Page 149 - Fundamentals of Air Pollution

III. Effects on Forests 117

Forests can act as sources of some of the trace gases in the atmosphere,
such as hydrocarbons, hydrogen sulfide, NO X, and NH 3. Forests have been
identified as emitters of terpene hydrocarbons. In 1960, Went (10) estimated
that hydrocarbon releases to the atmosphere were on the order of 108 tons
per year. Later work by Rasmussen (11) suggested that the release of
8
terpenes from forest systems is 2 x 10 tons of reactive materials per year
on a global basis. This is several times the anthropogenic input. Yet, it is
important to remember that forest emissions are much more widely dis-
persed and less concentrated than anthropogenic emissions. Table 8-2
shows terpene emissions from different types of forest systems in the
United States.
Forest systems also act as sources of CO 2 when controlled or uncontrolled
burning and decay of litter occur. In addition, release of ethylene occurs
during the flowering of various species. One additional form of emission
to the atmosphere is the release of pollen grains. Pollen is essential to the
reproductive cycle of most forest systems but becomes a human health
hazard for individuals susceptible to hay fever. The contribution of sulfur
from forests in the form of dimethyl sulfide is considered to be about
10-25% of the total amount released by soils and vegetation (12).
Trees and soils of forests act as sources of NH 3 and oxides of nitrogen.
Ammonia is formed in the soil by several types of bacteria and fungi. The
volatilization of ammonia and its subsequent release to the atmosphere are
dependent on temperature and the pH of the soil. Fertilizers are used as
a tool in forest management. The volatilization of applied fertilizers may
become a source of ammonia to the atmosphere, especially from the use
of urea.
Nitrogen oxides are formed at various stages of the biological denitrifica-
tion process. This process starts with nitrate; as the nitrate is reduced
through various steps, NO 2, NO, N 2O, and N 2 can be formed and, de-
pending on the conditions, released to the atmosphere.
The interactions of air pollutants with forests at low-dose concentrations
result in imperceptible effects on the natural biological cycles of these spe-
cies. In some instances, these interactions may be beneficial to the forest
ecosystem. Forests, as well as other natural systems, act as sinks for the
removal of trace gases from the atmosphere.

B. Intermediate-Dose Levels
The second level of interaction, the intermediate-dose level, can result
in measurable effects on forest ecosystems. These effects consist of a reduc-
tion in forest growth, change in forest species, and susceptibility to forest
pests. Both laboratory investigations and field studies show SO 2 to be an
inhibitor of forest growth. When various saplings have been exposed to SO 2
in the laboratory, they show reduction in growth compared with unexposed