Some of the most pervasive and least understood impacts on forests result from human-induced changes in the Earth’s atmosphere, globally, regionally, and locally. Long-term consequences of these changes may affect forest biodiversity as severely as fragmentation and loss of habitat do.
The effects of the atmosphere and air quality on vegetation are both local and large scale. Plants growing in the city are the most obviously stressed. The urban environment creates a heat island that, when combined with reflected heat, glare, and desiccating winds, creates a high water demand on plants, whose roots may be under paving or compacted soil and, hence, experiencing drought. Even when uncompacted, urban soils often exhibit restricted levels of water infiltration that may be attributed to hydrophobic (water-repelling) materials, like motor oil, in the soil. High levels of grime and particulate matter clog breathing pores on leaves. Ozone, hydrocarbons, and airborne pollutants, such as lead and other heavy metals, damage leaves, roots, and tissues and may increase opportunities for parasites and pathogens, such as tree fungi, to invade plants.
Although atmospheric changes, such as pollution or ozone thinning, have been known for some time as causes of serious health problems in humans, public awareness came slowly and change even more so. Despite our awareness, fine particulates, tailpipe exhaust, coal-fired power, and other forms of combustion still kill more people than car accidents or guns each year (Schwartz 1994). And we have consistently underestimated air pollution’s pervasive impacts on indigenous landscapes. If we are slow to act when it involves our own lives, is it any surprise that we are slow to recognize the overall impacts of atmospheric changes on forest systems — impacts that are unfamiliar to most people and regulatory agencies? The scenario is not unlike that of invasive exotics, whose impacts were denied until the evidence was overwhelming and the damage so excessive that some claim it is now too late to take action.
About the turn of the century, signs of air pollution began to show up in tree rings, the result of the advent of coal- and oil-fired engines and the emergence of large-scale industry. The enormous increase of carbon dioxide, sulfur, nitrogen, and many other contaminants released into the atmosphere since then has been accompanied by several other changes that have had profound effects on the environment.
First, the proliferation of trucks and autos meant that pollution sources were no longer confined to fixed locations, such as factories, where the affected area could be readily distinguished from more pristine landscapes. As greater and greater amounts of traffic moved far and wide across the landscape, the impacts became pervasive. By the 1950s trees began to die noticeably faster than they had in the first half of the century. By 1969 there was one mile of road for every square mile of land in the United States. By the mid-1980s more land was disturbed by roads than by all mining activities. Today, no area that we would have previously referred to as pristine still remains.
Another basic change came with a shift in the by-products of combustion, especially oxides of nitrogen, the so-called NOx pollutants. The use of wood, coal, and oil as fuel always produced some oxides of nitrogen, but the higher-temperature combustion processes that developed in industry created a huge new source of NOx pollutants from the nitrogen in the air itself. Little progress has been made in reducing the amount of NOx pollutants, and they are still largely unregulated.
Unfortunately, we still know far too little about such problems. The National Acid Precipitation Assessment Program (NAPAP 1991), carried out between 1981 and 1990 to evaluate acid rain-related problems, actually served to obscure the real issues by its assertion that the forest was healthy (Likens 1992). By limiting studies primarily to sulfur-related acidity and ignoring nitrogen-related acidity and other problems (Aber 1993), NAPAP deflected public concern as well as research efforts, especially in this country, except by a few persistent investigators.
The NOx constituents of air pollution tend to become fine nitric acid droplets or aerosols that wash out of the atmosphere as acid rain or fall directly onto the landscape as dry deposition, both of which add to overall acid problems that originate from other sources, such as the huge amounts of sulfur oxides also generated in fuel combustion. This complex problem, referred to by various names, such as acid precipitation, acid fog, dry acid deposition, and occult deposition, is one of the more important ways that we have modified forest conditions. Like other acid precipitants and ozone, NOx contaminants are directly injurious to humans, other animals, and plants alike.
Excess acidity creates many problems, such as the leaching away of important mineral nutrients like phosphorus, calcium, magnesium, and potassium, which may become factors limiting growth. High acidity also releases toxic elements such as aluminum and heavy metals. Long-term studies by Likens, Driscoll, and Buso (1996) document the loss of over half the pool of calcium in the soil in the past thirty years at the U.S. Forest Service’s Hubbard Brook Experimental Forest in New Hampshire’s White Mountains. The trees have nearly stopped growing, and the stream waters are still very acid because the soils have lost the ability to buffer these impacts.
William Sharpe of Penn State University has concluded that depleted and leached soils have weakened root systems in the Allegheny National Forest and that liming, fertilizing, and fencing will be necessary to regenerate them (Reidel 1995). In the Netherlands, calcium became so scarce in some woodlands affected by acid deposition that snail populations sharply declined. Without this source of calcium in the diets of birds, consequent eggshell thinning lowered reproductive success (Graveland et al. 1994).
Another phenomenon associated with high levels of nitrogen is a reduced proportion of lignin, which is the woody tissue in leaves and litter (McNulty, Aber, and Boone 1991). The lignin component of forest litter tends to decompose more slowly than cellulose and other components, so a decline in the lignin ratio increases the rate of decomposition that is already accelerated by higher nitrogen, resulting in reduced surface litter and increased soil exposure to erosion.
Some protection from plant-feeding insects is also lost with reduced lignin levels because leaves become more succulent. This nitrogen-induced succulence, which in turn cycles nitrogen more rapidly back to the forest floor, increases feeding by gypsy moths and other exotic and native herbivores. Experimental nitrogen fertilization of hemlock stands affected by the hemlock woolly adelgid, an exotic pest that is currently devastating hemlocks over large areas of their range, hastens their death (McClure 1991). Similarly, tests simulating acid rain worsened the rate and severity of fungus infection on dogwoods. Air pollution from industry and vehicles is in fact fertilizing the forest by adding nitrates and thereby accelerating its decline.
Despite the problems created for hemlock and other species, some plants benefit from increased nitrogen and are proliferating rapidly. An alien plant with a name like mile-a-minute vine, which is also known as dog-strangling vine, is not nutrient starved; rather, many such ruderals (plants of disturbed landscapes) and weed species are documented to be “nitrophilous,” or nitrogen-loving. Scientists in Nancy, France, demonstrated a consistent pattern of greater eutrophication (that is, increased nutrient levels) at the edge of a small woodland (Thimonier and Dupouey, 1992; Thimonier, Dupouey, and Becker 1994) on the upwind side, where pollutant deposition is greater. This “edge effect” was measured in the vigor of nitrophilous and acidophilous species and increased nitrogen and acidity in the soils. In northern Europe, a species of hairgrass, Deschampsia flexuosa, is a bioindicator of nitrogen deposition, especially in heathlands, where its replacement of the typical heather shrub layers during this century has been notable (Steuben 1992). Recent studies in the southern Appalachians (Wilson and Shure 1993) suggest that Rubus, a genus that includes many brambles, briars, and berries, may be a good bioindicator of nitrogen enrichment in eastern forests. In general the ground-layer vegetation in a forest is a good indicator of changes in the nutrient balance in the ecosystem, exerting significant pressure on species composition over time (Eichhorn and Hutterman 1994).
Nitrogen deposition in Europe has increased the production of forage plants for roe deer and, therefore, the population of roe deer in forests (Ellenberg 1987). We tend to think of overpopulations of deer as caused by lack of predators and change, in forest age and structure, but part of the increase is the result of added nitrogen, causing increased succulence and abundance of some food sources.
This rain of fertilizer and other once-uncommon substances has changed the character of soils as much as the vegetation. Some of the primary impacts of the overenrichment process now under way are due to changes in soil biology, including its most fundamental aspects, such as total soil respiration. Changes in soil respiration affect the oxygen available to root systems and in turn the health and vigor of the forest.
One of the most important studies being conducted today, at the Institute for Ecological Studies of the Carey Arboretum in Millbrook, New York, is examining the types of changes to soil systems occurring in the most urban to more natural landscapes. Scientists are monitoring the similarities and differences of oak woodlands growing on similar soils on sites extending from Central Park in Manhattan to a distance of about 200 miles north of New York, where the institute is located. This ongoing study has already shown several important urban-to-rural gradients in and around the New York City area (McDonnell, Pickett, and Pouyat 1993). Heavy metals in the soil were higher in urban areas and declined outwards. Salt in the soil and soil waters trended in the same way. Urban soils tended to be more hydrophobic, that is, more resistant to wetting, as well.
Urban soils also had higher rates of mineralization of nitrogen, which is the conversion of organic forms of nitrogen, such as soil organic matter, to inorganic forms such as nitrate. The authors suggested that the principal changes brought about by nitrogen deposition may be an increase in the abundance of bacteria in the soil and litter combined with reductions in fungal and invertebrate populations. Urban forest-floor litter also decomposes more quickly than that of countryside forests. The authors further suggest that higher nitrogen deposition on the urban end of the gradient leads to the faster release of organic nitrogen from soil and litter that they observed.
Soil structure is affected by these changes, too, particularly by a shift from fungal dominance to bacterial dominance. The webby mycelia that comprise the bulk of soil’s fungal component serves to knit together soil particles and bits of organic matter, while the substances secreted by bacteria (exopolysaccharides) are slippery and cause soil to slump when it is exposed to rain (Harris, Birch, and Short 1993).
Unfortunately, conventional soil tests do not measure such factors as total biomass, that is, the living component of the soil, nor do they identify levels of specific fungi or invertebrates. Conventional soils analyses do not reflect the high levels of nutrients cycling through ecosystems today because the analyses reflect what is present at the time rather than what is flowing through the system. Thus we have remarkably little information on the soil’s functional character.
Acidity is not the only problem occasioned by nitrogen as an air pollutant. Nitrate and ammonium ions, the important forms of nitrogen in deposition, are powerful stimulants to many biological processes.
Nitrogen levels have increased substantially, not just from automobiles and trucks, but from other sources as well: intensive agriculture, wastewater treatment, landfills, water pollution, and other activities that involve large quantities of organic waste. The levels of nitrogen loading on natural systems today are in many cases more than three times the natural level of loading.
This shift is an example of a general phenomenon that ecologists call “eutrophication,” an increase in levels of one or more nutrients. Although in the minds of regulators and the public, eutrophication is usually associated with aquatic or wetland habitats, eutrophication of terrestrial systems has occurred as well and will continue to severely affect forests as well as aquatic systems:
The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily by nitrogen fertilizer, also by legume crops and as a byproduct of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophication of the biosphere, and has effects on biological diversity in the most affected areas (Vitousek 1994, 1861-2).
Unfortunately, ecosystems are generally adapted to relatively low rates of nitrogen input. Nitrogen amounts above certain levels will drive a system to change. Prior to extensive human generation of nitrogen by industry, agriculture, transportation, and other activities, nitrogen from precipitation and deposition from the atmosphere was relatively low, and forested ecosystems were necessarily adapted to the efficient utilization of a limited flux of nitrogen in the system. Widespread association with mycorrhizal fungi by different kinds of forest trees is one of the adaptations that allowed forests to develop under natural conditions that were typically low in nitrogen. Certain trees, such as alder and locust, also have associations with nitrogen-fixing microorganisms. From the trunks and canopy branches of forest trees, lichen communities and their nitrogen-fixing symbionts shed a slow rain of nitrogen to the forest floor. At one time the total amount of nitrogen added to a system came from these and other small sources — such as minor amounts generated by lightning that then fall in rain. The shift to high nitrogen loading has been relatively swift, and the consequences are pervasive.
According to Orie Loucks, Director of the Lucy Braun Association for the Mixed-Mesophytic Forest, which monitors the impacts of air pollution:
In my view, the Mixed-Mesophytic has lost its immunity to diseases and insects from the impacts of these pollutants. In this sense, the death can be compared to the AIDS epidemic afflicting the human species. One only has to walk in the forest and see the premature leaf drop in early July to understand that ozone is dramatically impacting a predominance of species. As for nitrogen, we know that the forest is receiving three times the amount of its estimated tolerance level (quoted in Flynn 1994, 34).
Philip Wargo, a U.S. Forest Service plant pathologist, uses the term “full-force decline” to describe the combination of severe mortality and failing reproduction. European forests are even more strikingly affected. “The Dobris Assessment,” a comprehensive report on the environment of greater Europe, indicates that one-quarter of the forest trees in thirty-four countries are defoliated. More than 50 percent of the forest in the Czech Republic, for exmaple, may be irreversibly damaged.
Although discussion of the eutrophication of terrestrial systems is somewhat new, its effects on aquatic systems are all too familiar. Relationships between upland and wetland eutrophication are not, however. Much of the nitrogen that is causing eutrophication of rivers, lakes, and coastal water bodies derives from atmospheric deposition over the watersheds draining into them.
The bulk of the nitrogen in a forest soil is tied up in the soil’s organic matter and surface litter. Biological decomposition typically releases nitrogen in the form of the ammonium ion, in a process referred to as “ammonification.” But when excess nitrogen is available, certain microbes convert the ammonium to nitrate in a process referred to as “nitrification.” Plants or microbial populations may take up both the ammonium and nitrate forms of nitrogen; however, under more natural conditions, ammonium in limited quantities rather than high levels of nitrate would be typical. Ammonium, although less available, persists in the soil while large amounts of nitrate seep away in runoff in groundwater and contribute to downstream eutrophication. Presently, atmospheric deposition provides 10 to 50 percent of the nitrogen from human activities that is added to coastal waters, contributing to eutrophication and ecosystem changes (Paerl 1993). In the Chesapeake estuary, for instance, 30 percent of its nitrogen originates from atmospheric deposition in the watershed (EPA 1994). A study in southeast Australia found that deteriorating riparian eucalypt woodlands had high levels of nitrogen, especially as nitrate, and nitrophilous plants beneath the eucalypts had high foliar nitrogen and high levels of nitrate reductase activity (Granger, Kasel, and Adams 1994). Riparian woodlands, including swamplands, can be especially affected in this way since they receive burdens of nitrogen from agricultural runoff, wastewater, and atmospheric deposition, including not only that which falls directly on them but also that which washes down from upstream in the watershed.
Problems of nitrogen deposition at the global level go well beyond regional effects. Oxides of nitrogen in the atmosphere, especially where human-caused nitrogen deposition is high, contribute to global warming as well as to destruction of the ozone layer. High levels of nitrous oxide further increase incident ultraviolet radiation levels on forests and the biosphere owing to its role in the destruction of ozone in the stratosphere. Nitrous oxide is more or less stable in the lower atmosphere, but in the stratosphere it converts into reactive forms that become nitric acid aerosols. These droplets act as catalysts in the reactions of the various forms of chlorine with ozone and thus are implicated in the stratospheric ozone loss (Sander, Friedl, and Yung 1989).
Because coal and oil are fossil fuels, unlike firewood or other biomass fuels, their use releases carbon fixed long ago very rapidly back into the atmosphere as carbon dioxide, which is linked to the problem of global warming. The accelerated release of nitrous oxide from increased denitrification by soils receiving excess of nitrogen amplifies this greenhouse effect when high levels of nitrate are transformed by anaerobic soil microbes into gaseous nitrogen or gaseous oxides of nitrogen, especially N2O (nitrous oxide), which escape to the atmosphere.
Nitrous oxide is an important greenhouse gas itself, especially problematic because it is more persistent in the atmosphere than the other two prominent greenhouse gases, carbon dioxide and methane. Impacts of the greenhouse effect also have great relevance to the survival of woodlands. Temperate forest areas will possibly experience increased storminess, changed soil moisture conditions, changed hydrology, and increased growth of certain species over others, including many exotics. Associated problems will preoccupy managers of parks and other lands for many years to come. Blowdowns, for example, already a major problem in many parks and woodlands, are partly a consequence of fragmentation because forested areas have greater exposure to windstorms, especially when accompanied by soil saturating heavy rainfall. Should storms increase in frequency and intensity with global warming, the problem of blowdowns will worsen, especially if fragmentation continues.
Researchers have chronicled the plight of beech, an indigenous species that has shown a high degree of sensitivity to the climatic changes already under way. Early responses include reduced growth and seed production, increased insect and pathogen attacks, and, in older trees, direct physiological stress. Compounding these effects, beech also competes poorly with increased disturbance, such as trampling or windstorms. Their prognosis is very bleak:
With increased disturbance and the temperature scenarios we are using here, persistence of beech for longer than 50 years in reserves in the southeastern United States seems unlikely. To compound the difficulties of management, it may become difficult to protect forest reserves. Moribund beech forests will have little aesthetic appeal and may also be viewed as fire hazards or centers for disease that endanger surrounding vegetation (Davis and Zabinski 1992, 303).
As disturbing as some future concerns are, damage visible today in the landscape underscores the need to expand roadless areas, especially in national park and forest lands. Direct impacts along every road corridor amplify the consequences of regional air pollution and add to the complex of edge effects. The following description of forests in West Virginia should be specter enough to make everyone concerned with forests seriously reconsider our use of the automobile and of fossil fuels in general:
Aliff began noticing the decline in the 1950s — in native red mulberry and butternut walnut growing on the lower slopes, in the fields, and along the creek banks. Then some twenty-five years ago, it climbed to the ridges where it hit yellow locust at about the same time it struck the hickories and oaks. From there it invaded the coves and in the past five to eight years it has spread like wildfire until I don’t think there’s a single species unaffected. I used to think of survivor trees, like yellow locust and red maple. But that’s not the story anymore. The death is system-wide. Wherever I go, here at home and into other states, I walk among tombstones (Flynn 1994, 35).
Recent reports on the impact of atmospheric change on the growth of white pine in New England are at first glance encouraging. One study (Bennett et al. 1994) suggests that some of what we see is individual tree sensitivity, rather than the vulnerability of a whole species. This thorough review of field surveys of damage to white pine since 1900 suggests that many reports are flawed and that current inventories show that growth of white pine throughout its range is vigorous. It appears that not more than 10 percent of the population is hypersensitive and that these individuals in particular declined in the years between 1963 and 1973. Fewer losses are occurring now as this stock is gradually eliminated from the population. Still, is this good enough news? Is 10 percent too great a stress at this time? Is the sensitivity of other species much higher? Isn’t a more natural range of air and soil conditions more likely to sustain greater diversity? How much farther do we think we can deviate from historical conditions?
For many of us watching the treasured landscapes of our youth collapse before our eyes, it is hard to think what it might take to restore the landscapes of the ancient forests. As Bill McKibben (1996) noted in his afterword to a book about old-growth forest in the East,
When we look at a hemlock on a slope above Cold River in the Berkshires, or a towering white pine south of Cranberry Lake in the Adirondacks, or a massive tulip poplar in a cove in the Smokies, we must not imagine that its glory devalues the second- and third-growth birch and beech a quarter-mile distant. Instead the majesty of the ancient forest makes this tentative wildness all the more valuable, for it shows what it might become someday. Old growth is not simply a marker of past glory, an elegy for all that once was. It is a promise of the future, a glimpse of the systemic soundness we will not see completed in our lifetimes but that can fire our hopes for the timeliness to come (363).