H Y D R O C H E M I C A L B U D G E T S O F A S M A L L F O R E S T E D G R A N I T I C

C A T C H M E N T E X P O S E D T O A C I D D E P O S I T I O N : T H E

S T R E N G B A C H C A T C H M E N T C A S E S T U D Y ( V O S G E S M A S S I F ,

F R A N C E )

A. PROBST l, D. VIVILLE z, B. FRITZ 1, B. AMBROISE z, and E. DAMBRINE 3

Centre de GOochimie de la Surface (CGS/CNRS), 1 rue Blessig, F-67084 Strasbourg COdex, France 2 Centre d'Etudes et de Recherches Eco-GOographiques, (CEREG/ULP-CNRS), 3 rue de l'Argonne,

F-67083 Strasbourg COdex, France 3 Centre de Recherches Forestibres (CRF/INRA), Nancy-Amance, Champenoux, F-54280 Seichamps, France

(Received January 17, 1991; revised March 26, 1991)

Abstract. Hydrochemical budgets have been obtained for the 3-yr period 1986-89 at Strengbach, a small granitic basin in the Vosges mountains (north-eastern France). Here, the spruce forest shows both yellowing and crown thinning, symptoms of forest decline. Water amount and surface water chemistry were monitored in each ecosystem compartment. Bulk precipitation is acidic. Some pollution episodes occur in winter and early spring, but the annual bulk deposition acidity is rather low. Throughfall however, is much more concentrated, particularly for H § and associated strong acid anions. These inputs come as occult deposits which comprise major ecosystem inputs, as confirmed by the chloride balance for the catchment. Input-output budgets for the catchment indicate a net deficit of base cations, especially calcium. Sulfate also shows a net loss while N budget is well balanced. As the soil exchange capacity is nearly exhausted for base cations, and dominated by H + and A1, the neutralization of incident acid inputs occurs mainly in the weathered bedrock. Silicate weathering processes lead to high losses of cations and of silica. Aluminium hydroxide is precipitated; however, bicarbonate remains very low indicating poorly-buffered surface water.

1. Introduction

D u r i n g the 1980's, m a n y small , fo res ted ca tchments in the no r the rn hemisphere

have been m o n i t o r e d to assess the effects o f air po l lu t ion on ter res t r ia l ecosys tems

(Likens et al., 1977; Wr igh t and Johannessen , 1980; Pac~s, 1985; Hu l tbe rg , 1985).

Wi th the d e v e l o p m e n t of obvious forest decline symptoms , pa r t i cu l a r ly in spruce

s tands o f centra l Eu rope , new inves t iga t ions s ta r ted to s tudy the re la t ionship , if

any, be tween this type o f tree d a m a g e and a tmosphe r i c po l lu t ion (Hauhs and Wright ,

1986; Feger et al., 1988). In 1983 concern in F rance and in pa r t i cu l a r the nor th

eas tern a rea mos t affected by forest decline, led to the set t ing up of a scientific

research p r o g r a m m e ( D E F O R P A : D6p6r issement des For~ts et Po l lu t ion A t m o s -

ph6rique) . C a t c h m e n t inves t igat ions s ta r ted in 1985 with the a im to charac ter ize

the wate r chemis t ry in the different ecosys tem c o m p a r t m e n t s and, by means o f

hyd rochemica l budgets , to d o c u m e n t losses of nutr ients . We repor t here 3 yr of

i n p u t - o u t p u t budgets (1986-87 to 1988-89). Wate r qual i ty character is t ics of this

bas in have been de ta i led in Probs t et al. (1987a, b; 1990a).

Water, Air, and Soil Pollution 62: 337-347, 1992. �9 1992 Kluwer Academic Publishers. Printed in the Netherlands.

338 A. PROBST ET AL.

/ j . a ty station ~ / ~ ~eteorological shelter

/ ~ o rain gauge ~ ~ / ~ ~0 ip i ta t ion collector I 1146 / throughfall and soil solutions site I~1 ~ I old spruce stand (80 years) / ~ II young spruce stand (30 years)

�9 spring o ,. 2oo B , water level recorder and

automatical sampler

Fig. 1. Location map of the Strengbach basin.

2. Mater ia l and Methods

EXPERIMENTAL SITE

The Strengbach forested basin at Aubure is located on the eastern side of the Vosges massif (eastern France), 58 km south-west from Strasbourg. It ranges from 883 m at the outlet to 1146 m p.m.s.1, at the top (Figure 1). This small catchment (80 ha) lies mainly on the base-poor Br6zouard leucogranite (# 320 My old) characterized by two micas of which muscovite is the dominant. At the top edge of the basin, a banded gneiss lies in contact with this granite. Brown acid soils (acid distrocrepts) have developed on the south facing slope; here soil solution is collected. Podzolic soils (leptic podzols) cover the north facing slope. The valley bottom is occupied by a saturated area with hydromorphic conditions, but represents only 2% of the basin area.

Norway spruce dominates (2/3 of the basin area), and mixed white fir and beech cover the rest. Symptoms of decline began on the early 1980's. Yellowing mainly occurs on spruce and has been related to Mg deficiency (Probst et al., 1990a), as observed at several others European sites (Ulrich, 1984). These symptoms mainly affect the pole stage plantations. Crown-thinning affects both white fir and spruce; the mean level of needle loss is about 30%.

H Y D R O C H E M I C A L B U D G E T S 339

EQUIPMENT AND MEASUREMENTS

The Strengbach catchment has been monitored since the end of 1985. The catchment equipment has been detailed in Probst et al. (1990a) (Figure 1).

In this study, bulk precipitation and throughfall are collected fortnightly at 4 sites in the basin and in the old spruce stand (90 yr old), located in the upper part of the basin. Stream and spring waters are sampled at least weekly and more frequently during flood events.

Samples are collected in polyethylene bottles and filtered in the laboratory (0.45 ~tm Millipore membrane). Alkalinity is determined by Gran's titration; Ca, Mg, Na, K, by atomic absorption spectrometry; NH4 and silica by automatic colorimetry (Technicon apparatus); C1, NO3, SO 4 by ion chromatography (Dionex apparatus).

The hydrological year is defined as the period from October to September; autumn is usually dry. All calculated mean concentrations are volume weighted. Taking into account comparative measurements between different types of collectors, bulk precipitation amounts were increased by a 7% corrective factor to compensate for the capture deficit of rain collectors, particularly in the windy zones. Input element fluxes are calculated by multiplying the concentration for the period by the corresponding amount of water. Mean bulk precipitation flux for the basin was obtained by the Thiessen's method. Annual output flux (F) for each element is calculated as follows:

Ct + Ct+n e r t+n F 2 " Qt dt

Ct: element concentration (mg L -1) at time t Ct: element concentration (mg L -1) at time t+n Qt: instantaneous discharge (L s -1) at time t.

Several other methods of output flux calculations give comparable results.

3. Results

HYDROLOGY

The Strengbach basin is characterized by a temperate oceanic mountainous climate. The monthly average of daily mean temperature ranges about -2 to 14 ~ Westerly winds dominate, and the basin receives significant amount of precipitation through- out the year (Figure 2). Generally snow represents 25% of precipitation and covers the basin about 4 mo yr -1.

During the two first hydrological years (1986-87 and 1987-88), precipitation amounts were quite similar. The year 1989 was dryer and without snowfall. Stream flow is relatively important in this basin owing to rather low storage capacity due

340 A. PROBST ET AL.

(I/s) (Ol 250

200

150

100

50

0 ONE: 1986

t' 1 t l""J"r"T' " " r T , lt,-~l " ' l i ' r l

DJ SONDJ IJ FMAMJJ ASON FMAMJ J A FMAMJ 1987 1988 1989

AS

mm 40 8O

,(P)

Fig. 2. Hydrograph of the Strengbach mean daily discharge and corresponding hyetograph.

TABLE I

Water balance of the Strengbach basin

mm P TF Q P-Q ETP

86-87 cycle 1647 1070 1191 456 560 87-88 cycle 1613 1001 1117 496 545 88-89 cycle 1399 855 845 554 633 Mean cycle 1553 975 1051 502 579

P : Mean open field precipitation amount of the whole basin TF : Throughfall amount under the old spruce stand Q : Stream output amount at the outlet of the basin P-Q: Flow deficit ETP: Potential evapotranspiration (Penman's method)

to the thin superficial formations developed on this granitic bedrock. Moreover, the monitoring of sap fluxes (Granier, personnal communication) indicates that these stands have a relatively low transpiration capacity, probably linked to needle losses. As seen in Table I, the hydrological balance reveals a rather low deficit: the mean difference (P-Q) represents 32.5 % of the precipitation amount.

In this basin, the major hydrological events are due to snowmelt (Figure 2). Exceptional storms can produce important discharge but of short duration. Ge- nerally, after at least 50 mm of continuous precipitation, flood events are cha- racterized by a first peak of surface runoff, followed by a delayed subsurface peak flow several hours after the end of the precipitation event, due to groundwater contribution. This type of hydrograph also occurs in other similar European granitic spruce-forested basins (Dupraz, 1984; Hauhs, 1986).

The amount of water in throughfall is lower than the stream water output (Table I): in fact, the spruce stand located on the south-facing slope is only partly

HYDROCHEMICAL BUDGETS 341

TABLE II

Mean chemical composit ion of the surface waters in the Strengbach basin; BP: Bulk precipitation; TF: Throughfall; SW: Streamwater; Units: ~eq L-a; H4SiO4: ~mol L -1

pH H + NH4 + Na + K + Mg 2+ Ca 2+ Mn 2+ Alk.+H+C1 - NO3- SO42- H4SiO4

BP 4.5 30.3 26.5 12.5 2.9 5.0 16.7 <1 2.6 13.9 29.2 43.0 <1 TF 4.1 86.7 34.3 41.8 49.3 18.0 63.0 8.8 1.2 57.3 70.4 155.5 2.8 SW 6.1 0.8 1 83.5 23.2 60.3 186.2 <1 44.5 54.8 36.0 211.8 119.6

representative for the whole basin considering the water amount reaching the soil. Recent measurements indicate much higher amounts of throughfall on the north- facing slope.

SURFACE WATER CHEMISTRY

The mean annual chemical composition of the main elements are detailed in Table II.

Bulk precipitation is acidic. Dominant elements are H +, ammonium, sulfate and nitrate (Table II). These are directly derived from atmospheric pollution (Probst et al., 1990a). However, these waters are very dilute (mean TDS # 5 mg L-l). Because of lower precipitation amount during 1988-1989, bulk precipitation had higher levels of major ion concentrations than during the two previous hydrological years. In particular, ammonium, nitrate, sulfate, Mg and Ca were more concentrated (Figure 3). Compared to bulk precipitation, throughfall contains about 5 times higher concentrations of major ions (Figure 4). The mean pH value is 4.06. The

Si AIk.+H+

CI NO3 S04

Ca Mg Na

K NH4

H

! ' I L l l l l l i l l i i J l ~ l i k l J l l l l l l l l l l ~ l l l { ~ l ~ l i l i l l i l l ~ l L l l l l ] l ] ~ l ~ l l l I I ) l i ~ l l [ k l l ~ l H

~ l l l i l ~ l l l l ~ l ] I I I l l l l l l . . . . . . . . . . . . . . ~ ; ; ) ; ) ) . :

~ l l [ [ l i ] l l h l i l l [ i L i i l l J ~ L L l i I i V/ / / / / / / / / / / / / / / / / / / /A

V / i / / / / / ~ 7 I l l / l / / / / / / / / / / / / / / / / Z / I / I / ~

~ l l~ id } l l l l ] L i l l J l~ [ I kk l l i l l 41 ] l l ]F , i l i i L~kF l i [ l i L i l l : l , i i , i i Lk l ] l V/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / i / / / / / / / / / / / / i / / / / / / / / / / / / / / / ~

I I I I I I I I

0 5 10 15 20 25 30 35

BP 87

BP 88

I I BP 89

I

40 45

,ueq/I Fig. 3. Annua l volume weighted mean concentration of major ions in bulk precipitation (BP) in 1986-

87, 1987-88 and 1988-89.

342 a . PROBST ET AL.

MnSi ~ .................. / ........... ~j~ BP a l l

A I k . + H + T F a l l

CI

N O 3 !- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S04 ~ ..................................................................................... ) / / . . . . . . . a ~'/////////////////////////////////////~

Mg t.. -.-...-...-...-.-., :

K ~l///////////////////z,////z///~ N H 4 . . . . . . . . . . . . . . . . . . . . .

H ~////,'/f////'////.","///////////////////////////////////A I J = I I i I i

0 20 40 60 80 1 O0 120 140 160 /ueq/!

Fig. 4. Comparison between weighted mean bulk precipitation chemistry (BP ALL) and throughfall chemistry (TF ALL) in the old spruce stand for the main components.

elements in throughfall have different origins (Miller, 1984; Matzner, 1986; Probst et al. 1990a). Some are derived from atmospheric pollution and are mainly due to dry and occult deposits (Na +, SO42-, C1- and NO3-), others are recycled by the trees (K +) or some have both origin (Ca ++ and Mg++). The important difference between throughfall and bulk precipitation, particularly for the first group of ions indicates that dry and occult deposits are rather important in this basin, especially in the upper part of the basin where clouds and fog are typical during the winter.

The Strengbach waters are neutral and dominated by Ca and sulfate, while alkalinity is low (Probst et al., 1987b). Mean streamwater chemistry was rather stable during the studied period. The total dissolved solid content is 7 times that of rainwater (# 35 mg L-l). The comparison between Strengbach water chemistry and nearby Ringelbach, a small stream draining a granitic grassland catchment indicates that the Strengbach waters are poorly-buffered and uncharacteristic of alkaline waters draining such a type of rock in pristine areas (Probst et al., 1987b). In the Strengbach catchment, the pollutant sulfate inputs enhanced by the spruce forest have led to soil acidification; as a consequence sulfate is the dominant anion in the streamwaters which now have a low buffering capacity (i.e. weak alkalinity).

ELEMENT BUDGETS

Element fluxes are given in Table III. As a first approach, occult deposits are estimated as the difference between bulk precipitation fluxes and throughfall fluxes in the old spruce stand. The amount of throughfall water measured under the old spruce stand appears to be underestimated for the whole basin. Sulfur and N deposition, in particular, is greater in old spruce stands (Stevens, 1987). On the basis of recent

HYDROCHEMICAL BUDGETS

TABLE III

Mean annual element budget for the whole Strengbach basin (October 1986 - September 1989)

343

kg ha 1 a-1 Ca Mg K Na N C1 S Si

Open field 4.0 0.9 1.5 4.3 10.2 8.2 10.0 <0.5 inputs (OF1) (1) Occult deposits (2) 6.4 0.7 3.4 5.7 3.7 11.9 14.1 0.8 (estimated) Throughfall 12.0 2.2 18.3 10.0 14.0 20.1 24.1 1.3 (inputs (TF) (3) Streamwater 39.1 7.7 9.2 20.6 5.5 20.3 36.3 35.8 outputs (SWO) (4) Biomass removal (5) 7.4 1.3 4.8 1.0 8.5 0.9 2.0 Budget (1+2-4-5) --36.1 -7 .4 -- 9.1 --11.6 - 0.1 -- 0.2 -13.1 --36.5

measurements under a younger spruce stand and under a beech stand (higher amount of throughfall water, but less deposition), the throughfall fluxes, extrapolated to the whole basin from the old spruce stand measurements, appear to be rather suitable. Following the procedure of Matzner (1986), Lindberg and Garten (1988) and Probst et al. (1990a), occult deposits are estimated from the 'canopy difference' as follows: 100 % for Na, C1, NO3, and SQ, 80% for Ca, 50% for Mg and 20% for K of this difference was assumed to be due to occult deposits.

Bulk deposition is relatively low at Strengbach, compared to some other European Iocations (Buldgen, 1984; Ulrich, 1984; Pac~s, 1985); concentrations of major ions are low, but of the same order of magnitude as those measured in the Black Forest,

ko .ha'1.yr-I Strengbach basin / /

60 /

:i Ca Mg K Na N CI S Si

Fig. 5. Mean catchment budget for the major ions. T[: bulk precipitation inputs + occult deposits. TO: streamwater outputs + biomass storage. Balance = TI-TO.

344 A. PROBST ET AL.

kg .ha" 1.yr-1 Stfengbach basin lO

o

-lO

-20

-3o

-4o i e 7

-50

Ca Mg K Na N CI S $i

Fig. 6. Comparison of the three annual hydrochemical budgets (1986-87, 1987-88 and 1988-89) for the major ions.

south-western Germany (Z6ttl et al., 1989). Throughfall inputs on the other hand, are relatively important. In particular,

sulfate concentrations can reach up to 10 to 12 times sulfate concentrations in bulk precipitation. Mean throughfall inputs are 4 to 5 times greater than bulk precipitation. This contrasts with the situation in the Schluchsee basin (south of the Black Forest) where throughfall inputs are similar to open field inputs for pollutant constituents (Z6ttl et al., 1989).

Outputs of major constituents in streamwater are dominated by Ca and sulfate (Table III). Input-output budgets indicate a high net loss of cations, mainly for Ca, from the forest ecosystem. Except for N, outputs of all anions exceed inputs. There is a large output of silica. The large deficit between chloride inputs in bulk precipitation and outputs in streamwater confirms the importance of occult de- position in the Strengbach basin (Table III).

Whole catchment budgets have been calculated (Figure 5), taking into account total inputs (bulk deposition + calculated occult deposition) and total outputs (steamwater output + net uptake in biomass). Storage in biomass has been calculated for the whole catchment using element concentrations of the trees and annual biomass production of the forest (Dambrine and Pr6vosto, 1988).

A large net output of base cations was shown in each of the three hydrological cycles studied, in particular for calcium and to a lesser extent magnesium (Figure 6). The N budget is almost balanced; the N inputs from the atmosphere is not utilized by tree growth, or by micro-organisms, or enhanced mineralization of organic matter may be taking place - either or both could reflect needle loss over the period 1983-86; this could explain the rather important output of N. No upward

H Y D R O C H E M I C A L B U D G E T S 345

trend can be detected in the nitrate concentration of streamwater, in contrast to the situation at Dicke and Lange Bramke catchments (Germany), where forest decline is associated with increased nitrate concentrations and fluxes (Hauhs and Wright, 1986). Chloride is conservative; total outputs equal inputs. However, there is currently a net output of sulfate. There are four hypothesis to explain the origin of this extra sulfate:

(1) Throughfall underestimates total inputs. The chloride budget indicates, however, this is not the case.

(2) Streamwater outputs may be overestimated. Both chemical analysis and hydro- logy measurements, however, appear to be correct.

(3) Only 3 yr of measurements are available for the budgets, and this may be too short to obtain an accurate sulfate balance.

(4) Sulfate concentrations in streamwater currently show a downward trend reflecting reduced atmospheric inputs and low S emissions. The net output from the catchment suggests release of sulfate retained in soils, during periods of larger acid atmospheric inputs, as the result of adsorption/desorption processes in soils and/or by the destabilization of aluminium sulfate phases such as jurbanite (Reuss and Johnson, 1985). However, this explanation seems weak here, the soil solution being far undersaturated with respect to this mineral (Dambrine and Pr~vosto, 1988). As no primary sources of S have been indicated in this catchment (Dahire, 1988), the mechanism controlling retention and release of sulfate requires further studies.

At Strengbach, the soils have a very low base saturation, the exchange complex being dominated by H+ and A1 (Probst et al., 1990a), and base cation release by ion exchange is not an important mechanism in neutralization of acidic inputs. The large export of silica suggests that the major neutralization occurs by silicate hydrolysis in the weathering zones. The ratio between net output of silica and the net output of the base cations is about 0.4; this ratio is very low as compared to other undisturbed granitic basins where it is about 1 (Meybeck, 1984).

The large net outputs of base cations and silica, and mobilization of sulfate from the soils confirm that the hydrochemistry of the Strengbach basin is, and has been influenced by atmospheric acid inputs. These inputs are still buffered but the buffering takes place in the weathering zone not within the soils; as a result, also it has weak alkalinity, streamwater is still not acidic and has only small measured concentrations of A1. Unfortunately, at least 15 other streams in the Vosges mountains are already acidic with large A1 concentrations (up to 800 ppb L -1) and, as a consequence no longer support a trout population (Probst et al., 1990 b). For these streams, there is a clear relationship between the acidification, the spruce cover of the drainage basin, and the base-poor acid bedrock and soils, in response to acid atmospheric inputs.

These observations suggest that this situation is not of recent origin. Climatic conditions have played a role in the sudden appearance of forest decline symptoms; but soil acidification, as a result of silvicultural practices and acidic atmospheric

346 g. PROBST ET AL.

inputs, has been the main predisposing factor (Bonneau, 1989) leading to the nutrient deficiencies, particularly for Ca and Mg, currently observed in spruce.

4. Conclusions

Within the framework of the DEFORPA Programme, the integrated site studies at the Strengbach catchment has led to increased understanding of the phenomenon of decline in Norway spruces.

The research presented in this paper leads to several conclusions: (1) This basin, located in the north-eastern part of France receives acidic

precipitation (mean pH=4.5). Bulk precipitation is rather low owing to low TDS, but throughfall inputs are much larger, particularly for ions derived from atmospheric pollutants. The streamwaters are still circumneutral, however dominated by strong acid anions, and poorly buffered.

(2) The difference between bulk precipitation and throughfall inputs particularly for chloride and sulfate underlines the importance of occult deposits in this basin, in contrast to the nearby southern Black Forest (80 km East).

(3) Most of the acidity input from the atmosphere is neutralized in the weathering zone, as the soils are highly acidified and can not participate actively in acid neutralization.

(4) This neutralization results from silicate weathering processes and leads to large losses of basic cations out of the basin. Sulfate dominates the anionic charge in streamwaters and is also currently being released out of the basin.

(5) At present, At is below detection limit ( < 1 ~mol L -1 level) in the Strengbach stream. Other streams of the Vosges mountains have lost trout populations due to very low pH and high levels of dissolved A1.

It is likely that the interaction of acid atmospheric inputs and the intensive plantation of spruce since the end of the last century on acid soils developed on base-poor granite, has led, to soil solution acidification, loss of base cations from soils, and hence nutrient deficiencies for trees, and streamwater acidification. This scheme explains part of the present forest decline problem.

Acknowledgments

Yvette Hartmeier, G6rard Krempp and Daniel Million from the C.G.S./CNRS are thanked for the chemical analysis. Within the framework of the DEFORPA Programme, this research has been supported by the ECC-DGXII, the French Ministory of the Environment, the French Ministory of Research and the R6gion Alsace.

The authors would like to thank particularly R. F. WRIGHT (NIVA, Oslo) for his useful comments.

HYDROCHEMICAL BUDGETS 347

References

Bonneau, M.: 1989, Rev. For. Fr. XLI, 5, 367. Buldgen, R: 1984, Th&e Doc. Sci. Botaniques, Univ. Liege, 218 p. Dahire, M.: 1988, Th~se de 3~me cycle. Universit6 Nancy I, 158 p. Dambrine, E. and P%vosto, B.: 1988, Rapport Scientifique, Ministbre de l'Environnement, annde 1987-

1988 INRA-CRF, Nancy, 25 p. Dupraz, C.: 1984, Th~se Doc. Ing., Grologie appliqu~e (hydrologie), Univ. Orlrans, 363 p. Hauhs, M.: 1986, NHP Seminar 29-31 January 1986, As Norway, Report NHP n ~ 15,207. Hauhs, M. and Wright, R. F.: 1986, Proceedings of Mid-South Symposium on acid deposition. Little

Rock, Arkansas, April 20-21, 1986, 15. Hultberg, H.: 1985, Ecol. Bull. 37, 133. Likens, G. E., Bormann, F. H., Pierce, R. C., Eaton, J. S., and Johnson, W. M.: 1977, Springer Verlag,

New York, 147 p. Lindberg, S. E. and Garten, C. T.: 1988, Nature, 336 (6195): 148. Matzner, E.: 1986, in H. W. Georgii (ed.), Atmospheric Pollutants in Forest Areas, Reidel, pp. 247-

262. Meybeck, M.: 1984, Th&e de Doctorat d'Etat, Universit6 de Paris 6, 554 p. Miller, H. G.: 1984, Phil. Trans. R. Soc. Lond. B 305, 339. Pac&, T.: 1985, Nature 315 (6014):31. Probst, A., Fritz, B., Ambroise, B., and Viville, D.: 1987a, in Mathy (ed.), Proceedings of the International

Symposium Air Pollution and Ecosystems, Grenoble 18-22 Mai 1987, Reidel Publ. Co., pp. 829- 834.

Probst, A., Fritz, B., Ambroise, B., and Viville, D.: 1987b, Proceedings of the Vancouver Symposium, August 1987, IAHS, 167, 109.

Probst, A., Dambrine, E., Viville, D., and Fritz, B.: 1990a, J. of Hydrology, 116, 101. Probst, A., Massabuau, J. C., Probst, J. L., and Fritz, B.: 1990b, C.R. Acad. Sci. Paris, 311, srrie

II, 405-411. Reuss, J. O. and Johnson, D. W.: 1985, Ecologicalstudies 59, Springer Verlag (N.Y.). 120 p. Stevens, R: 1987, Plant and Soil 101, 291. Ulrich, B.: 1984, Atmospheric Environment 18, 621. Wright, R. F. and Johannessen, M.: 1980, in D. Drablos and A. Tollan (eds.), Ecological Impact of

Acid Precipitation. SNSF Project, Proceedings of the Sandefjord International Conference, Oslo, March 11-14, pp. 250-251.

Zrttl, H. W., Brahmer, G., and Feger, K. H.: 1989, in 'Projekt EuropNsches Forschungszentrum f/ir Massnahmen zur Luftreinhaltung, 5, RE.F. Statuskolloquium', Kartsruhe, 7-9 M~irz 1989, K.f.K., RE.F. Berichte, 5ll (1), 23-34.