Environmental Engineering Reference
In-Depth Information
in much the same way. Needles turn yellow, dry
up and fall off the branches; new buds fail to
open or, if they do, produce stunted and distorted
growth (Park 1987). The trees gradually weaken
during these changes and become more and more
vulnerable to insect attack, disease and the
ravages of weather, all of which contribute to
their demise (Norton 1985).
The maple groves of Ontario, Quebec and
Vermont have been suffering progressive
dieback since 1980 (Norton 1985), and it is now
estimated that, in Quebec alone, more than 80
per cent of the maple stands show signs of
damage (Robitaille 1986). Mortality rates for
maples growing in Quebec have increased from
2 per cent per year to 16 per cent (presenting a
potential $6 million loss for the province's
maple sugar producers), and regeneration is
well below normal (Norton 1985). There is as
yet no conclusive proof that acid rain is the
cause of dieback, nor that current acid rain
levels are sufficient to prevent or diminish the
germination and growth of maple seeds (Pitelka
and Raynal 1989). Maple seedlings are sensitive
to the presence of aluminium in the soil, but in
most of the affected area aluminium levels are
not high enough to be toxic (Thornton
et al.
1986). Alternative explanations for the dieback,
such as poor sugar bush management or
disease—with or without a contribution from
acid rain—have been put forward. However, the
problem is common to both natural and
managed groves, while disease usually follows
rather than precedes the onset of dieback
(Norton 1985). Dieback is now becoming
prevalent in beech and white ash stands, but it is
the damage to the maple which is causing
greatest concern, particularly in Quebec,
currently the source of 75 per cent of the world's
supply of maple sugar (Norton 1985).
Deciduous trees are not the only victims of
dieback. The coniferous forest of eastern North
America has also suffered considerable decline
(Johnson and Siccama 1983). The red spruce
forests in the Adirondack, Green and White
Mountains were particularly badly hit between
the early 1960s and mid-1980s (Johnson
et al.
1989). Since the greatest dieback occurred at
higher elevations, where the trees are often
enveloped in cloud for days at a time, it was
hypothesized that persistent exposure to the high
levels of acid common in these clouds was the
main cause of the damage. Subsequent studies,
however, provided no proof of a direct link
between acidity and dieback in that setting
(Johnson
et al.
1989). There is evidence that
winterkill played an important part in initiating
the spruce decline (Johnson
et al.
1989), and it
has been suggested that exposure to acid stress
caused the trees to be less capable of with-
standing the extreme cold of winter (Haines and
Carlson 1989).
Dieback is extensive in Europe also. By the
mid-1980s, the symptoms had been recognized
across fifteen countries in forests covering some
70,000 sq km (Park 1991). Damage was
particularly extensive in what was then West
Germany, where Ulrich and his colleagues first
linked it to acid rain (Ulrich
et al.
1980). It was
estimated that one third of the trees in that
country had suffered some degree of dieback; 75
per cent of the fir trees and 41 per cent of the
spruce trees in the state of Baden-Wurttemberg,
which includes the Black Forest, were damaged
(Anon. 1983); 1,500 hectares of forest died in
Bavaria in the late 1970s and early 1980s (Pearce
1982d). For a nation which prides itself in good
forest management, such figures were
horrendous. The death of the forests, or
Waldsterben
as it has became known, was
progressing at an alarming, and apparently
quickening, pace in the mid-1980s (Park 1987).
Some recovery took place in the German forests
after 1985 (Blank
et al.
1988), but by the end of
the decade perhaps as many as half the trees in
the country were showing signs of damage (Park
1991). Other nations in Europe are affected, from
Sweden with 38 per cent of its trees damaged, to
Greece with 64 per cent. As more information
becomes available for eastern Europe, it is clear
that forest decline is also a serious problem in
the Czech and Slovak republics, eastern Germany
and Poland. In contrast, Norway does not fit the
pattern, despite the exposure of its forests to acid
