Environmental Engineering Reference
In-Depth Information
Sphagnum
mosses will colonize the lake bottoms
(Pearce 1982d). Rapid
Sphagnum
growth has
been reported in Scandinavia, but in eastern
North America large quantities of green algae
are more common. The absence of insect larvae
and other organisms which normally graze on
the algae may in part explain its abundance
(Stokes
et al.
1989). Leaves or twigs falling into
the water will be slow to decompose, because
the bacteria which would normally promote
decay have been killed by the acidic conditions
(Park 1987). The absence of phytoplankton and
the general reduction in organic activity allows
greater light penetration, which makes acid lakes
unnaturally clear and bluish in colour (LaBastille
1981). This ethereal appearance may suggest
death, but even the most acid lakes have some
life in them.
the relationship seems obvious in many cases,
the growth, development and decline of plants
has always reflected the integrated effects of
many variables, including site microclimatology,
hydrology, land-use change, age and species
competition. Acid rain has now been added to
that list. Even those individuals or groups with
greatest concern for the problem admit that it is
next to impossible to isolate the impact of any
one element from such a combination of variables
(Ontario: Ministry of the Environment 1980).
Thus it may not be possible to establish definitive
proof of the link between acid precipitation and
vegetation damage. The body of circumstantial
evidence is large, however, and adverse effects
have been produced in laboratory experiments.
Together, these support the view that the
terrestrial environment is under some threat from
acid rain.
Assessment of the threat is made difficult by
the complexity of the relationships in the
terrestrial environment. In areas experiencing
acid rain, dry and wet deposition over land is
intercepted initially by the vegetation growing
there. The effects of this precipitation on the
plants may be direct, brought about by the
presence of acid particles on the leaves, for
example, or indirect, associated with changes in
the soil or the biological processes controlling
plant growth (see Figure 4.11). Acid precipitation
intercepted by trees may promote necrosis of leaf
tissue, leaching of leaf nutrients and chlorophyll
degradation (Shriner & Johnston 1985), all of
which cause visible damage. Vegetation growing
at high altitudes, and therefore enveloped in cloud
for long periods, frequently displays such
symptoms, since cloud moisture is often more
acidic than rain (Hendrey 1985). Ultimately, the
acid particles will be washed off the vegetation
and into the soil, where they can begin to affect
the plants indirectly, but no less seriously.
By intercepting acid deposition, plants,
particularly trees, also act as concentrators. For
example, dry deposition allowed to accumulate
on the leaves or needles of the forest canopy,
will increase the acidity of the rainfall which
washes it out to the forest floor (Park 1987).
ACID RAIN AND THE TERRESTRIAL
ENVIRONMENT
Terrestrial ecosystems take much longer to show
the effects of acid rain than aquatic ecosystems.
As a result, the nature and magnitude of the
impact of acid precipitation on the terrestrial
environment has been recognized only recently.
As early as 1965, however, air pollution was
known to be killing oak and pine trees on Leo
Tolstoy's historic estate at Yosnaya Polyana
(Goldman 1971). There is growing evidence that
those areas in which the waterbodies have already
succumbed to acidification must also face the
effects of increasing acid stress on their forests
and soils. The threat is not universally recognized,
however and there remains a great deal of
controversy over the amount of damage directly
attributable to acid rain. Reduction in forest
growth in Sweden (LaBastille 1981), physical
damage to trees in West Germany (Pearce 1982b),
and the death of sugar maples in Quebec and
Vermont (Norton 1985) have all been blamed
on the increased acidity of the precipitation in
these areas. Some of the soils developed on the
hill-peats of Scotland appear to have become so
acid that they can longer support the acid-tolerant
heather native to the area (Last 1989). Although
