Air pollution—effects on the structure and function of the temperate forest ecosystem

Abstract Air pollution has had, is having, and will continue to have an influence on forest ecosystems throughout the temperate regions of the world. The nature of this relationship can be divided into three classes. Under conditions of low dosage—Class I relationship—the vegetation and soils of forest ecosystems presumably function as a very important sink for air contaminants. When exposed to intermediate dosage— Class II relationship—individual tree species or individual members of a given species may be adversely and subtly affected by nutrient stress, reduced photosynthetic or reproductive rate, predisposition to entomological or microbial stress, or direct disease induction. Exposure to high dosage—Class III relationship—may induce acute morbidity or mortality of specific trees. The ecosystem impact of these various relationships would be very variable. In the Class I relationship, pollutants would be transferred from the atmospheric compartment to the biotic (organic) or available nutrient compartments. Depending on the nature of the pollutant, the ecosystem impact of this transfer could be undetectable (innocuous effect?) or stimulatory (fertilising effect). If the effect of air pollution exposure on some component of the ecosystem biota is inimical then a Class II relationship is established. The ecosystem impact in this case could include reduced productivity or biomass, shifts in species composition, increased secondary effects, such as insect outbreaks or disease epidemics, or increased morbidity and reduced vigour. The ecosystem impacts of Class II relationships are extraordinary important because of their potentially widespread significance. In the presence of high air pollution dosage—Class III relationship—impact on the structure of the ecosystem may be gross simplification, and disturbances to the function of the ecosystem may include basic changes in hydrology, nutrient cycling, erosion, microclimate and overall stability. While these numerous ecosystem impacts, resulting from air pollution stress, have been identified, few have been quantified in the field. We are especially deficient in our ability generally to assess Class I and II relationships. This hiatus of knowledge is due to several factors in addition to the obvious difficulty of making accurate measurements of subtle processes in expansive and frequently remote forest ecosystems. Among the most important factors are: (1) the extraordinary variable response different plant species and individuals within species have to individual air pollutants, (2) the strong controlling influence that local edaphic, topographical and meteorological conditions exert on plant response to air contaminants, (3) the fact that numerous tree species and most forest shrub and herb species have not been evaluated in regard to response to air pollutants, (4) the realisation that most of our data stem from studies with a very few pollutants reacted singly and that some gaseous and particulate contaminants and mixtures of pollutants have received little research attention, (5) that much of the research has been conducted employing air pollution dosages in considerable excess of ambient forest levels, and that (6) most of the investigations have been carried out under highly controlled—and hence artificial—environments. These six difficulties must be taken into consideration in future research. This research should concentrate on Class I and II relationships as these are considerably more significant than Class III situations. With the recognised deficiencies in our information we can speculate on the total impact of air pollution on forest ecosystems, but at present we cannot model or quantify it. Research to determine the relationships between air pollution and forest ecosystems must be given high priority because of the size and significance of these ecosystems in temperate regions and the numerous potentially-damaging interactions that have been identified.