Page 152 - Fundamentals of Air Pollution

120 8. Effects on Vegetation and Animals

susceptible than the other members of this forest system, and continued
exposure to photochemical oxidant may very well shift it from the dominant
species to a minor one.
The interactions in the intermediate-dose category may result in effects
on the reproduction cycle of species, the utilization of nutrients, the produc-
tion of biomass, and the susceptibility to disease.

C. High-Dose Levels
The third category for interactions is high dose (III). The effects produced
by this level of interaction can be seen by the casual observer. The result
of high-dose exposure is destruction or severe injury of the forest system.
High-dose conditions are almost always associated with point source emis-
sions. The pollutants most often involved are SO 2 and hydrogen fluoride.
Historically, the most harmful sources of pollution for surrounding forest
ecosystems have been smelters and aluminum reduction plants.
One example of high-dose interaction is the impact of a smelter on the
surrounding area in Wawa, Ontario, Canada. This smelter began operating
about 1940. Gordon and Gorham (16) documented the damage in the pre-
vailing downwind northeast sector for a distance of 60 km. They analyzed
vegetative plots and established four zones of impact in the downwind
direction: Within 8 km of the plant, damage was classified as "very severe"
where no trees or shrubs survived; "severe damage" occurred at —17 km,
where no tree canopy existed; "considerable damage" existed at —25 km,
where some tree canopy remained, but with high tree mortality; and "mod-
erate damage" was found at —35 km, where a tree canopy existed but was
put under stress and where the number of ground flora species was still
reduced.
This type of severe air pollution damage has occurred several times in
the past. If care is not taken, additional examples will be documented in
the future.

D. Acid Deposition

Acid deposition refers to the transport of acid constituents from the
atmosphere to the earth's surface. This process includes dry deposition of
SO 2, NO 2, HNO 3, and particulate sulfate matter and wet deposition ("acid
rain") to surfaces. This process is widespread and alters distribution of
plant and aquatic species, soil composition, pH of water, and nutrient
content, depending on the circumstances.
The impact of acid deposition on forests depends on the quantity of
acidic components received by the forest system, the species present, and
the soil composition. Numerous studies have shown that widespread areas
in the eastern portion of North America and parts of Europe are being