leachate. This suggests that hydrolysis of the abundant primary minerals in these youthful soils rapidly resupplies these cations under the conditions of the experiment. Clayton and others (in press b) concluded that the column leaching experiments indicated considerable capacity to neutralize acids in the soils studibd, although they expressed concern for areas where meltwaters are not routed through soil before reaching a stream or lake.

The western-montane forest area is characterized as having some of the cleanest air in North America, yet it clearly has localized disaster areas resulting from smallplume air pollutants. These point sources are associated with smelting and refining activities, and stricter controls are improving this situation.
Regionally dispersed 03 is increasing in the West, and it has the potential to damage forest ecosystems. Ozone's effect on trees is direct, and it is not processed by soils.
There are concerns about the impact of acids and acid precursors in the West, particularly in alpine ecosystems.
In general, S02 and H2SO" emissions are declining in the
West, but this is not the case with NO
• x
More study on the acid-neutralizing capacity of western alpine soils is needed, with particular emphasis on acid processing by rock and saprolite. We also know little about long-term time trends in soil properties and how soils might be affected by air pollutants. Johnson and others (1989) recommended soil archiving as a means of detecting gradual changes in soil properties as a result of human activities.
This activity would be of particular benefit in the montane west where we suspect there are remnants of unpolluted soils.
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