IMPACTS OF A C I D PRECIPITATION ON CONIFEROUS

FOREST ECOSYSTEMS l GUNNAR ABRAHAMSEN ,

RICHARD HORNTVEDT~ and BJORN T V E I T E ~ . The

SNSF-project (Acid P r e c i p i t a t i o n Fish) , 1432 is-NLH,Norway

-

E f f e c t s on Forest and

ABSTRACT

This paper summarizes t h e r e s u l t s from c u r r e n t s t u d i e s i n Norway. One main approach i s t h e a p p l i c a t i o n of a r t i f i c i a l a c i d " r a i n " and of lime t o f i e l d p l o t s and lysimeters. Application during two growth seasons of 50 mrn p e r month of " r a i n water" of pH 3 t o a podzol s o i l increased t h e a c i d i t y of t h e humus and decreased t h e base s a t u r a t i o n . The reduct i o n i n base s a t u r a t i o n was mainly due t o leaching of calcium and magnesium. Laboratory experiments revealed t h a t decomp o s i t i o n of pine needles was n o t a f f e c t e d by any a c i d " r a i n " treatment of t h e f i e l d p l o t s . Liming s l i g h t l y retarded t h e decomposition. No n i t r i f i c a t i o n occurred i n unlimed s o i l s (pH 4.4 4.1). Liming increased n i t r i f i c a t i o n . The s o i l enchytraeid (Oligochaeta) fauna was n o t much a f f e c t e d by t h e a c i d i f i c a t i o n . Germination of spruce seeds i n a c i d i f i e d mineral s o i l was negatively a f f e c t e d when s o i l pH was 4.0 o r lower. Seedling establishment was even more s e n s i t i v e t o i n c r e a s i n g s o i l a c i d i t y . Analysis of t h r o u g h f a l l and stemflow water i n southernmost Norway r e v e a l s t h a t t h e t o t a l deposition of sulphuric a c i d beneath spruce and pine i s approximately two times t h e deposition i n open t e r r a i n . A l a r g e p a r t of t h i s i n c r e a s e i s probably due t o dry deposit i o n . Increased a c i d i t y of t h e r a i n seems t o increase t h e leaching of c a t i o n s from t h e t r e e crowns. Tree-ring analysis of spruce (Picea a b i e s ( L . ) Karst.) and p i n e Pinus s y l v e s t r i s L.) has been based on comparisons between regions d i f f e r e n t l y s t r e s s e d by a c i d p r e c i p i t a t i o n and a l s o between s i t e s presumed t o d i f f e r i n s e n s i t i v i t y t o a c i d i f i c a t i o n . No e f f e c t t h a t can be r e l a t e d t o a c i d p r e c i p i t a t i o n has y e t been detected on diameter growth.

-

'SNSF-contribution FA 1/75 ' ~ u t h o r s' address : Norwegian F o r e s t Research I n s t i t u t e . 1432

As-NLH, Norway

1.

INTRODUCTION

Southern Norway receives acidic pollutants with air and precipitation far in excess of what could be expected from the emissions within the region. The monthly mean pH of the precipitation varies between 4 and 5. The deposition of acids represents a possible threat to forest and fresh-water ecosystems. An increasing number of rivers and lakes are becoming too acidic for trout (Jensen & Snekvik 1972). Occasional mass-dying of trout has been observed. Estimates of an anticipated reduction in forest growth have been given (Dahl & Skre 1971). Lysimeter studies have indicated a dramatic increase in the leaching of calcium from soil when the percolating water pH drops below 3 (Overrein 1972). The great concern over these problems in Norway has resulted in the establishment of a large nationwide project named "Acid Precipitation - Effects on Forest and Fish" (Overrein & Abrahamsen 1975). The aim of the forest-research portion of this project is primarily to study the effects of acid precipitation on forest growth and development, secondly to study the effects on main processes influencing forest growth and development. The present paper summarizes the main approaches, and the results obtained so far, without comprehensive discussion.

2.

EFFECTS ON SOIL

The effects of acid precipitation on soil properties and processes are undoubtedly very complicated. Experiments with simplified systems in the laboratory must therefore be controlled by field experiments where the complex interaction of climate, organisms, and organic and inorganic matter are largely undisturbed. Because most field experiments have,to be pursued over a long period of time, these experiments have been given priority in the starting phase of the project. The soil studies include investigations on soil fauna, decomposition and soil chemistry. 2.1.

Experimental design

The soil studies are based on the application of lime and of simulated acid rain, to field plots and lysimeters. The aim of these experiments is to obtain information on the effect of acid precipitation on soil properties, soil processes, and plant growth. At present, results from one plot experiment and one lysimeter study, both started in 1972, are available. These particular experiments were located in a

semipodzolic s o i l (intermediate p r o f i l e between podzol and brown e a r t h ) , developed i n a sandy, g l a c i f l u v i a l sediment. S o i l chemical c h a r a c t e r i s t i c s appear i n Table 1. The experimental a r e a , which i s s i t u a t e d

Table 1.

Soil horizon

Soil depth cm

Soil chemical characteristics of the field experiments.

Loss on ignitlon

PH(n20)

0

Soil chemlcal properties- ( c - 2 5)-_-Catlon exchanqe Base capacity saturation meq/100 g 8

-N in 0 of ovendry materlal

I

approximately 40 km north of Oslo, was c l e a r c u t about 1960 and r e f o r e s t e d with Pinus contorta Douglas, i n 1965. I n 1974 t h e t r e e s were between 2 and 4 m high. The ground f l o r a i s completely dominated by Deschampsia flexuosa (L ) Trin

.

.

Three by four meter p l o t s were t r e a t e d with lime and " r a i n " of various a c i d i t y (Table 2 ) . "Rain water" q u a l i t y was a d j u s t e d with s u l p h u r i c acid. The simulated r a i n was applied i n a d d i t i o n t o t h e n a t u r a l p r e c i p i t a t i o n , and was applied once a month i n t h e non-frozen period of t h e year. The lysimeters a r e 29.5 cm wide and 45 cm high f i b e r g l a s s cylinders. Each i s f i l l e d with an undisturbed s o i l monolith according t o t h e procedure described by Overrein (1968). Treatments appear i n Table

The l e a c h a t e from t h e lysimeters was c o l l e c t e d i n 5 - l i t e r p l a s t i c containers. Each week, 100 m l of t h e leachate was analysed f o r pH, conductivity and colour. The remaining leachate was s t o r e d i n t h e f i e l d and preserved by adding H C 1 (10 m l p e r 1). A t t h e end of each month, one l i t e r of t h e preserved leachate f o r t h e month was used f o r chemical a n a l y s i s .

Treatment of the field plot (P) and the lysimeter

Table 2.

(L) experiment. Lime was applied as ground limestone. Experimental design: Complete randomization with 3 replicates in the plot experiment and 2 in the lysimeter study. i RR lGAf

ION

No i r r igat ion

Liming

P

N o l ims

25 m per m o n t h pH4

pH3

pH5.7

P L

P L

P

P

P

P L P P P

pH5.7

1500 k g CaO/ha

2.2.

3000

"

6000

"

" "

5 0 mm per m o n t h pH4

P

pH3

P

Soil zoology

Soil samples collected from the field experiment in October 1974 were extracted for enchytraeids (Oligochaeta) which is one of the most abundant groups of animals in coniferous forest soils. The procedures used in sampling, extraction, etc. are described elsewhere (Abrahamsen 1972). Other groups of soil animals were not considered. Statistical analyses did not reveal any significant influence of the various treatments on the total abundance of enchytraeids. The three main species, however, seem to respond differently to the various treatments. Cognettia spagnetorum (Vejdovsky, 1877) is a dominant species in acidic raw-humus habitats (e.g. Abrahamsen 1972). Populations of this organism were significantly reduced by liming, but not by the'bcid rain". Enchytronia parva Nielsen & Christensen, 1959, is confined to more fertile soils (Abrahamsen 1972). This organism increased in numbers with increasing acidity of the rain at a high lime concentration and decreased with increasing acidity at a low lime concentration. (Significant interaction at 1% level). The abundance of the third main species, Enchytraeus norvegicus Abrahamsen, 1969, has not been significantly influenced by any treatment. 2.3.

Soil microbiology

Studies on microbiological processes have been focused on the decomposition of organic matter, nitrification and nitrogen fixation. No results are yet available for nitrogen fixation.

&

Decomposition has been studied on needles of Pinus contorta (Ishac Hovland 1975). The needles were collected from the field in 1973 and

1974 in the month of November. They were incubated at 15' and 25' on moistened glass-wool in conical flasks. Weight loss was measured after 90 days. Decomposition was significantly increased by increasing temperature from 15' to 25' C, significantly decreased by liming (3000 kg CaO per ha) and not significantly influenced by the acidity (pH 5.7 and pH 3) of the "rain". Decomposition was also examined on needles from the control plots. The needles were moistened with various dilutions of sulphuric acid. The experiment showed that the decomposition rate (weight loss) was significantly (P3500

Birch

800-)3500

C1

NO3

NH4

stemflow.

Ibl, o r lower

(U,

than t h e

values are indicated.

Birkenes,

mg p e r 1 . PO4 SO4 Ea

Ng

Na

K

1.0h

4.7

Norway,

a Strong cid

1974

pH

0.2

1.4

0 . 9 4 1 11.7

4.0

1

0.07

4.9

21.2b 0.3

0.3

0.03

20.3h 4.8h

1.7h 10.3h

3.7

h

0.42

3.8

1 0.1

10.13

15.0h 2.3h

1.2h

1.5

0.34h

3.7

8.5 11.4h

0.1

3.5h

>-

4.6

1 1

ammonium. I t i s f u r t h e r noteworthy t h a t while b i r c h t h r o u g h f a l l was l e s s a c i d i c than i n c i d e n t r a i n f a l l , stemflow was more a c i d i c . A conclusive i n t e r p r e t a t i o n of t h e r e s u l t s appears t o be impossi-

b l e a t present. I t i s probable t h a t a l a r g e r p a r t of t h e t h r o u g h f a l l enrichment i n c h l o r i d e , sulphate, calcium, sodium, and hydrogen i o n s i n Birkenes i s derived from dry d e p o s i t s than from leached metabolites. F u r t h e r , hydrogen ions from dry and/or wet d e p o s i t s might r e p l a c e o t h e r c a t i o n s i n t h e t r e e crowns. C

3.2.

Germination and establishment

The r e s u l t s presented here a r e based experiments c a r r i e d o u t with seeds of spruce (Picea a b i e s (L.) Karst.) and pine (Pinus s y l v e s t r i s L.) i n a r t i f i c i a l l y a c i d i f i e d mineral s o i l (Teigen 1975). S o i l s of d i f f e r e n t pH were obtained by p e r c o l a t i n g mineral s o i l with 1500 mm water of d i f f e r e n t pH values. Additional s o i l pH l e v e l s were obtained by liming. The s o i l used was derived from a podzolized morainic s o i l with a low content of p l a n t n u t r i e n t s ( c a t i o n exchange capacity 10 meq/100 g and base s a t u r a t i o n 2 p e r c e n t ) . Seeds from one open p o l l i n a t e d t r e e of each s p e c i e s were t r a n s f e r r e d t o t h e a c i d i f i e d s o i l s . An adequate moisture regime was obtained by adding d i s t i l l e d water. The number of germinated seeds and establ i s h e d seedlings was recorded a f t e r 7 weeks. Seeds were defined t o have germinated when t h e emerged embryo was a t l e a s t 3 xnm long. Seedlings were defined a s e s t a b l i s h e d when t h e primary needles were developed. The experiments with pine covered a s o i l pH range of 4.0 t o 4.6. No e f f e c t s upon germination o r establishment were found within t h i s range. The experiments with spruce covered a s o i l pH range of 3.8 t o 5.6. S i g n i f i c a n t e f f e c t s of s o i l pH upon both germination and establishment were demonstrated (Figures 2 and 3 ) . Germination seems t o have a r a t h e r broad optimum around pH 4.8. Establishment seems t o have a more narrow optimum around pH 4.9. About 80 p e r c e n t of t h e seeds d i d n o t develop

y

=

- 3 9 3 . 0 6 + 2 0 5 . 34. ( p H ) R'

= 0,405

21.65, ( p ~ ) Z

s = 7.38

Figure 2. Relation between s o i l reaction and percentage of spruce seed germination.

m

o

0 y

0

-

= - I 1 8 8 . 0 5 + 5 1 9 . 4 9 . (pH) 5 2 . 8 6 , ( p ~ ) Z R~ = 0 , 7 7 1 s = 10.78

Figure 3 . Relation between s o i l reaction and percentage of spruce seedlings established. normal seedlings a t pH 3 . 8 . A large number of these plants had developed roots, but the roots did not penetrate i n t o the s o i l . Variations i n the nutrient content of the s o i l s had no apparent e f f e c t upon germinat i o n or establishment. 3.3.

Tree growth

Possible e f f e c t s of acid precipitation upon t r e e growth are p a r t l y being studied i n f i e l d experiments with simulated acid r a i n and p a r t l y by analysing past t r e e growth i n different regions and on d i f f e r e n t s i t e s . A t present only preliminary r e s u l t s from t r e e ring analyses are available.

T r e e r i n g a n a l y s e s have been used i n s t u d i e s of a i r p o l l u t i o n e f f e c t s around l o c a l emission s o u r c e s (e.g. Pollanschutz 1971, Sundberg 1974). A s f a r a s we know t h e o n l y a t t e m p t t o use t h i s approach i n a n a l y s e s o f r e g i o n a l a c i d i f i c a t i o n e f f e c t s has been made by Jonsson & Sundberg (1972)

.

Our approach and models l e a n h e a v i l y upon t h e Swedish s t u d y of Jonsson & Sundberg (1972). Tree r i n g development i s compared: 1) b e t ween r e g i o n s presumed o r known t o have d i f f e r e n t i n p u t s of a c i d p r e c i p i t a t i o n and 2 ) w i t h i n r e g i o n s between s i t e s supposed t o d i f f e r i n s e n s i t i v i t y owing t o s o i l p r o p e r t i e s . Increment c o r e s of spruce (Picea a b i e s ( L . ) K a r s t . ) and p i n e (Pinus s y l v e s t r i s L . ) a r e o b t a i n e d from t h e National F o r e s t Survey which annually c o l l e c t s information about t h e Norwegian f o r e s t s according t o a s y s t e m a t i c c l u s t e r e d sampling scheme. Sample p l o t information i s used t o s t r a t i f y t h e m a t e r i a l i n t o s i t e o r r e g i o n a l groups. I t i s necessary i n t h e models used t o have a r e f e r e n c e p e r i o d b e f o r e t h e a c i d i f i c a t i o n e f f e c t s s t a r t e d . A t t h i s s t a g e of t h e s t u d y , t h e p e r i o d from t h e y e a r 1927 and onwards has mainly been i n v e s t i g a t e d . Trees of t h e same s p e c i e s covering t h e whole i n v e s t i g a t i o n p e r i o d a r e p u t t o g e t h e r w i t h i n sample p l o t s t o form an average-plot t r e e - r i n g s e r i e s . I n t h e models adopted, t h e f a c t o r s i n f l u e n c i n g growth a r e supposed t o a c t m u l t i p l i c a t i v e l y . Additive models a r e o b t a i n e d by using l o g a r i t h m i c r i n g width a s t h e dependent v a r i a b l e .

P l o t s w i t h i n t h e same region o r s i t e group a r e combined t o form average t r e e r i n g s e r i e s f o r t h e s e p a r a t e groups. Two groups a r e f u r t h e r compared by forining a d i f f e r e n c e s e r i e s which shows t h e r e l a t i v e t r e e r i n g development of t h e groups. The d i f f e r e n c e s e r i e s a r e analysed f o r p o s s i b l e t r e n d changes. The a n a l y s e s a r e complicated by a number of f a c t o r s . I d e a l l y t h e r e should be a n e g l i g i b l e i n t e r a c t i o n between group and growth y e a r . This i s probably n o t t h e c a s e i n comparisons between g e o g r a p h i c a l l y s e p a r a t e d r e g i o n s o r i n comparisons between r e g i o n s which a r e extreme i n e c o l o g i c a l f a c t o r s . A u t o c o r r e l a t i o n i n t h e time s e r i e s i s a n o t h e r fact o r which d e c r e a s e s t h e s e n s i t i v i t y of t h e a n a l y s e s . Other complicating f a c t o r s a r e p o s s i b l e d i f f e r e n c e s i n c u t t i n g p r a c t i c e s and i n t e n s i t y , s t a n d h i s t o r y , and t h e normal time-trend between r e g i o n s o r S i t e groups t h a t a r e compared. The e f f e c t of t h e s e f a c t o r s must be e v a l u a t e d i n a d d i t i o n t o t h e time s e r i e s a n a l y s e s . F i g u r e s 4 and 5 show t h e average tree r i n g development i n p i n e and spruce i n two geographic r e g i o n s S d r l a n d e t and & s t l a n d e t , i n South Norway and E a s t Norway r e s p e c t i v e l y - a s w e l l a s t h e d i f f e r e n c e s e r i e s between t h e regions. S d r l a n d e t r e c e i v e s more a c i d p r e c i p i t a t i o n and i s a l s o supposed t o be more s e n s i t i v e t o a c i d i f i c a t i o n due t o s h a l l o w s o i l s .

-

-

Diff.logmi, +

0.10

1

Difference series Ssrlandet Ostlandet

-

-

SPRUCE

Logmi,

F i g u r e 4. Tree r i n g development of spruce i n d i f f e r e n t r e g i o n s and a d i f f e r e n c e s e r i e s between t h e r e g i o n s .

-

Difference series Ssrlandet Ostlandet

Diff.log,i,

-

+ 0.10

- 0.10

-

PINE

-

Logloi, 1.20 1.10

-

am

-

I

1930

1940

1950

.--..

1960

SOrlMdml

a.tlmda

817 plots -- 163 plots

1970

Figure 5. Tree r i n g development o f p i n e i n d i f f e r e n t r e g i o n s and a d i f f e r e n c e s e r i e s between t h e r e g i o n s . The main f e a t u r e s o f ring-width development a r e similar i n t h e two r e g i o n s p o i n t i n g back t o common components of c l i m a t i c growth f a c t o r s and time t r e n d s . The d i f f e r e n c e s e r i e s r e v e a l some f l u c t u a t i o n s i n t h e r e l a t i v e tree r i n g development. I n t e r a c t i o s between r e g i o n and y e a r

seem t o be a main component of t h e s e f l u c t u a t i o n s according t o prelimi n a r y analyses. These i n t e r a c t i o n s decrease t h e s e n s i t i v i t y of analys e s of t r e n d changes. The year 1950 has been used a s a s t a r t i n g p o i n t f o r p o s s i b l e a c i d i f i c a t i o n e f f e c t s i n t h e Swedish study. There a r e , however, no i n d i c a t i o n s of a r e l a t i v e l y slower growth within Sfirlandet a f t e r t h i s year. The region p s t l a n d e t i s , on t h e o t h e r hand, a l s o influenced by a c i d p r e c i p i t a t i o n and does n o t give an i d e a l reference. The d a t a from one region (Sdrlandet) have been more f u l l y analysed i n an attempt t o e l u c i d a t e t h e p o s s i b l e e f f e c t of d i f f e r e n t productivit y ( t h e s i t e - c l a s s concept i n f o r e s t terminology), vegetation type, water regime, s o i l depth, and t r e e s p e c i e s i n r e l a t i o n t o t h e a c i d i f i c a t i o n problem. Although t h e analyses a r e not completed, t h e d i f f e r e n c e s e r i e s between extreme groups do not support t h e hypotheses t h a t l e s s productive s i t e s , poor vegetation types, ombrogen s i t e s , o r shallow s o i l s should be more s e n s i t i v e t o a c i d i f i c a t i o n . Neither does an a n a l y s i s of t h e development of spruce r e l a t i v e t o pine support t h e hypothesis t h a t spruce i s more s e n s i t i v e t o a c i d i f i c a t i o n when growing on poor s i t e s . Figure 6 shows a s an example t h e t r e e r i n g development

Diff.logloi,

ot

1

Difference series poor type rich type

-

SPRUCE

.-•

,

Vaccinium myrt. Herb

I

-

-

117 plots

45 P l o t s

Figure 6. Tree r i n g development of spruce within d i f f e r e n t vegetation types and a d i f f erence s e r i e s between t h e types. Stands from S#rlandet (South Norway) below 300 m a l t i t u d e .

of spruce w i t h i n two groups o f v e g e t a t i o n t y p e s (a V a c c i n i u m m y r t i l l u s type and a herb-rich type) from s t a n d s w i t h i n S 4 r l a n d e t and below 300 m a l t . Figurc 7 shows f i n a l l y t h e development of p i n e on two groups of

-

Diff.logloi, + 0.10

Difference series shallow soil deep soil

-

1

I

PINE

I

'

.

I

-Sail

depth

d

20cm

- 209plots

Figure 7 . Tree r i n g development of p i n e on d i f f e r e n t s o i l depth groups and a d i f f e r e n c e s e r i e s between t h e groups. Stands from ~ d r l a n d e t (South Norway) below 300 m a l t i t u d e . s o i l d e p t h s ( < 20 cm and > 70 cm) w i t h i n t h e same region and h e i g h t zone. The reason f o r t h e apparent l a c k of r e l a t i o n s h i p between t r e e r i n g development and t h e hypotheses about a c i d p r e c i p i t a t i o n e f f e c t s can be The e f f e c t s may be masked by o t h e r f a c t o r s which a r e n o t many-fold. s a t i s f a c t o r i l y i s o l a t e d by t h e approach. The hypotheses about p o s s i b l e e f f e c t s a r e based upon meagre f a c t s and may be f a l s e . More d a t a may be needed. It a l s o may be t h a t no l a r g e e f f e c t s of a c i d p r e c i p i t a t i o n upon t r e e growth have y e t occurred.

4.

SUMMARY AND CONCLUSIOfV

The p r e s e n t paper summarizes t h e c u r r e n t r e s u l t s of ongoing s t u d i e s i n Norway on t h e impact of a c i d p r e c i p i t a t i o n and d r y d e p o s i t s on conif e r o u s f o r e s t ecosystems. These s t u d i e s i n c l u d e i n v e s t i g a t i o n s on s o i l ,

leaching from tree crowns, germination, establishment of seedlings and tree ring analyses. The soil studies are entirely based on simulated acid rainfall (pH range 5.7 - 3.0) and the application of lime, in field plots and lysimeters. Results from these experiments are at present available from an experiment running since September 1972. The results concern soil fauna, decomposition of organic matter, and soil chemistry. The abundance of one important group of the soil fauna - the Enchytraeidae has been studied. Two species had not been influenced by the acid rain. One species decreased in abundance with increasing acidity of the "rain" at a low lime level and increased in abundance at a high lime level. Laboratory experiments with pine needles (Pinus contorts) from the field plots have not revealed any influence of acid "rain" (pH range 5.7 - 3.0) on decomposition. Liming on the other hand, has reduced the decomposition rate significantly. During the period from September 1972 to October 1974 significant reduction in the degree of base saturation has occurred in the topmost layer of the soil of the plots where the highest amounts of acid have been applied. This reduction is reflected by the increased leaching of Ca and Mg in the lysimeter experiments. Leaching from tree crowns has been studied in Birkenes, South Norway, being heavily exposed to long transported air pollutants, and in MBlselv, North Norway, which is supposed to serve as a control area. Both the amount of various chemical components in throughfall and the ratio between concentration in throughfall and in incident rainfall are significantly higher in Birkenes than in ~dlselv. Conclusive interpretation of these results is impossible at present. However, the high concentrations in Birkenes may derive from dry deposition, the leaching of metaboli-s and the exchange of ions between plant tissue and deposited pollutants. Green-house experiments with spruce (Picea abies) on artificially leached (with dilute sulphuric acid) mineral soil indicate that germination and especially establishment are negatively affected when soil reaction drops below pH 4.0 4 . 2 .

-

Regions supposed to differ in exposure to acid deposition have been compared with regard to tree ring development. So far no differences have been found that can be related to the pollution. Nor have clear effects of acid precipitation been observed on sites supposed to be most sensitive to acidification (poor vegetation types, shallow soils, etc. )

.

The experiments d e s c r i b e d i n t h e p r e s e n t p a p e r i n d i c a t e t h a t s e v e r e n e g a t i v e i n f l u e n c e s on s o i l and organisms a r e n o t t o be expected o v e r a p e r i o d of a few y e a r s when t h e a c i d i t y of t h e r a i n i s above c a . pH 4. Since t h e a c i d i t y of t h e p r e c i p i t a t i o n i n s o u t h e r n Norway v a r i e s g r e a t l y between pH 3.5 and 5.7 w i t h a p r e v a i l i n g pH of c a . 4.5 (1974) it seems l i k e l y t h a t p o s s i b l e e f f e c t s on t r e e growth up t o now have been t o o s m a l l t o b e d e t e c t e d by t r e e r i n g a n a l y s i s . However, longer exposure time and i n c r e a s e d a c i d i t y of t h e p r e c i p i t a t i o n seem t o be a s e v e r e t h r e a t t o s o i l c o n d i t i o n s , t h e h e a l t h o f v e g e t a t i o n , and f i n a l l y t o f o r e s t production.

5.

LITERATURE

Abrahamsen, G. 1972. E c o l o g i c a l s t u d y of Enchytraeidae (Oligochaeta) i n Norwegian c o n i f e r o u s f o r e s t s o i l s . PEDOBIOLOGIA 12: 26-82.

-

Dahl, E. & 0. Skre. 1971. En u n d e r s e e l s e over v i r k n i n g e n a v s u r nedbprr p a p r o d u k t i v i t e t e n i landbruket. Pp. 27-40 i n : Konferens om a v s v a v l i n g , Stockholm 11. november 1969. Nordforsk, Miljzvardssekretariatet, P u b l i k a t i o n 1971: 1. Hovland, J. & Y. 2 . I s h a c . 1975. EFFECT OF A C I D PRECIPITATION AND LIMING ON NITRIFICATION. SNSF-prosjektet, I R ( i n p r e s s ) . I s h a c , Y. 2. & J. Hovland. 1975. EFFECT OF A C I D PRECIPITATION AND LIMING ON LITTER DECOMPOSITION. SNSF-prosjektet, I R ( i n p r e s s ) . Jensen, K. W. & E. Snekvik. 1972. Low pH l e v e l s wipe o u t salmon and t r o u t p o p u l a t i o n s i n southernmost Norway. AMBIO l ( 6 ) : 223-225.

-

Jonsson, B. & R. Sundberg. 1972. Has t h e a c i d i f i c a t i o n by atmospheric p o l l u t i o n caused a growth r e d u c t i o n i n Swedish f o r e s t s ? A comparison between r e g i o n s w i t h d i f f e r e n t s o i l p r o p e r t i e s . Skogshagskolan, I n s t . fijr skogsproduktion. Rapporter och Uppsats e r , N r . 20, 48 pp. Nihlgard, B. 1970. P r e c i p i t a t i o n , i t s chemical composition and e f f e c t on s o i l water i n a beech and s p r u c e f o r e s t i n South Sweden. OIKOS 21: 208-217.

-

-

Oden, S. 1971. Nederb6rdens f z r s u r n i n g e t t g e n e r e l l t h o t mot ekosystemen. Pp. 63-98 i n : Mysterud, I. ( e d . ) . Forurensning og b i o l o g i s k miljgfvern. U n i v e r s i t e t s f o r l a g e t , Oslo 1971.

Overrein, L. N. 1968. Lysimeter studies on tracer nitrogen in forest soil: I. Nitrogen losses by leaching and volatilization after addition of urea-~l~.SOIL SCI. 106: 280-290.

-

Overrein, L. N. 1972. Sulphur pollution. Patterns observed, leaching of calcium in forest soil determined. AMBIO l(4): 145-147. Overrein, L. N. & G. Abrahamsen. 1975. A presentation of the Norwegian project "Acid precipitation - effects on forests and fish". Paper presented at "The first international symposium on acid precipitation and the forest ecosystem". May 12 - 15, 1975. The Ohio State University, Columbus, Ohio. Pollanschutz, J. 1971. Die ertragskundlichen Messmethoden zur Erkennung und Beurteilung von forstlichen ~auchschzden. MITT. FORSTL. BUNDVERSANST. WIEN 91: 153-206. Royal Ministry for Foreign Affairs & Royal Ministry of Agriculture 1971. AIR POLLUTION ACROSS NATIONAL BOUNDARIES. THE IMPACT ON THE ENVIRONMENT OF SULFUR IN AIR AND PRECIPITATION. Sweden's case study for the United Nations conference on the human environment. Stockholm 1971. 96 pp. Sundberg, R. 1974. On the estimation of pollution-caused growth reduction in forest trees. Pp. 167-175 in: Pratt, J. W. (ed.). STATISTICAL AND MATHEMATICAL ASPECTS OF POLLUTION PROBLEMS. New York. Teigen, 0. 1975. Spire- og etableringsfors$k med gran og furu i kunstig forsuret mineraljord. SNSF-prosjekt IR (in press). Ulrich, B. 1968. Ausmass und Selektivittit der NBhrelementaufnahme in Fichten- und Buchenbestxnden. ALLG. FORSTZ. 23: 815.