Air Pollution, Photosynthesis and Forest Decline: Interactions and Consequences
291
Union of Forest Research Organisations, Vienna, 1978) is 25 flgm-3 or
8.7 ppb (annual means). This limit is doubled for forest sites with moderate
or good growth conditions. The WHO (World Health Organization) and the
UNECE (United Nations Economic Commission for Europe, Paris) recommend, in 1987 and 1988, a tolerance limit of 30flgm-3 (1O.5ppb, annual
means) S02 for sensitive forest plants. The German TA-Luft (TA =
Technische Anleitung) of 1986 is less restrictive permitting emissions up to
50flgm-3 (17.4ppb) on an annual basis. The same limit is proposed by the
German VDI-Richtlinie 2310 (VDI = Verein Deutscher Ingenieure) of 1978
for "very sensitive" plants and a higher limit of 80 flg m -3 (28 ppb) for
"sensitive" plants within vegetation periods of 7 months.
In 1987, the VDI considered 8-h averages of 40ppb, 85ppb, and 150ppb
ozone tolerable for very sensitive, sensitive, and less sensitive plants, respectively. The corresponding 1!2-h averages were 150,250, and 500ppb. In
1988, the UNECE recommended 30 ppb (8-h-average) and 150ppb (l/2-h
average) ozone as limits for sensitive plants and ecosystems. During the
summer of 1992, ozone values measured in various locations in Bavaria
exceeded lower limits in many cases.
For N0 2 , tolerance limits recommended in 1978 by the VDI for sensitive
plants were 180 ppb averages during a vegetation period of 7 months. The
UNECE distinguished between the vegetation period with a tolerance limit
of 30 ppb average and the winter period with a lower tolerance limit of
20 ppb average. In combination with S02 and ozone, a less than 15 ppb
average was considered tolerable.
Such recommended limits are usually based on fumigation experiments
with elevated concentrations of the pollutants performed in climatic
chambers or open top chambers. The duration of such experiments is
necessarily restricted. They are performed under climatic or growth conditions which rarely correspond to the climatic or growth conditions of
exposed forest sites.
For S02, Slovik et al. (1992a,b) have recently attempted to derive limits
of tolerance of healthy spruce on the basis of measured physiological data.
Form measurements of cation contents in xylem sap during the course of a
year, and considering the necessity of cation circulation in the tree, they
calculated a limit of long-term acidification tolerance which was below lOppb
S02 during part of the winter, but much higher during the summer. At any
time of the year, tolerance levels are lowered when cation availability is
restricted. In this situation, cation deficiency symptoms will develop even
when cation determinations in leaf extracts fail to indicate deficiency because
of vacuolar cation sequestration. In comparing the calculations with accepted
tolerance limits which permit averaging of pollution levels on an annual
basis, it must be emphasized that the highest levels actually occur during
the winter, when sensitivity of S02 is particularly high because metabolic
activity is low and cation transport in the xylem is slow. High winter levels
of S02 are presently legally tolerable because low summer levels produce an
291
Union of Forest Research Organisations, Vienna, 1978) is 25 flgm-3 or
8.7 ppb (annual means). This limit is doubled for forest sites with moderate
or good growth conditions. The WHO (World Health Organization) and the
UNECE (United Nations Economic Commission for Europe, Paris) recommend, in 1987 and 1988, a tolerance limit of 30flgm-3 (1O.5ppb, annual
means) S02 for sensitive forest plants. The German TA-Luft (TA =
Technische Anleitung) of 1986 is less restrictive permitting emissions up to
50flgm-3 (17.4ppb) on an annual basis. The same limit is proposed by the
German VDI-Richtlinie 2310 (VDI = Verein Deutscher Ingenieure) of 1978
for "very sensitive" plants and a higher limit of 80 flg m -3 (28 ppb) for
"sensitive" plants within vegetation periods of 7 months.
In 1987, the VDI considered 8-h averages of 40ppb, 85ppb, and 150ppb
ozone tolerable for very sensitive, sensitive, and less sensitive plants, respectively. The corresponding 1!2-h averages were 150,250, and 500ppb. In
1988, the UNECE recommended 30 ppb (8-h-average) and 150ppb (l/2-h
average) ozone as limits for sensitive plants and ecosystems. During the
summer of 1992, ozone values measured in various locations in Bavaria
exceeded lower limits in many cases.
For N0 2 , tolerance limits recommended in 1978 by the VDI for sensitive
plants were 180 ppb averages during a vegetation period of 7 months. The
UNECE distinguished between the vegetation period with a tolerance limit
of 30 ppb average and the winter period with a lower tolerance limit of
20 ppb average. In combination with S02 and ozone, a less than 15 ppb
average was considered tolerable.
Such recommended limits are usually based on fumigation experiments
with elevated concentrations of the pollutants performed in climatic
chambers or open top chambers. The duration of such experiments is
necessarily restricted. They are performed under climatic or growth conditions which rarely correspond to the climatic or growth conditions of
exposed forest sites.
For S02, Slovik et al. (1992a,b) have recently attempted to derive limits
of tolerance of healthy spruce on the basis of measured physiological data.
Form measurements of cation contents in xylem sap during the course of a
year, and considering the necessity of cation circulation in the tree, they
calculated a limit of long-term acidification tolerance which was below lOppb
S02 during part of the winter, but much higher during the summer. At any
time of the year, tolerance levels are lowered when cation availability is
restricted. In this situation, cation deficiency symptoms will develop even
when cation determinations in leaf extracts fail to indicate deficiency because
of vacuolar cation sequestration. In comparing the calculations with accepted
tolerance limits which permit averaging of pollution levels on an annual
basis, it must be emphasized that the highest levels actually occur during
the winter, when sensitivity of S02 is particularly high because metabolic
activity is low and cation transport in the xylem is slow. High winter levels
of S02 are presently legally tolerable because low summer levels produce an