Air Pollution and Forests

Forests of the world are of enormous importance to mankind for wood, watershed, wildlife, recreation, wilderness, and aesthetic and amenity values. Over the next several decades improved understanding of forests, wilderness stewardship, improved natural forest management, and increased acreages of intensively managed artificial forests will significantly improve these values. During this same period, unfortunately, other human activities will exert a variety of stresses on forest ecosystems. One of the most important stresses of widespread significance is air pollution. This book is an exploration of the various interactions between air pollution and temperate forest ecosystems.

Despite the fundamental and enormous importance of carbon, sulfur, and nitrogen to the biota, our appreciation of the cycling, residence, and flux of these elements among various ecosystems, oceans, and the atmosphere is incomplete. Despite our deficiencies efforts are being made to refine our understanding of major element cycles and to more accurately estimate global budgets. This is especially important for those interested in air pollution as compounds containing these elements are extremely significant air contaminants under certain circumstances. Forest ecosystems, because of their extensive distribution and their varied functions, play important roles in global element cycles. The imprecision of our estimates of global nutrient budgets is large and conclusions and predictions based on them must be qualified and cautious.

In addition to whatever contribution forests may make to the atmospheric burden of carbon, sulfur, and nitrogen oxides, they are known to be important natural sources of hydrocarbons and particulates. Volatile hydrocarbons are released by a variety of woody plants during the course of normal metabolism. Pollen, the most significant particulate contaminant released by forests from the standpoint of human health, is also produced, of course, during normal reproductive metabolism. Hydrocarbon aerosols are viewed as an increasingly important particulate emission from forests. Forest burning, whether naturally occurring or artificially ignited, also produces hydrocarbons, particulates as well as carbon oxides. Even though forest fires may be a natural recurring event in most forest ecosystems, the pollutants generated by this process are not the result of normal metabolism but rather are generated by combustion of forest biomass. As a result, the latter are discussed in Section C.

Air contaminants may be removed from the atmosphere by a variety of mechanisms. The primary processes are precipitation scavenging, chemical reaction, dry deposition (sedimentation), and absorption (impaction) (Rasmussen et al., 1974). Loss via precipitation may occur in two ways: “rainout” which involves both absorption and particle capture by falling raindrops. Primary and secondary contaminants are subject to a large number of chemical reactions in the atmosphere that may ultimately transform them into an aerosol or oxidized or reduced product. Attachment by aerosols and subsequent deposition on the surface of the earth is termed dry deposition. Absorption by water bodies, soils, or vegetation at the surface of the earth is an additional extremely important removal process.

In addition to the soil compartment, the vegetative compartment of forest ecosystems functions as a sink for atmospheric contaminants. As in the case of soils a complex variety of biological, chemical, and physical processes are involved in the transfer of pollutants from the air to the surfaces of vegetation. For certain contaminants, for example, persistent heavy metal particles, the repository functions of vegetation and soils are intimately linked as a portion of the heavy metals input to the soil are derived from vegetative sources contributing litter to the forest floor. Interest in the ability of plants to remove pollutants from the air has grown considerably in recent years as individuals have become increasingly aware of the amenity functions (Heisler, 1975; Smith, 1970a) of woody plants, particularly in urban and suburban areas. The capability of plants to act as a sink for air contaminants has been addressed by a variety of recent reviews, for example, U.S. Environmental Protection Agency (1976a), Smith and Dochinger (1976), Bennett and Hill (1975), Hanson and Thorne (1972), Hill (1971), Environmental Health Science Center (1975), Keller (1978), and Warren (1973). These papers indicate that the surfaces of vegetation provide a major filtration and reaction surface to the atmosphere and importantly function to transfer pollutants from the atmosphere to the biosphere.

The Class I relationship between forest ecosystems and air pollution is of primary importance when the atmospheric load of air contaminants from anthropogenic sources is relatively low. This situation exists locally and regionally when the sources of air pollutants produced by the activities of human beings are not operating or operating at low level or when meteorological conditions are not conducive to atmospheric accumulation. On a global scale, the Class I relationship may be extensive throughout those regions relatively remote from the activities of people. The specific concentration of air contaminants under “low” conditions is variable depending on the pollutant, but in general is meant to approximate “background,” clean-air concentrations as, for example, presented by Rasmussen et al. (1975) for the major trace gases in μg m^-3 : sulfur dioxide (1–4), hydrogen sulfide (0.3), dinitrogen oxide (460–490), nitric oxide (0.3–2.5), nitrogen dioxide (2–2.5), ammonia (4), carbon monoxide (100), ozone (20–60), and reactive hydrocarbons (<1). Since the majority of air contaminants of greatest significance to vegetative and human health (Table 1-1) originate from, and are removed by, both anthropogenic and natural agents it is essential to evaluate the importance of forest ecosystems within the latter group.

Sexual reproduction of forest trees is critically important for maintenance of genetic flexibility and the persistence of most species in natural forest communities. Reproductive strategies, however, are typically beset by a variety of “weak points” and reproductive growth of many forest trees is, at best, irregular and quite unpredictable. Generally there is a very good correlation between tree vigor and the capacity for flowering and fruiting (Kozlowski, 1971). A variety of environmental constraints imposes restrictions on tree reproductive processes. Because air contaminants may reduce tree vigor and in view of the fact that numerous potential points of interaction have been identified between air pollutants and reproductive elements (Figure 7-1), it has been hypothesized that air contaminants may impact forest ecosystems by influencing reproductive processes.

Nutrients must move into, within, and out of forest ecosystems in appropriate amounts, at appropriate rates and along established pathways for normal forest growth to occur. The two major sources of nutrients for temperate forest ecosystems are (1) meteorologic input of dissolved, particulate, and gaseous chemicals from outside the ecosystem ; and (2) release by weathering of nutrients from primary and secondary minerals stored within the ecosystem (Bormann and Likens, 1979). Healthy forest ecosystems conserve these nutrients and continually recycle them through the system via an elaborate litterfall-decomposition-uptake intrasystem cycle.

Acid precipitation is defined as rain or snow having a pH of less than 5.6. The pH parameter is a measurement of the difference in hydrogen ion activity between an unknown solution and a standard buffer of assigned pH value. Upon ionization water yields hydrogen and hydroxyl ions. When the activity of these ions is equal, water is neutral and the pH recorded will be 7. At pH values below 7 water becomes increasingly acid, and above 7 increasingly alkaline. In the absence of air pollutants, the pH of precipitation is presumed to be dominated by carbonic acid formed from ambient carbon dioxide, which produces a pH of approximately 5.6–6.0. The pH of precipitation presently falling in North and Central Europe and in the northeastern United States and adjacent portions of Canada is commonly in the range of 3–5.5. Individual storm events have been recorded with pH values between 2.0 and 3.0.

Symbiotic microorganisms have roles of very great importance in nutrient relations in forest ecosystems. Forests frequently flourish in regions of low, marginal, or poor soil nutrient status. In addition to nutrient conservation and tight control over nutrient cycling, trees have evolved critically significant symbiotic relationships with soil fungi and bacteria that enhance nutrient supply and uptake. The interaction between air contaminants, symbiotic microbes, and their relationship with host trees is of critical importance. Adverse impact on mycorrhizae by air pollution has been hypothesized (Sobotka, 1968).

Photosynthesis is the most important metabolic process of forest ecosystems. In simple outline the process amounts to the reduction of carbon dioxide to CH₂O and the oxidation of water to molecular oxygen and results in the derivation of approximately 95% of the dry weight of plants. Photosynthesis is the process primarily responsible for forest productivity.

Arthropods have roles of enormous importance in the structure and function of terrestrial ecosystems. Forest ecosystems, in particular, typically have large and diverse arthropod populations. The importance of pollinating (Chapter 7) and litter metabolizing (Chapter 8) species has already been introduced. The damaging influence of high population densities of certain insects can be very visible and cause widespread forest destruction; witness contemporary North American situations involving the Douglas fir tussock moth, the gypsy moth, the eastern spruce budworm, and the southern pine bark beetle. It is critically important, however, to keep in perspective that there is substantial evidence to support the notion that forest insects, even those that cause massive destruction in the short run, may play essential and beneficial roles in forest ecosystems in the long run. These roles may involve regulation of tree species competition, species composition and succession, primary production, and nutrient cycling (Huffaker, 1974; Mattson and Addy, 1975).

Abnormal physiology, or disease, in woody plants follows infection and subsequent development of an extremely large number and diverse group of microorganisms internally or on the surface of tree parts. All stages of tree life cycles and all tree tissues and organs are subject, under appropriate environmental conditions, to impact by a heterogeneous group of microbial pathogens including viroids, viruses, mycoplasmas, bacteria, fungi, and nematodes. The influence of a specific disease on the health of an individual tree may range from innocuous to mild to severe. Over extended time periods, the interaction of native pathogens with natural forest ecosystems is significant, and frequently beneficial, in terms of ecosystem development and metabolism. As in the instance of insect interactions (Chapter 12), microbes, and the diseases they cause, play important roles in succession, species composition, density, competition, and productivity. In the short term, the effects of microbial pathogens may conflict with forest management objectives and assume a considerable economic or managerial as well as ecologic significance (Smith, 1970).

Under conditions of sufficient dose, air pollutants directly cause visible injury to forest trees. The accumulation of particulate contaminants on leaf surfaces or the continued uptake of gaseous pollutants through leaf stomata will eventually result in cell and tissue damage that will be manifest in foliar symptoms obvious to the trained, but unaided eye. This direct induction of disease in trees by air pollutants is the most dramatic and obvious individual tree response of all Class II interactions. It is the only Class II interaction that can be detected in the field by causal observation. Unlike altered reproductive strategy, nutrient cycling, tree metabolism, or insect and disease relationships; the degree of foliar symptoms induced by air pollutants can be relatively easily observed, inventoried, and quantified. In the presence of sufficient dose, tree damage may be of sufficient severity to cause mortality. Tree death directly induced by ambient air pollution exposure is considered a Class III interaction and is treated in Chapter 16.

In the presence of a sufficient dose of an air pollutant, forest trees will be adversely impacted. When this occurs the threshold between Class I and Class II interactions is crossed. At intermediate dose, the specific contaminant concentration and time of exposure varying greatly with specific pollutant and forest situation, the influence on individual forest components may range from extremely subtle to visibly dramatic. Chapters 7 through 14 have reviewed the evidence available to support the hypotheses that intermediate air pollution loads may alter or inhibit forest tree reproduction, alter forest nutrient cycling, alter tree metabolism, or change forest stress conditions by influencing insect pests, microbial pathogens, and by directly damaging foliar tissue. All but the latter of these impacts would be extremely subtle, visibly asymptomatic, and detectable only by very careful forest monitoring.

Under conditions of excessive dose, that is, atypically high atmospheric concentrations of one or more contaminants for extended (or continuous) time periods, the impact on forest ecosystems may be very severe and dramatic. This response is designated a Class III interaction. The reaction of vegetation in this case is characterized by severe morbidity and mortality caused directly by air pollutants.

Large areas of the temperate forest ecosystem are currently experiencing major perturbation from air pollution. The influence of a variety of air contaminants on biogeochemical cycling, patterns of succession and competition, and individual tree health, designated Class II interactions in this book, are causing significant forest change in the temperate zone. At the ecosystem level the major perturbations include decreased productivity, biomass and diversity; at the community level, reduced growth; and at the population level, altered species composition. Early and midsuccessional forests are concluded to be at particular risk. Temperate forests have historically been subjected to major change resulting from the activities of human beings. For centuries the major influence was gross destruction for agricultural, fuel, or other wood-product purposes. In the present century reduced need for agricultural land and increased forest management have reduced the adverse impact on forests in temperate latitudes. Human activities of primary contemporary importance to forest structure and function have included the introduction of exotic arthropod and microbial tree pests into forest systems lacking evolutionary exposure to these destructive agents, enhancement of native and natural stresses by cultural practices, and the creation of artificial forests of one or a few commercially important species. In the past several decades, however, we have accumulated sufficient evidence to indicate that an additional major anthropogenic modifier of temperate forest ecosystem development is air pollution.