Page 148 - Fundamentals of Air Pollution

116 8. Effects on Vegetation and Animals

caused by air pollution will not result in a determination of damage if there
is no loss in yield. Individual circumstances determine whether air pollution
damage has occurred.
The costs of air pollution damage are difficult to estimate. However,
estimates indicate crop losses of $1 billion to $5 billion for the United States
(6). When compared to the crop losses due to all causes, this percentage
is small. However, for particular crops in specific locations, the economic
loss can be very high. Certain portions of the Los Angeles, California,
basin are no longer suitable for lettuce crops because they are subject to
photochemical smog. This forces producers either to move to other locations
or to plant other crops that are less susceptible to air pollution damage.
Concern has been expressed regarding the future impact of air pollution
on the much larger Imperial Valley of California, which produces up to
50% of certain vegetables for the entire United States.

HI. EFFECTS ON FORESTS
2
10
Approximately 1.95 x 10 km of the earth's surface has at least 20% or
more crown tree cover, representing about one-third of the total land area
(7). Several different types of forest ecosystems can be defined based on
their location and the species present. The largest in area are tropical forest
systems, followed by temperate forests, rain forests, and tidal zone systems.
The temperate forest systems are located in the latitudes where the greatest
industrialization is occurring and have the most opportunity to interact
with pollutants in the atmosphere. The impact of air pollution on forest
ecosystems ranges from beneficial to detrimental. Smith (8, 9) classified
the relationship of air pollutants with forests into three categories: low dose
(I), intermediate dose (II), and high dose (III). With this classification
scheme, seemingly contradictory statements on the impact of air pollution
on forests can be understood.

A. Low-Dose Levels

Under low-dose conditions, forest ecosystems act as sinks for atmo-
spheric pollutants and in some instances as sources. As indicated in Chapter
7, the atmosphere, lithosphere, and oceans are involved in cycling carbon,
nitrogen, oxygen, sulfur, and other elements through each subsystem with
different time scales. Under low-dose conditions, forest and other biomass
systems have been utilizing chemical compounds present in the atmosphere
and releasing others to the atmosphere for thousands of years. Industrializa-
tion has increased the concentrations of NO 2, SO 2 , and CO 2 in the "clean
background" atmosphere, and certain types of interactions with forest
systems can be defined.