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Sulphur dioxide and the scarcity of Pinus sylvestris in the industrial pennines
SULPHUR DIOXIDE AND THE SCARCITY OF PINUS SYLVESTRIS IN THE INDUSTRIAL PENNINES
,J. F. FARRAR,'~J. RELTON & A. J. RUTTER
Department of Botany and Plant Technology,i Imperial College, London, Great Britain
ABSTRACT
The followingi observations, consistent with the hypothesis that SO z is causal in restricting the distribution of Pinus sylvestris, were made: (a) the occurrence of P. sylvestris is negatively correlated with SO 2 levels; ( b ) the occurrence of P. sylvestris is negatively correlated with its needle S content and (c) needle S content is positively correlated with SO 2 levels. Criteria necessary to establish the hypothesis are defined.
INTRODUCTION
The growth of several plant species can be reduced by SO2 under controlled conditions: Lolium perenne by 191 #g SO 2 m -3 (Bell & Clough, 1973), Raphanus sativus by 131 #g m - 3 (Tingey e t al., 1971) and Pinus sylvestris by 153 and 165 #g m - 3 (Farrar et aL, 1977). It is possible that the growth of these species is reduced in those areas of Britain experiencing comparable mean SO2 levels. Indeed, it has been suggested for 75 years that plant growth in the industrial Pennines is affected by atmosphericlpollution~(Moss, 1901 ; IRuston,, 1921 ; Scurfield, !1955; Grayson,1956; Lines & Phillips, 1963). This paper adduces several lines of evidence consistentlwith',the !hypothesis that SO2 adversely affects the performance of Pinus sylvestris (a tree of economic importance) in the industrial Pennines and defines the criteria which need to be fulfilled before that hypothesis can be considered established. t Present address: Department of Botany, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, Great Britain.
63
Environ. Pollut. (14) (1977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
64
J. F. FARRAR, J. RELTON, A. J. RUTTER
MATERIALS A N D M E T H O D S
We recorded the distribution of P. sylvestris in an area extending from Liverpool to Leeds and from Sheffield to Preston. Young and old trees, distinguished by the presence or absence of a single terminal leader, were recorded separately. In the centre of this area, all woods marked on the 1 : 63,360 Ordnance Survey mapl(seventh series) were visited, whilst as many roads as possible were traversed throughout the entire region. In each 10 km square of the National Grid the number of 2 km squares in which P. sylvestris was found was divided by the number of kilometres of road traversed in that 10km square. A figure of 'occupied 2 k m squares per 10km traverse' was thus obtained for each 10km square; this measure is termed 'occurrence'. A minimum of 10km was traversed in each 10km square. If P. sylvestris could be seen up to 1 km from a road, the maximum possible occurrence would be 5.0, but the area that could be seen varied widely. It is assumed that low occurrence can be a reflection of adverse conditions for establishment or survival. In order to relate occurrence to SO2 levels, data from the National Survey of Air Pollution were used to provide a comparative estimate of SO2 levels in each 10 km square. Such an estimate is approximate because the pollution sampling sites are mainly in urban areas and record total titratable acidity rather than SO2. Of the seventy-nine 10 km squares in the survey, forty contained pollution sampling sites. In each of these squares, the winter mean level of SO2 for the years 1968-70 (National Survey of Air Pollution, 1972) was averaged over all sites in the square to obtain an SO 2 level for that square. Levels of sulphur in P. sylvestris needles collected in the area were assayed tubidimetrically (Basson & Bohmer, 1972; Farrar et al., 1977) in acid digests of unwashed needles.
RESULTS
The resultant map (Fig. 1) shows P. sylvestris to have a low occurrence in a corridor of land extending north and east from Liverpool and Manchester, whilst it is abundant in similar terrain north and south of this corridor. This region of scarcity is similar in shape and extent to the distribution of high rural SO 2 levels (Meetham, 1956; Saunders & Wood, 1973) and the distribution of certain pollution-sensitive lichen species (Hawksworth & Rose, 1970). In the forty squares containing pollution sampling sites, the winter mean level of SO 2, averaged over all sites in a 10km square for the period 1968-70, showed a significant negative correlation with occurrence of P. sylvestris (Fig. 2). Young and old trees considered separately both show similar correlations (legend to Fig. 2), in spite of the changing pattern of pollution in the area (National Survey of Air
SO 2 AND OCCURRENCE OF PINES
4
5
6
7
8
9
0
I
65
2
3
4
Key: occurrence of Pinus sy/vesfris
II 0 - 0.9
1- 1.9
2-2.9
3-3.9
4-4.9
Fig. 1. Distribution of Pinus sylvestris in the industrial Pennines. The marginal numbers refer to the National Grid. The area surveyed is enclosed by the heavy line, and tfeintilines demarcate 10 km squares. The towns markedare: Bo, Bolton; Br, Bradford; H, Huddersfield; Ha, Halifax; K, Kirkby; L, Leeds; Li, Liverpool; M, Manchester; P, Preston; S, Sheffield; W, Wigan; Wi, Widnes. The data used in compiling this and the other figures can be obtained from the authors. 'Occurrence' is explained in the text. P o l l u t i o n , 1972). Y o u n g trees are, however, f r e q u e n t in M a n c h e s t e r with high SO2, as a d e t a i l e d investigation by J. W . K i n n a i r d (pers. c o m m . ) has shown. T h e few m e a s u r e m e n t s available indicate t h a t m e a n winter rural SO2 levels in the a r e a s a r o u n d c o n u r b a t i o n s are 1Off- 150 pg m - 3 ( M e e t h a m , 1956'; N a t i o n a l Survey o f A i r P o l l u t i o n , 1972). M a t u r e P. sylvestris is a b s e n t f r o m a r e a s o f the R u h r with a n a n n u a l m e a n SO2 c o n c e n t r a t i o n o f m o r e t h a n 2 0 0 / ~ g m - 3 ( K n a b e , 1970).
J. F. FARRAR, J. RELTON, A. J. RUTTER
66
3-
eel •
§
"
.
o
.'. ;
•
. 11
I00
e•
• •
200
300
Mean Winter SO2 in tO km square (,u.g m-3) Fig. 2. R e l a t i o n s h i p between occurrence of P, syluestris a n d S O 2 levels. Occurrence is p l o t t e d a g a i n s t the m e a n winter S O 2 level for each l 0 k m s q u a r e (averaged over all sites in t h a t s q u a r e for the period 1968-70). The linear regression is y = 5.36 - 0.0216 ( + 0 - 0 0 5 5 ) x ; r = - 0 . 6 2 6 , P < 0-001, n = 40. Regression e q u a t i o n s for the y o u n g a n d old trees c o n s i d e r e d s e p a r a t e l y are; young, y = 3.5t - 0.0140 (_+0-0042)x; r = - 0 - 5 4 0 , P < 0.001; old, y = 3.57 - 0.0157 (_+0.0041)x; r = - 0 . 6 2 1 , P < 0.001; n = 40 in each case. 5
O 2-
,~
a'o
~'.o
40
Needle S content (mg g-l) Fig. 3. Needle S c o n t e n t s a n d the occurrence of P. sylvestris. The needle ~S contentm were averaged il-3 sites per 10 k m square. A t each site needles were collected f r o m three trees 1-2 m from the g r o u n d . The coefficient o f v a r i a t i o n at each site was a b o u t 15 %. The values are for needles p r o d u c e d in 1973 a n d collected in J u n e a n d October, 1974. The regression e q u a t i o n i s y = 6.75 - 1.864 ( + 0 . 2 3 9 ) x ; r = 0.903, P < 0.02, n = 15.
SO 2 AND OCCURRENCE OF PINES
67
7en
2" cO
Z
,o'o Mean Winter
2(o
SO2 in I0 km square
300 (,u.g rn- 3 )
Fig. 4. A t m o s p h e r i c S O 2 levels a n d needle S c o n t e n t s o f P. sylvestris. S O 2 levels a n d needle S c o n t e n t s were derived as described in the text. The regression e q u a t i o n is y = 0.438 + 0 . 0 1 1 9 ( + 0 . 0 0 6 3 ) x ; r = 0-903, P < 0-02, n = 6.
The correlation betweeen needle S level and occurrence is highly significant (Fig. 3), levels of needle S rising from 1.5-2-0 mg S g- ~needle dry weight in 10 km squares showing a high occurrence to 3.0-4.0 mg g- 1 in squares with a low occurrence (Fig. 3). Relationships between sulphur levels in the atmosphere and in foliage have been established elsewhere (Johansson, 1959; McGovern & Balsillie, 1973) but the present data (Fig. 4) are inconclusive because only six of the needle collections were made in squares containing SO 2 sampling sites. The highest needle S levels found here approach those reported to be associated with SO2 damage (Ishin, 1971). DISCUSSION
These data on distribution, correlation of occurrence with S O 2 levels and needle S contents are consistent with SO 2 being a causal factor in the distribution of P. sylvestris but are hardly proof of it. Many other explanations could be found for the relative scarcity of an introduced species in an environmentally and historically heterogeneous area. Some other reports of supposed SO 2 damage are also based solely on correlative evidence, no causal relationship being demonstrated (e.g. Scheffer & Hedgcock, 1955; Johansson, 1959; Guderian & Stratmann, 1968; Linzon, 1971). Whilst we favour the direct involvement of SO 2 in explaining our data, particularly in view of the known adverse effects of low jlevelsof SO 2 on the growth of P. sylvestris (Farrar et al., 1977), less equivocal evidence is necessary.
68
J. F. FARRAR,J. RELTON,A. J. RUTTER
This can best be provided by the use of a physiological test, the results o f which (a) are positive when, a n d only when, growth is being reduced by SO 2 a n d (b) are correlated with the intensity of effect of SO 2. I n conclusion, we believe that e n o u g h d a t a s u p p o r t the idea o f SO 2 reducing tree growth in the industrial P e n n i n e s to m a k e a n intensive study worth while. Such a study should examine the possibility that SO 2 is particularly i m p o r t a n t where factors such as exposure are themselves adversely affecting tree growth. ACKNOWLEDGEMENTS This research was supported by a g r a n t from the N a t u r a l E n v i r o n m e n t Research Council. We t h a n k J. W. K i n n a i r d for u n p u b l i s h e d data, S. G. G a r s e d for some pine records a n d J. N. B. Bell a n d J. R. T h o m p s o n for discussions. REFERENCES
BASSON,W. D. & BOHMER,R. G. (1972). An automated procedure for the determination of sulphur in plant tissue. Analyst, Lond., 97, 266-70. BELL, J. N. B. & CLOUGH,W. S. (1973). Depression of yield in ryegrass exposed to sulphur dioxide. Nature, Lond., 241, 47-9. FARRAR,J. F., RELTON,J. & RUTTER,A. J. (1977). Sulphur dioxide and the growth ofPinus sylvestris. J. Appl. Ecol., 14. GRAYSON,A. J. (1956). Effects of atmospheric pollution on forestry. Nature, Lond., 178, 719-21. GUDERIAN,R. & STRATMANN,H. 0968). Freilandversuchezur Ermittlung yon Schwefeldioxydwirkungen!aufldieVegetation.~III. Grenzwerte.sch/idlicherSO2-Immissionenfor Obst- und Forstkulturen sowie fiir landwirtschaftlicheund g/irtnerisehe Pflanzenarten. ForschBer. Landes NRhein-Westj~, l-ll4. HAWKSWORTH,D. L. & ROSE,F. (1970). Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature, Lond., 227, 145-8. lsulr~, Y. D. (1971). Content of sulfur in needles of pine stands in the Moscow forest-park belt. Abstr. Bull. Inst. Pap. Chem. 42, 5400. JOHANNSON, O. (1959). On sulfur problems in Swedish Agriculture. K. LantbrHi~gsk. Annlr, 25, 59-169. KNAaE, W. (1970). Kiefernwaldverbreitingund Schwefeldioxid-lmmissionenim Ruhrgebeit. Staub, 39, 32-5. LINES, R. & PHILLIPS,D. W. (1963). Air pollution in forestry. J. For. Commn, 32, 97-100. LINZON,S. N. (197l). Economiceffects of sulfur dioxide on forest growth. J. Air Pollut, Control Ass., 21, 81-6. McGOVERN,P. C. & BALSILLIE, D. (1973). Sulphur dioxide-heavy metal levels and vegetation effects in the Sudbury area. Sudbury, Ontario, Air Management Branch, Ontario Ministry of the Environment. MEETHAM, A. R. (1956). Atmospheric pollution: Its origin and prevention. 2nd edition. London, Pergamon Press. Moss, C. E. (1901). Changes in the Halifax flora in the last century and a quarter. Naturalist, 26, 99-107. NATIONALSURVEYOFAIR POLLUTION,1961--71(1972). Volume 2, The South West, Wales, and NorthWest. Warren Spring Laboratory, HMSO. RUSTOH,IA. G. (1921).[The[plant haslan indicator of smoke pollution. Ann. appl. Biol, 7, 390-402. SAUNDERS,P. W. & WOOD,C. W. (1973). Sulphur dioxide in the environment: Its production, dispersal and fate. In Air pollution and lichens, ed. by B. W. Ferry, M. S. Baddeley and D. L. Hawksworth, 6-37. London, Athlone Press. SCHEFFER,T. C. & HEDC,COCK,G. C. (1955). ~njuryto northwestern forest trees by sulfur dioxide from smelters. Tech. Bull. United States Department of Agriculture Forest Services, No. 1117. SCUgFIELD,G. (1955). Atmospheric pollution considered in relation to horticulture. J. R. hort. Soc., 80,23-101. TINGLY,D. T., HECK,W. W. & REINERT,R. A. (1971). Effect of low concentrations of ozone and sulfur dioxide on foliage, growth and yield of radish. J. Am. Soc. hort. Sci., 96, 369-71.
,J. F. FARRAR,'~J. RELTON & A. J. RUTTER
Department of Botany and Plant Technology,i Imperial College, London, Great Britain
ABSTRACT
The followingi observations, consistent with the hypothesis that SO z is causal in restricting the distribution of Pinus sylvestris, were made: (a) the occurrence of P. sylvestris is negatively correlated with SO 2 levels; ( b ) the occurrence of P. sylvestris is negatively correlated with its needle S content and (c) needle S content is positively correlated with SO 2 levels. Criteria necessary to establish the hypothesis are defined.
INTRODUCTION
The growth of several plant species can be reduced by SO2 under controlled conditions: Lolium perenne by 191 #g SO 2 m -3 (Bell & Clough, 1973), Raphanus sativus by 131 #g m - 3 (Tingey e t al., 1971) and Pinus sylvestris by 153 and 165 #g m - 3 (Farrar et aL, 1977). It is possible that the growth of these species is reduced in those areas of Britain experiencing comparable mean SO2 levels. Indeed, it has been suggested for 75 years that plant growth in the industrial Pennines is affected by atmosphericlpollution~(Moss, 1901 ; IRuston,, 1921 ; Scurfield, !1955; Grayson,1956; Lines & Phillips, 1963). This paper adduces several lines of evidence consistentlwith',the !hypothesis that SO2 adversely affects the performance of Pinus sylvestris (a tree of economic importance) in the industrial Pennines and defines the criteria which need to be fulfilled before that hypothesis can be considered established. t Present address: Department of Botany, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, Great Britain.
63
Environ. Pollut. (14) (1977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
64
J. F. FARRAR, J. RELTON, A. J. RUTTER
MATERIALS A N D M E T H O D S
We recorded the distribution of P. sylvestris in an area extending from Liverpool to Leeds and from Sheffield to Preston. Young and old trees, distinguished by the presence or absence of a single terminal leader, were recorded separately. In the centre of this area, all woods marked on the 1 : 63,360 Ordnance Survey mapl(seventh series) were visited, whilst as many roads as possible were traversed throughout the entire region. In each 10 km square of the National Grid the number of 2 km squares in which P. sylvestris was found was divided by the number of kilometres of road traversed in that 10km square. A figure of 'occupied 2 k m squares per 10km traverse' was thus obtained for each 10km square; this measure is termed 'occurrence'. A minimum of 10km was traversed in each 10km square. If P. sylvestris could be seen up to 1 km from a road, the maximum possible occurrence would be 5.0, but the area that could be seen varied widely. It is assumed that low occurrence can be a reflection of adverse conditions for establishment or survival. In order to relate occurrence to SO2 levels, data from the National Survey of Air Pollution were used to provide a comparative estimate of SO2 levels in each 10 km square. Such an estimate is approximate because the pollution sampling sites are mainly in urban areas and record total titratable acidity rather than SO2. Of the seventy-nine 10 km squares in the survey, forty contained pollution sampling sites. In each of these squares, the winter mean level of SO2 for the years 1968-70 (National Survey of Air Pollution, 1972) was averaged over all sites in the square to obtain an SO 2 level for that square. Levels of sulphur in P. sylvestris needles collected in the area were assayed tubidimetrically (Basson & Bohmer, 1972; Farrar et al., 1977) in acid digests of unwashed needles.
RESULTS
The resultant map (Fig. 1) shows P. sylvestris to have a low occurrence in a corridor of land extending north and east from Liverpool and Manchester, whilst it is abundant in similar terrain north and south of this corridor. This region of scarcity is similar in shape and extent to the distribution of high rural SO 2 levels (Meetham, 1956; Saunders & Wood, 1973) and the distribution of certain pollution-sensitive lichen species (Hawksworth & Rose, 1970). In the forty squares containing pollution sampling sites, the winter mean level of SO 2, averaged over all sites in a 10km square for the period 1968-70, showed a significant negative correlation with occurrence of P. sylvestris (Fig. 2). Young and old trees considered separately both show similar correlations (legend to Fig. 2), in spite of the changing pattern of pollution in the area (National Survey of Air
SO 2 AND OCCURRENCE OF PINES
4
5
6
7
8
9
0
I
65
2
3
4
Key: occurrence of Pinus sy/vesfris
II 0 - 0.9
1- 1.9
2-2.9
3-3.9
4-4.9
Fig. 1. Distribution of Pinus sylvestris in the industrial Pennines. The marginal numbers refer to the National Grid. The area surveyed is enclosed by the heavy line, and tfeintilines demarcate 10 km squares. The towns markedare: Bo, Bolton; Br, Bradford; H, Huddersfield; Ha, Halifax; K, Kirkby; L, Leeds; Li, Liverpool; M, Manchester; P, Preston; S, Sheffield; W, Wigan; Wi, Widnes. The data used in compiling this and the other figures can be obtained from the authors. 'Occurrence' is explained in the text. P o l l u t i o n , 1972). Y o u n g trees are, however, f r e q u e n t in M a n c h e s t e r with high SO2, as a d e t a i l e d investigation by J. W . K i n n a i r d (pers. c o m m . ) has shown. T h e few m e a s u r e m e n t s available indicate t h a t m e a n winter rural SO2 levels in the a r e a s a r o u n d c o n u r b a t i o n s are 1Off- 150 pg m - 3 ( M e e t h a m , 1956'; N a t i o n a l Survey o f A i r P o l l u t i o n , 1972). M a t u r e P. sylvestris is a b s e n t f r o m a r e a s o f the R u h r with a n a n n u a l m e a n SO2 c o n c e n t r a t i o n o f m o r e t h a n 2 0 0 / ~ g m - 3 ( K n a b e , 1970).
J. F. FARRAR, J. RELTON, A. J. RUTTER
66
3-
eel •
§
"
.
o
.'. ;
•
. 11
I00
e•
• •
200
300
Mean Winter SO2 in tO km square (,u.g m-3) Fig. 2. R e l a t i o n s h i p between occurrence of P, syluestris a n d S O 2 levels. Occurrence is p l o t t e d a g a i n s t the m e a n winter S O 2 level for each l 0 k m s q u a r e (averaged over all sites in t h a t s q u a r e for the period 1968-70). The linear regression is y = 5.36 - 0.0216 ( + 0 - 0 0 5 5 ) x ; r = - 0 . 6 2 6 , P < 0-001, n = 40. Regression e q u a t i o n s for the y o u n g a n d old trees c o n s i d e r e d s e p a r a t e l y are; young, y = 3.5t - 0.0140 (_+0-0042)x; r = - 0 - 5 4 0 , P < 0.001; old, y = 3.57 - 0.0157 (_+0.0041)x; r = - 0 . 6 2 1 , P < 0.001; n = 40 in each case. 5
O 2-
,~
a'o
~'.o
40
Needle S content (mg g-l) Fig. 3. Needle S c o n t e n t s a n d the occurrence of P. sylvestris. The needle ~S contentm were averaged il-3 sites per 10 k m square. A t each site needles were collected f r o m three trees 1-2 m from the g r o u n d . The coefficient o f v a r i a t i o n at each site was a b o u t 15 %. The values are for needles p r o d u c e d in 1973 a n d collected in J u n e a n d October, 1974. The regression e q u a t i o n i s y = 6.75 - 1.864 ( + 0 . 2 3 9 ) x ; r = 0.903, P < 0.02, n = 15.
SO 2 AND OCCURRENCE OF PINES
67
7en
2" cO
Z
,o'o Mean Winter
2(o
SO2 in I0 km square
300 (,u.g rn- 3 )
Fig. 4. A t m o s p h e r i c S O 2 levels a n d needle S c o n t e n t s o f P. sylvestris. S O 2 levels a n d needle S c o n t e n t s were derived as described in the text. The regression e q u a t i o n is y = 0.438 + 0 . 0 1 1 9 ( + 0 . 0 0 6 3 ) x ; r = 0-903, P < 0-02, n = 6.
The correlation betweeen needle S level and occurrence is highly significant (Fig. 3), levels of needle S rising from 1.5-2-0 mg S g- ~needle dry weight in 10 km squares showing a high occurrence to 3.0-4.0 mg g- 1 in squares with a low occurrence (Fig. 3). Relationships between sulphur levels in the atmosphere and in foliage have been established elsewhere (Johansson, 1959; McGovern & Balsillie, 1973) but the present data (Fig. 4) are inconclusive because only six of the needle collections were made in squares containing SO 2 sampling sites. The highest needle S levels found here approach those reported to be associated with SO2 damage (Ishin, 1971). DISCUSSION
These data on distribution, correlation of occurrence with S O 2 levels and needle S contents are consistent with SO 2 being a causal factor in the distribution of P. sylvestris but are hardly proof of it. Many other explanations could be found for the relative scarcity of an introduced species in an environmentally and historically heterogeneous area. Some other reports of supposed SO 2 damage are also based solely on correlative evidence, no causal relationship being demonstrated (e.g. Scheffer & Hedgcock, 1955; Johansson, 1959; Guderian & Stratmann, 1968; Linzon, 1971). Whilst we favour the direct involvement of SO 2 in explaining our data, particularly in view of the known adverse effects of low jlevelsof SO 2 on the growth of P. sylvestris (Farrar et al., 1977), less equivocal evidence is necessary.
68
J. F. FARRAR,J. RELTON,A. J. RUTTER
This can best be provided by the use of a physiological test, the results o f which (a) are positive when, a n d only when, growth is being reduced by SO 2 a n d (b) are correlated with the intensity of effect of SO 2. I n conclusion, we believe that e n o u g h d a t a s u p p o r t the idea o f SO 2 reducing tree growth in the industrial P e n n i n e s to m a k e a n intensive study worth while. Such a study should examine the possibility that SO 2 is particularly i m p o r t a n t where factors such as exposure are themselves adversely affecting tree growth. ACKNOWLEDGEMENTS This research was supported by a g r a n t from the N a t u r a l E n v i r o n m e n t Research Council. We t h a n k J. W. K i n n a i r d for u n p u b l i s h e d data, S. G. G a r s e d for some pine records a n d J. N. B. Bell a n d J. R. T h o m p s o n for discussions. REFERENCES
BASSON,W. D. & BOHMER,R. G. (1972). An automated procedure for the determination of sulphur in plant tissue. Analyst, Lond., 97, 266-70. BELL, J. N. B. & CLOUGH,W. S. (1973). Depression of yield in ryegrass exposed to sulphur dioxide. Nature, Lond., 241, 47-9. FARRAR,J. F., RELTON,J. & RUTTER,A. J. (1977). Sulphur dioxide and the growth ofPinus sylvestris. J. Appl. Ecol., 14. GRAYSON,A. J. (1956). Effects of atmospheric pollution on forestry. Nature, Lond., 178, 719-21. GUDERIAN,R. & STRATMANN,H. 0968). Freilandversuchezur Ermittlung yon Schwefeldioxydwirkungen!aufldieVegetation.~III. Grenzwerte.sch/idlicherSO2-Immissionenfor Obst- und Forstkulturen sowie fiir landwirtschaftlicheund g/irtnerisehe Pflanzenarten. ForschBer. Landes NRhein-Westj~, l-ll4. HAWKSWORTH,D. L. & ROSE,F. (1970). Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature, Lond., 227, 145-8. lsulr~, Y. D. (1971). Content of sulfur in needles of pine stands in the Moscow forest-park belt. Abstr. Bull. Inst. Pap. Chem. 42, 5400. JOHANNSON, O. (1959). On sulfur problems in Swedish Agriculture. K. LantbrHi~gsk. Annlr, 25, 59-169. KNAaE, W. (1970). Kiefernwaldverbreitingund Schwefeldioxid-lmmissionenim Ruhrgebeit. Staub, 39, 32-5. LINES, R. & PHILLIPS,D. W. (1963). Air pollution in forestry. J. For. Commn, 32, 97-100. LINZON,S. N. (197l). Economiceffects of sulfur dioxide on forest growth. J. Air Pollut, Control Ass., 21, 81-6. McGOVERN,P. C. & BALSILLIE, D. (1973). Sulphur dioxide-heavy metal levels and vegetation effects in the Sudbury area. Sudbury, Ontario, Air Management Branch, Ontario Ministry of the Environment. MEETHAM, A. R. (1956). Atmospheric pollution: Its origin and prevention. 2nd edition. London, Pergamon Press. Moss, C. E. (1901). Changes in the Halifax flora in the last century and a quarter. Naturalist, 26, 99-107. NATIONALSURVEYOFAIR POLLUTION,1961--71(1972). Volume 2, The South West, Wales, and NorthWest. Warren Spring Laboratory, HMSO. RUSTOH,IA. G. (1921).[The[plant haslan indicator of smoke pollution. Ann. appl. Biol, 7, 390-402. SAUNDERS,P. W. & WOOD,C. W. (1973). Sulphur dioxide in the environment: Its production, dispersal and fate. In Air pollution and lichens, ed. by B. W. Ferry, M. S. Baddeley and D. L. Hawksworth, 6-37. London, Athlone Press. SCHEFFER,T. C. & HEDC,COCK,G. C. (1955). ~njuryto northwestern forest trees by sulfur dioxide from smelters. Tech. Bull. United States Department of Agriculture Forest Services, No. 1117. SCUgFIELD,G. (1955). Atmospheric pollution considered in relation to horticulture. J. R. hort. Soc., 80,23-101. TINGLY,D. T., HECK,W. W. & REINERT,R. A. (1971). Effect of low concentrations of ozone and sulfur dioxide on foliage, growth and yield of radish. J. Am. Soc. hort. Sci., 96, 369-71.