the chemistry of precipitation, throughfall and stemflow in two holm oak (quercus ilex l.) forests...

The Science of the Total Environment 305(2003) 195–205

0048-9697/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0048-9697Ž02.00470-9

The chemistry of precipitation, throughfall and stemflow in twoholm oak(Quercus ilex L.) forests under a contrasted pollution

environment in NE Spain

A. Rodrigo*, A. Avila, F. Roda` `

Centre de Recerca Ecologica I Aplicacions Forestals, Universitat Autonoma de Barcelona 08193, Bellaterra, Spain` `

Received 26 July 2002; received in revised form 9 September 2002; accepted 15 September 2002

Abstract

Atmospheric deposition was studied through measurements of bulk deposition, throughfall and stemflow at twosites of contrasted exposure to pollution in the Montseny mountains(Northeastern Spain). To explore the contributionof local or distant sources at both sites, concentration data and precipitation amounts(log transformed) for both bulkdeposition and net throughfall were fitted by linear regression. These models indicated the more important contributionof washout scavenging processes and dry deposition at the pollution exposed site. This is relevant in the context ofMediterranean holm oak forests: up to now, most of the studies have been conducted in protected remote sites andwere little representative of the fluxes in forests close to industrial activity, traffic, agriculture and residential areas.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Bulk deposition; Throughfall; Holm oak forest; Dry deposition; Leaching; N uptake

1. Introduction

The chemistry of precipitation is relevant tomonitor air pollution, particularly in places distantfrom source areas, as most atmospheric pollutantsmay eventually be scavenged by rainwater. Precip-itation chemistry is also relevant in ecosystemnutrient cycles(e.g. Johnson and Lindberg, 1992;Likens and Bormann, 1995; Roda et al., 2002).`Nutrient inputs in precipitation can make a majorcontribution to total ecosystem inputs and even tothe total nutrient capital of the ecosystem in

*Corresponding author. Tel.:q34-93-581-29-70; fax:q34-93-581-13-12.

E-mail address: [email protected](A. Rodrigo).

nutrient-poor sites. Nutrients in precipitation canoriginate from a variety of natural and anthropo-genic sources, including air pollution.The role of forest canopies in modifying the

chemistry of rainfall has long been recognised(Eaton et al., 1973; Parker, 1983; Lindberg et al.,1986; Lovett, 1994). Element enrichment inbelow-canopy precipitation is mostly due to bothdry deposition and leaching of intercellular solutesfrom leaves. However, some nutrients do not showsuch enrichment because they are absorbed byplant surfaces and by epiphytic organisms. Dis-criminating among these canopy processes isimportant because dry deposition represents mostlyan external input of nutrients to the ecosystem

196 A. Rodrigo et al. / The Science of the Total Environment 305 (2003) 195–205

Fig. 1. Idealised topographical profile showing the two study sites in the Montseny Mountains. Exposure to polluted air massesdecreases from the topographical exposed site(Riera de Sant Pere, RP) to the topographical sheltered forest site(La Castanya, LC).For clarity, a deep valley between RP and LC has been omitted. See the text for relevant distances.

while leaching represents mostly a within-systemrecirculation of nutrients previously taken up bytrees from the soil. It is also important to determinethe magnitude of the canopy uptake flux, which isa net input to the ecosystem that cannot be inferredfrom net throughfall(Eaton et al., 1973; Parker,1983; Lindberg et al., 1986; McCune and Botce,1992).In different forests throughout the world, atmos-

pheric deposition has been studied through themeasurement of wetybulk deposition, throughfalland stemflow(Parker, 1983; Freedman and Prager,1986; Draaijers and Erisman, 1993; Lovett, 1994).In Mediterranean holm oak forests, atmosphericdeposition has been mostly measured in mountainsites little affected by air pollution(Roda et al.,`1990; Bellot and Escarre 1991; Escarre et al.,´ ´1999). Many of these sites lie in protected naturalareas in mountain ranges of moderate altitude.However, the Mediterranean landscape is veryheterogeneous, with a mosaic of forest, cultivated,urban or industrial areas, and studies in relativelyremote rural mountain sites may scarcely representthe general situation in the country.Atmospheric pollutants can travel for long dis-

tances and be deposited in remote zones. In con-trast, the role of dry deposition decreases withincreasing distance to the source(Lindberg et al.,1986; Fahey et al., 1988; Pucket, 1990; Lovett,1994). Therefore, it is possible that forests closerto pollution sources receive higher deposition flux-es, and this has not been appropriately studied inNortheastern Spain because of the usual protectedsituation of the study sites.

Here, we compare the deposition fluxes of themajor ions at two holm oak forests in the MontsenyMountains(Northeastern Spain). The two sites liein contrasted positions relative to the influence ofthe Barcelona conurbation, a major urban, indus-trial and transport area: one is exposed and theother one is in a topographically protected position,and they have been selected in order to determinethe effect the pollution exposure on the amount ofdeposition.

2. Material and methods

2.1. Site description

The Montseny massif lies 40 km to the NNE ofBarcelona and 25 km to the W of the Mediterra-nean sea. The highest altitude is 1707 m. Themassif covers about 400 km , with 75% of this2

area being protected as a natural park. The bedrockis mostly metamorphic phyllites and schists. Theclimate is Mediterranean, with maximum precipi-tation in spring and autumn. Mean annual precip-itation is around 900 mm year and mean annualy1

temperature is 9–108C (1983–1995) for bothsites.One sampling station(named LC) was in La

Castanya valley in the centre of the Montsenymassif(418469N, 28219E). LC is at 730 m asl in asheltered position from the influence of the pollut-ed atmosphere of Barcelona: it is separated fromthis conurbation area by the elevated ridges of LaCalma(1350 m asl) (Fig. 1). The other samplingsite, at Riera de Sant Pere, hereafter named RP(418439N, 28239E), was located at 535 m asl in

197A. Rodrigo et al. / The Science of the Total Environment 305 (2003) 195–205

Table 1Study site characteristics(average of 4 throughfall plots per site)

Site Altitude Aspect Slope Density Basal area Tree height(m) (degrees) (tree ha )y1 (m ha )2 y1 (m)

LC(sheltered site) 731 N 37.5 2127 26.5 5.9RP (exposed site) 535 S 20.5 1753 22.3 6.4

the southern reaches of the Montseny massif, at7.2 km to the SSW from the LC sheltered site.This site was more exposed to air pollution origi-nated in the Barcelona conurbation, in 4 mid-sizecities of 50 000–100 000 inhabitants(St. Celoni,Mataro, Granollers and Mollet), and a highway´that runs parallel to the coast from SW to NE witha perpendicular distance of 8.7 km from RP(Fig.1). In spring, fertilisation with animal manure ofthe cultivated fields in the low reaches of themassif is probably a source of NH affecting more3

acutely the exposed site.The sites chosen for the present study are highly

representative of the montane holm oak(Quercusilex L.) forest type in the Mediterranean regionsof France, Italy, Greece and Eastern Spain(Terra-das, 1999). They are quite dense forests(Table 1)of a resprout origin after coppicing or selectionthinning in practice until the 1950–1960 decades.The two forests are similar in stand structure buthave different aspects, and LC, steeper slopes(Table 1). They are established on different soils:the RP site has deep soils classified as TypicXerochrepts(Hereter, 1990), while at LC, the soilsare quite heterogeneous, shallow, with depths rang-ing between 0.2–1.5 m on very steep and ruggedslopes. They are classified as Entisols(LithicXerorthents; Hereter, 1990). The bedrock is thesame(phyllites) at both sites.

2.2. Field sampling and laboratory analysis

Precipitation was collected at each site in aclearing of the forest. Four collectors consisting ofa funnel connected to a 10-l polyethylene bottlewere used for bulk deposition sampling. The sam-ples were combined to provide a composite one.As the funnels were permanently open to theatmosphere, precipitation thus collected has beennamed bulk deposition and comprises the wet

deposition flux and the portion of the dry deposi-tion flux of gravitatory sedimentation(Eriksson,1953). However, bulk deposition inputs have beencomplemented with the measurement of dry dep-osition accumulated since the last rain or duringrain-free weeks. Therefore, the input flux we aredealing with comprises wet deposition and the partof dry deposition of the gravitatory sedimentation.A more extended description of the bulk depositionequipment and measurements at Montseny can befound in Avila (1996).`

Throughfall was sampled in 4 replicate plots inthe two forest sites at a distance of 50–100 mfrom the bulk deposition collectors. The plots werecircular, with 7-m radius(area of 154 m). Each2

plot contained 8 throughfall collectors, consistingof a 10-cm diameter funnel connected to a 2-lpolyethylene bottle. The 8 samples for each plotwere bulked in proportion to the collected volumeinto a single composite sample: for each weekwith rain, 4 throughfall samples were obtained ateach site.Stemflow was sampled in 10 trees in one of the

throughfall plots from each site. A spiral tubearound each selected tree conveyed the stemflowinto a 25–200-l polyethylene container. Theamount of water accumulated by each tree wasmeasured in situ and one aliquot(1 l) was takento the laboratory. The 10 samples for each sitewere bulked in proportion to the collected volumein a single composite sample.The containers and bottles for bulk deposition,

throughfall and stemflow were in the dark duringthe sampling period to avoid biological growth inthe sample, and a nylon mesh was placed in thefunnel necks and at the end of the stemflow tubesto prevent insects or vegetal debris from fallinginside the sampling container. The sampling sched-ule was weekly for a 16-month period for bulkdeposition and throughfall, extending from 23


January 1995 to 24 June 1996, and for a 12-monthperiod for stemflow, extending from 6 June 1995to 24 June 1996. In this period, 57 weekly samplesof bulk deposition, 54 of throughfall and 34 ofstemflow were collected at LC and 56, 51 and 39,respectively at RP.On the day of collection, the samples were taken

to the laboratory where pH, conductivity andalkalinity were measured within 24 h. The pH wasmeasured potentiometrically using a Ross com-bined electrode for low ionic strength solutions.Alkalinity was measured by Gran titration(Gol-terman et al., 1978). Samples were then filteredthrough 0.45mm pore size Millipore filters andtwo aliquots of each sample(one acidified with0.3% HNO) were deep-frozen aty20 8C in the3

dark for later analysis.In non-acidified samples, concentrations of

PO , SO , NO and Cl were determined by3y 2y y y4 4 3

ion chromatography, and NH by flow injectionq4

analysis and gas diffusion(FIAstar ApplicationNote 50-01y84, Tecator). Concentrations of Ca ,2q

Mg , Na and K in acidified samples were2q q q

measured by ICP-MS at the ‘Serveis Cientıfico-´Tecnics de la Universitat de Barcelona’. For all`ICP-MS samples, the final concentration was themean(with a standard deviation lower than 1%)of three measurements. A rhodium internal stan-dard was used during the ICP-MS analysis tocorrect for baseline drift. All analytical runs includ-ed synthetic samples of known concentrations ofthe analysed ions to check for precision andaccuracy of the results. The analytical quality ofthe data was checked by a cation–anion balanceand by comparison of the measured electric con-ductivity with conductivity calculated from theconcentration of all measured ions and their spe-cific conductivity. The results were satisfactory(Rodrigo, 1998).Volume-weighted mean concentrations in bulk

precipitation, throughfall and stemflow were cal-culated by weighting each sample by the appro-priate amount of precipitation, throughfall orstemflow. At our sites, where pH values of weeklysamples ranged between 4.0 and 8.0, the calcula-tion of mean pH from H concentrations does notq

provide a proper estimate of the rainwater acidity(Liljestrand, 1985). In such cases, alkalinity is the

conservative property to average(Hantschel andKlemm, 1987; Pinol, 1990).˜Differences between sites were tested witht-

Student tests of paired observations for ion con-centrations and fluxes in BD, T and S. Weeks withcollected precipitation lower than 2 mm wereexcluded from the analysis. The sequential Bon-ferroni method was employed to control group-wide type I error rates(Rice, 1989). Prior tostatistical analysis, data were log-transformed tonormalise the distributions(Sokal and Rohlf,1995). In all cases, inspection of residuals wascarried out to check for normality andhomocedasticity.

3. Results and discussion

3.1. Chemistry of bulk deposition and input fluxes

Mean volume-weighted bulk deposition at bothsites was characterised by positive alkalinity, mod-erate marine influence and moderate strong acidanion concentrations. In fact, the most abundantanion, SO , was almost totally balanced by2y

4

Ca at LC and by NH at RP(Table 2), a2q q4

typical feature of the precipitation chemistry in theN and NE of the Iberian peninsula(Camarero andCatalan, 1993; Avila, 1996; Escarre et al., 1999).` ´This contrasts with the most frequent situation inCentral and Northern Europe and Eastern NorthAmerica where precipitation is acidic and H isq

the major counterpart to SO and NO(Reuss2y y4 3

and Johnson, 1986; Lovett, 1994; Likens andBormann, 1995).Volume-weighted mean(VWM) concentrations

in bulk deposition were slightly higher at RP thanat LC (except for alkalinity, Table 2), probablyreflecting the lower precipitation amount collectedat RP for the period of study(1048 mm at RPcompared to 1275 mm at LC). However, only forCl the difference in concentrations was statisti-y

cally significant.The annual deposition of major ions in bulk

precipitation is shown in Table 3. Because of thehigher precipitation amount at LC we would expecthigher deposition fluxes at this site. However, forthe ions H , Cl , NO -N and Mg depositionq y y 2q

3


Table 2Conductivity(ms cm ) and ion concentrations(meq l ) in bulk deposition, throughfall and stemflow at two holm oak forests iny1 y1

the Montseny mountains exposed to contrasted air pollution

Cond. pH Alk. PO3y4 Cly SO2y4 NOy3 NHq

4 Ca2q Mg2q Naq Kq

LC (sheltered site)Bulk deposition 14.3 6.58 19.6 1.03 22.3 38.9 27.9 31.9 35.9 7.00 18.2 3.45Throughfall 27.9 7.17 76.7 6.05 50.0 60.3 25.0 27.5 73.9 24.8 27.5 47.1Stemflow 20.2 7.18 77.6 4.13 36.1 41.2 10.7 17.5 47.5 16.2 18.1 41.4

RP (exposed site)Bulk deposition 18.4 6.50 16.1 1.25 30.9 48.2 35.0 37.9 34.9 8.10 21.6 4.26Throughfall 43.5 7.34 111.6 5.10 72.5 93.7 49.4 56.5 106 32.9 41.2 89.9Stemflow 40.1 7.25 91.3 2.06 66.3 89.2 39.8 42.9 101 29.4 37.2 74.7

Period of study, from 23 January 1995 to 25 June 1996 for bulk deposition and throughfall, and from 6 June 1995 to 25 June1996 for stemflow. Significant differences between weekly mean concentrations(applying Studentt-test of paired observations withBonferroni criteria) are shown in bold characters.

Table 3Annual fluxes in bulk deposition, throughfall, stemflow and Net Throughfall(NTsTqSyBD) at two holm oak forests in theMontseny mountains exposed to contrasted air pollution

Rainfall Alk. H PO -P3y4 Cly SO -S2y

4 NO -Ny3 NH -Nq

4 Ca2q Mg2q Naq Kq

LC (sheltered)Bulk depositona 1275 23.1 .111 0.246 10.8 7.62 4.71 5.36 9.69 1.16 5.77 1.70Throughfall 962 73.3 .0122 0.632 15.4 8.48 3.02 3.67 12.8 2.68 5.77 16.0Stemflow 33.9 2.63 .0003 0.014 0.433 .224 .0507 .0833 .0323 .0668 0.141 0.548Net throughfall y279 52.8 y.0985 0.400 5.03 1.08 I1.64 I1.61 3.14 1.59 0.14 14.8

RP (exposed)Bulk depositiona 1048 15.1 .126 0.152 11.4 7.58 4.80 5.17 7.52 1.05 5.32 1.71Throughfall 755 85.0 .0096 0.336 17.6 10.6 4.74 6.00 14.0 2.70 6.54 25.3Stemflow 55.7 5.07 .0009 0.014 1.31 .797 .312 .335 1.13 0.199 0.476 1.63Net throughfall y337 75.0 y.115 0.198 7.51 3.82 .252 1.16 7.61 1.85 1.70 25.2

Units in kg ha year , except for alkalinity(meq m year ) and rainfall(mm year ) Period of study, from 6 June 1995y1 y1 y2 y1 y1

to 25 June 1996. Significant differences in mean weekly values are shown in bold characters when applying a Studentt-test ofpaired observations and the Bonferroni determination of significance.

Includes a minor input through gravitational sedimentation of particles onto funnel collectors during rainless sampling periods.a

was higher at RP, but again, only for Cl they

difference was significant.

3.1.1. Distinction between local sources and longrange transportIt is generally considered that ions in rainwater

are incorporated through the processes of rainout(incorporation as condensation nuclei during orafter droplet formation) and washout(incorpora-tion by impaction as the drops fall). It is assumedthat ions incorporated by rainout are originated atgreat distances from the collecting point whilethose incorporated by washout would be originatedfrom closer local sources. The relationship between

element concentration and amount of precipitationhas the shape of a negative hyperbola with thevertical axis representing the below-cloud scaveng-ing plus concentration by evaporation, and thehorizontal axis, the in-cloud processes, in additionto dilution depending on the typeyintensity ofprecipitation(Wolaver and Lieth, 1972). In spiteof the simplifications of the model, it is useful todescribe the main physico–chemical processesaffecting the incorporation of ions in precipitation,and as such has been used here. When plotting theion concentrations vs. the inverse of precipitation(in log transformed data) a positive linear relation-ship would be expected for washout-incorporated


Table 4Regression parameters:(a) interception, (b) coefficient ofregression and(R ) coefficient of determination of the regres-2

sions between the inverse of precipitation volume as independ-ent variable and conductivity, alkalinity and ion concentrationsin bulk deposition as dependent variables

LC (Sheltered site) RP (Exposed site)

a b R2 a b R2

Hq 10.1 8.8 0.01 13.1 18.7 0.01Alkalinity 17.4 26.6 0.01 y0.94 297.2 0.13PO -3y

4 0.17 24.0 0.12 1.72 3.31 0.01Cly 21.2 43.0 0.05 24.1 178.2 0.17SO2y4 42.1 98.8 0.13 42.9 266.8 0.31NOy

3 27.8 133.0 0.21 28.2 260.6 0.41NHq

4 33.7 105.7 0.10 32.8 221.7 0.18Ca2q 34.2 88.8 0.06 20.9 336.5 0.33Mg2q 7.2 14.3 0.09 4.03 88.7 0.29Naq 18.2 33.5 0.03 15.7 137.4 0.20Kq 4.1 7.1 0.06 1.26 74.2 0.22

The significance values, corrected with the Bonferroni cri-teria, are shown in bold characters.

ions. For rainout incorporated ions, the concentra-tions in rainwater should remain approximatelyconstant independently of the precipitation amount.On the other hand, dry deposition by gravitationalsedimentation usually moves elements from localsources(Tanner, 1990; Lovett, 1994; Kopacek etál., 1997) which are exhausted with increasingprecipitation, therefore, showing a behaviour sim-ilar to washout incorporated ions.To explore these effects, the concentration data

and precipitation amounts were fitted by linearregression to a simple model, described by thefollowing equation:

C sCq(AyV ) (1)w i w

whereC is the concentration of a given ion for aw

given week,V is the weekly volume of precipi-w

tation, whileC (the ‘rainout’ concentration, of ai

constant level during the rain) andA (the ‘wash-out’ scavenged below the cloud, which is inverselyrelated to precipitation volume) are the parametersto be fitted. Similar models have been used byother authors(e.g. Baeyens et al., 1990; Pinol,˜1990; Camarero and Catalan, 1993, 1996). Theregression parameters are shown in Table 4. At thesheltered site(LC), all regressions(except for

alkalinity, PO and Ca ) had a significant inter-3y 2q4

cept value. The intercept being representative ofthe rainout, this would indicate the contribution ofthe long-range transport of elements. It is alsonoteworthy that these intercept values are verysimilar in magnitude and also statistically signifi-cant at RP(Table 4). This would imply a distantprovenance source that had similar effects at aregional scale comprising both stations.On the other hand, at the sheltered site, only

NO presented a significant correlation with they3

inverse of precipitation amount, leading to theconclusion that the effect of below-cloud processeswas of minor importance. Similar results havebeen found for bulk deposition studies at othersheltered sites on the Iberian peninsula(Pinol,˜1990; Camarero and Catalan, 1993).In contrast, at the exposed site of RP, all

regressions(except for PO ) showed significant3y4

slopes(Table 4) indicating the important role ofwashout scavenging processes or gravitational drydeposition in this pollution exposed site. Nitrate,NH and SO are usually incorporated in rainq 2y

4 4

mostly from anthropogenic emission sources. AtRP these ions would originate from local emissionsfrom the industry and urban complexes aroundBarcelona. For Na and party for Cl , the localq y

influence can be related to the more open positionof RP respect to the Mediterranean coast.On the other hand, in the Western Mediterrane-

an, African red rains are responsible for most ofthe inputs of alkalinity and base cations in bulkdeposition(Loye-Pilot et al., 1986; Chester et al.,¨1993; Roda et al., 1993; Le Bolloch and Guerzoni,`1995; Avila et al., 1997, 1998). These rains are`

sporadic and have a most variable ionic contentdepending on the strength of the emission sourcesin Africa and the transport patterns(Loye-Pilotänd Martin, 1995; Avila and Roda, 2002).` Àlthough the ions deposited by African rains areoriginated in the distant Sahara and probably areincorporated during the transport of the air massesproducing the rain(the rainout mechanism), thehigh variability of occurrence of red rains alongthe period and the variability in ionic load mayhave produced the lack of significance of theindependent term in Eq.(1) for alkalinity and basecations, the characteristic ions of red rains(Table


Fig. 2. Net throughfall fluxes(the difference between bulk deposition and throughfallqstemflow fluxes) in a pollution topograph-ically sheltered(LC) and an exposed site(RP) in the Montseny mountains. Study period from 23 January 1995 to 25 June 1995.

4). However, at RP the slope term is significantfor alkalinity and base cations(Table 4), a resultindicating that there is a local source for thesecomponents at the exposed site, probably from theimportant activity in the agriculture and construc-tion sectors.

3.2. Chemistry of throughfall and stemflow

At both sites, throughfall and stemflow in holmoak canopy is enriched relative to rainfall(Table2), as also found in other studies in Mediterraneanholm oak forests(Roda et al., 1990; Bellot and`Escarre, 1991) and in forests throughout the world´(Eaton et al., 1973; Prebble and Stirk, 1980;Parker, 1983; Tsusumi and Nishitani, 1984; Freed-man and Prager, 1986). In fact, the fluxes in netthroughfall(NT), calculated as NTs(TqS)yBD(Parker, 1983), allow a quantification of the can-opy effect. For all analysed ions at the samplingsites, except for NO -N and NH -N at LC, nety q

3 4

throughfall was positive(Fig. 2). Net throughfallinputs are, however, the result of an equilibriumbetween three processes that modify the rainwatercontents. Two of these processes, leaching and drydeposition, increase the concentration in through-fall as compared to bulk deposition. The thirdprocess, uptake at the leaves, produces a negativenet throughfall flux (Parker, 1983). To evaluatethe relative contribution of these three processes,

a simple regression model was used that regressesthe net throughfall fluxes to the weekly volume oftotal throughfall (TqS), an approach based inLovett and Lindberg(1984a) proposal. We haveinterpreted the results in the light of the followinghypothesis:1. When net throughfall displays a positive rela-tionship with precipitation volume, this enrich-ment is assumed to be produced by leaching.This implies that leaching is active along thewhole rain episode, despite the fact that theprocess is probably weaker as the rain eventproceeds(Lindberg and Lovett, 1985; Shanley,1989; Rodrigo and Avila, 2002). Alkalinity and`

K at the two sites and PO -P at LC followq 3y4

this pattern(Table 5).2. If net throughfall is independent of precipitationvolume, the enrichment is linked to dry depo-sition. We assume that dry deposition accumu-lated in the canopy during dry spells is washedquite effectively at the beginning of the rainepisodes and is quickly exhausted, so that higherrainfall depths do not imply higher enrichments(Lindberg and Lovett, 1985; Shanley, 1989;McCune and Botce, 1992). This behaviour isshown by Cl , SO -S, NH -N, Ca , Mgy 2y q 2q 2q

4 4

and Na at both sites, H only at LC andq q

PO -P and NO -N at RP(Table 5).3y y4 3

3. If net throughfall decreases significantly withincreasing precipitation volumes, we assume


Table 5Values of the coefficient of regression(b) and the coefficientof determination(R ) of the regressions between the through-2

fallqstemflow volume as independent variable and alkalinityand ion net throughfall fluxes(in meq m ) as dependenty2

variables

LC (Sheltered site) RP (Exposed site)

b R2 b R2

Hq y5.6 0.12 y16.9 0.36Alkalinity 28.5 0.23 73.8 0.20Cly y0.92 0.03 1.1 0.01PO3y4 3.5 0.25 1.3 0.02SO2y4 y1.3 0.01 7.2 0.04NOy

3 I6.8 0.15 y9.2 0.07NHq

4 y6.3 0.09 1.0 0.03Ca2q y2.9 0.01 11.7 0.15Mg2q 4.0 0.09 9.2 0.14Naq y1.9 0.01 3.6 0.13Kq 13.7 0.15 62.6 0.15

The significance values, corrected with the Bonferroni meth-od, are shown in bold characters.

there is uptake in the canopy. Here we assumethat the uptake process is active throughout therain event. Only net throughfall for NO at LCy

3

and H at RP follow this behaviour(Table 5).q

Dry deposition has been described as the mainatmospheric input at sites close to pollutionsources, its role decreasing with the distance tothe source(Lindberg et al., 1986; Fahey et al.,1988; Pucket, 1990; Lovett, 1994). Our resultsshow that the behaviour of ions of anthropicprovenance, either from industry and traffic(suchas Cl , SO -S and NO -N) or from agriculturaly 2y y

4 3

activities (such as NH -N) is consistent withq4

hypothesis 2, indicating that the enrichment inthroughfall is related to dry deposition. Consis-tently, the exposed site shows higher net through-fall fluxes than the sheltered site(Fig. 2) andthese differences(except for Cl ) are significantlyy

higher at RP when comparing mean weekly valuesbetween sites(Table 3, Fig. 3)

3.2.1. Nitrogen net fluxesFor both inorganic nitrogen components(NO -y

3

N and NH -N) net throughfall fluxes were nega-q4

tive at the sheltered site for most of the studyperiod (Fig. 3a,b) with the result that the totalannual fluxes were negative at LC(Table 3).According with the above hypothesis, for NO -Ny

3

at LC, this indicates uptake at the canopy, sug-gesting that there is retention of N by the epiphyticflora or by the leaves themselves in this holm oakforest where N has been found to be a limitingnutrient(Roda et al., 2002).`At the exposed site RP, net annual throughfall

fluxes for NO -N and NH -N were positivey q3 4

(Table 3). In spring, however, periods of negativenet fluxes were observed(Fig. 3). Particularly forNH -N, negative net fluxes were specifically pro-q

4

duced in May(Fig. 3b). Since N can be taken upin the canopy, the extent of the dry deposition fluxcannot be estimated by net throughfall fluxes.However, gaseous or aerosol dry deposition of Ncompounds to forests has been widely reported indifferent forest sites(Lovett and Lindberg, 1984b;Bytnerowicz and Fenn, 1996). At RP and LC, anapproach based on the washing of branches andthe recovery from surrogate surfaces placed in thecanopy has shown that dry deposition of NO andy

3

NH is important in both forests(Rodrigo andq4

Avila, 2002). This suggests that dry deposition is`

a relevant input for N at both sites, although netthroughfall fluxes are negative or slightly positiveas a result of canopy uptake. The result of thebalance between dry deposition and uptake ispositive for RP. Assuming similar N canopy uptakefluxes at both sites, which is reasonable given thesimilarities in soil type and stand structure, thepositive fluxes at RP would indicate higher N drydeposition at RP. Roda et al.(2002) have recently`calculated an enhanced dry deposition flux at theexposed site greater by 2 and 3 kg N hay1

year for NO and NH , respectively, than aty1 y q3 4

the sheltered site.

3.2.2. Base cation net fluxesAs indicated by the regression analysis and the

associated hypothesis, dry deposition was also themain process in the enrichment of the cationsCa , Mg and Na at our sites. At the exposed2q 2q q

site, the net throughfall fluxes for these ions was


Fig. 3. Monthly net throughfall fluxes of:(a) NO -N, (b) NH -N, (c) SO -S and(d) Ca in a pollution topographicallyy q 2y 2q3 4 4

sheltered(LC) and an exposed site(RP) in the Montseny mountains. Study period from 23 January 1995 to 25 June 1995. BeforeJune 1996 stemflow values were not included in net throughfall fluxes.

higher (Fig. 2), and for Ca , the difference was2q

significant when comparing weekly valuesbetween sites(Table 3). In this case, the enhanceddeposition at RP is probably due to the proximityof this site to the expanding towns of the valleyand to cultivated fields in a calcareous soilenvironment.Potassium was the most enriched ion in net

throughfall (Table 3). According to the abovehypothesis, this enrichment is the result of leaching(Table 5). The leaching of K has been extensive-q

ly found around the world(Parker, 1983; Leonardiand Fluckiger, 1987; Pucket, 1990; Bellot et al.,1999). Also, some authors have proposed thatleaching accounts for most of the net throughfallfluxes of Ca and Mg (Fahey et al., 1988;2q 2q

Pucket, 1990; Parker, 1983; Ragsdale et al., 1992;Tsusumi and Nishitani, 1984), but for our sites anexperiment with surrogate surfaces and leaf wash-

ings indicated its negligible role for these ions(Rodrigo and Avila, 2002).`

It is important to note the alkalinization role ofthe canopy: the interaction of bulk deposition withthe canopy consumed H and produced an impor-q

tant increase of alkalinity in net throughfall at bothsites(Table 3), a similar result to other studies inMediterranean holm oak forests(Roda et al., 1993;`Bellot et al., 1999). This effect has been relatedto the leaching of organic acids from the canopy(McClaugherty, 1983; McDowell and Likens,1988; Hoffman et al., 1980). We do not havemeasurements of organic acids; however, based onthe above hypothesis, leaching would be the mainenrichment pathway for alkalinity in net through-fall (Table 5). At the exposed site, probably thereis also dry deposition of carbonates(contributingalso with Ca and Mg ) that can add to the net2q 2q

alkalinity flux in throughfall.


4. Conclusions

The Mediterranean landscape where these holmoak forests are included is composed of a hetero-geneous mosaic of forested mountain ranges sur-rounded by more flat areas where most agriculture,industry and cities are found. In this context, thedeposition to holm oak forests next to these pol-lution source areas has not been properly addressedup to now, since most studies in holm oak forestshave only considered protected sites(Bellot andEscarre, 1991; Roda et al., 1993).´ `The results of the present study show that the

site more exposed to agriculture, industry andtraffic receives higher atmospheric deposition flux-es than the sheltered site. This enhanced depositionis due to higher ‘washout’ which influences thebulk deposition composition, and the filteringeffect of the forest canopy enhancing drydeposition.

Acknowledgments

The research has been funded by the SpanishCICYT project AMB92-0349, and by MCYTREN01-0659yCli. The collaboration of the Depar-tament de Medi Ambient de la Generalitat deCatalunya is fully acknowledged. We thank thetechnical personnel from CREAF and the ServeisCientıfico-Tecnics of the Universitat de Barcelona´ `for field and laboratory assistance. A. Rodrigoenjoyed a post-graduate grant from the SpanishMinisterio de Ciencia y Tecnologıa.´

References

Avila A. Time trends in the precipitation chemistry at a`

mountain site in Northeastern Spain for the period 1983–1994. Atmos Environ 1996;30:1363–1373.

Avila A, Alarcon M, Queralt I. The chemical composition of` ´dust transported in red rains: its contribution to the biogeo-chemical cycle of a holm oak forest in Catalonia(Spain).Atmos Environ 1998;32:179–191.

Avila A, Queralt-Mitjans I, Alarcon M. Mineralogical com-` ´position of African dust delivered by red rains over North-eastern Spain. J Geophys Res 1997;102:21977–21996.

Avila A, Roda F. Assessing decadal changes in rainwater` `alkalinity at a rural Mediterranean site. Atmos Environ2002;36:2881–2890.

Bellot J, Avila A, Rodrigo A. Throughfall and stemflow. In:`

Roda F, Retana J, Gracia CA, Bellot J, editors. Ecology of`

Mediterranean evergreen oak forests. Berlin: Springer, 1999.p. 209–220.

Bellot J, Escarre A. Chemical characteristics and temporal´variations of nutrients in throughfall and stemflow of threespecies in Mediterranean holm oak forest. For Ecol Manage1991;41:125–135.

Baeyens W, Dehairs F, Dedeurwaerder H. Wet and dry depo-sition fluxes above the North Sea. Atmos Environ1990;24:1693–1703.

Bytnerowicz A, Fenn ME. Nitrogen deposition in Californiaforests: a review. Environ Pollut 1996;92:127–146.

Camarero L, Catalan J. Chemistry of bulk precipitation in thecentral and Eastern Pyrenees, Northeast Spain. Atmos Envi-ron 1993;27:83–94.

Camarero L, Catalan J. Variability in the chemistry of precip-itation in the Pyrenees(Northeastern Spain): Dominance ofstorm origin and lack of altitude influence. J Geophys Res1996;101:491–498.

Chester R, Nimmo M, Alarcon M, Saydam C, Murphy KJT,Sanders GS, Corcoran P. Defining the chemical character ofaerosols from the atmosphere of the Mediterranean Sea andsurrounding regions. Oceanologica Acta 1993;16:231–246.

Draaijers GP, Erisman JW. Atmospheric sulphur deposition toforest stands: Throughfall estimates compared to estimatesfrom inference. Atmos Environ 1993;27A:43–55.

Eaton JS, Likens GE, Bormann FH. Throughfall and stemflowchemistry in a northern hardwood forest. J Ecol1973;61:495–508.

Eriksson E. Composition of atmospheric precipitation. Tellus1953;4:215–232.

Escarre A, Carratala A, Avila A, Bellot J, Pinol J, Millan M.`´ ` ˜ ´Precipitation chemistry and air pollution. In: Roda F, Retana`J, Gracia CA, Bellot J, editors. Ecology of Mediterraneanevergreen oak forests. Berlin Heidelberg: Springer-Verlag,1999.

Fahey TJ, Yavitt JB, Joyce G. Precipitation and throughfallchemistry inPinus contorta ssplatifolia ecosystems, South-eastern Wyoming. Can J For Res 1988;18:337–345.

Freedman B, Prager U. Ambient bulk deposition, throughfalland stemflow in a variety of forest stand in Nova Scotia.Can J For Res 1986;16:854–860.

Golterman HL, Clymo RS, Ohnstad MAM. Methods forphysical and chemical analysis of fresh waters. Oxford:Blackwell Scientific, 1978. (210 pp).

Hantschel R, Klemm O. Characterization of weak acidity inselected precipitation samples from a forest ecosystem.Tellus 1987;39B:354–361.

Hereter, A(1990) Els sols forestals del Massıs del Montseny.` ´Ph.D. Thesis. Universitat de Barcelona, 292 pp.

Hoffman WA Jr, Lindberg SE, Turner RR. Some observationsof organic constituents in rain above and below a forestcanopy. Environ Sci Technol 1980;14:999–1002.

Johnson DW, Lindberg SE. Atmospheric deposition and forestnutrient cycling. New York: Springer-Verlag, 1992. p. 707.

Kopacek J, Prochazkova L, Hejzlar J, Blazka P. Trends and´ ´ ´seasonal patterns of bulk deposition of nutrients in theCzech Republic. Atmos Environ 1997;31:797–808.


Le Bolloch O, Guerzoni S. Acid and alkaline deposition inprecipitation on the western coast of Sardinia, centralMediterranean (408N, 88E). Water, Air, Soil Pollut1995;85:2155–2160.

Leonardi S, Fluckiger W. Short-term canopy interactions ofbeech trees: mineral ion leaching and absorption duringrainfall. Tree Physiol 1987;3:137–145.

Likens E, Bormann EH. Biogeochemistry of a forested eco-system. 2nd ed. New York: Springer-Verlag, 1995. (146 pp)

Liljestrand HM. Average rainwater pH, concepts of atmos-pheric acidity, and buffering in open systems. Atmos Envi-ron 1985;19:487–499.

Lindberg SE, Lovett GM. Field measurements of particle drydeposition rates to foliage and inert surfaces in a forestcanopy. Environ Sci Technol 1985;19:238–244.

Lindberg SE, Lovett GM, Richter DD, Johnson DW. Atmos-pheric deposition and canopy interactions of major ions ina forest. Science 1986;231:141–145.

Lovett GM, Lindberg SE. Dry deposition and canopy exchangein a mixed oak forest as determined by analysis of through-fall. J Appl Ecol 1984a;21:1013–1027.

Lovett GM, Lindberg SE. Atmospheric deposition and canopyinteractions of nitrogen in forests. Can J For Res1984b;23:1603–1616.

Lovett GM. Atmospheric deposition of nutrients and pollutantsin North America: an ecological perspective. Ecol Appl1994;4:629–650.

Loye-Pilot MD, Martin JM, Morelli J. Influence of Saharan¨dust on the rain acidity and atmospheric input to theMediterranean. Nature 1986;321:427–428.

Loye-Pilot MD, Martin JM. Saharan dust input to the Western¨Mediterranean: an eleven years record in Corsica. In: Guer-zoni S, Chester R, editors. The impact of desert dust acrossthe Mediterranean. Dordrecht: Kluwer, 1995. p. 191–199.

McCune DC, Botce RL. Precipitation and the transfer of water,nutrients and pollutants in tree canopies. Tree 1992;7:4–7.

McDowell WH, Likens GE. Origin, composition, and flux ofdissolved organic carbon in the Hubbard Brook Valley. EcolMonograph 1988;58:177–195.

McClaugherty CA. Soluble polyphenols and carbohydrates inthroughfall and leaf litter decomposition. Oecol Plant1983;4:375–385.

Parker GG. Throughfall and stemflow in the forest nutrientcycle. Adv Ecol Res 1983;13:57–133.

Pinol, J(1990) Hidrologia i biogeoquımica de conques fores-˜ ´tades de les Muntanyes de Prades. Ph.D. Thesis. Universitatde Barcelona. 232 pp.

Prebble RE, Stirk GB. Throughfall and stemflow on silverleafironbark (Eucalyptus melanophloia) trees. Aust J Ecol1980;5:419–427.

Pucket LJ. Estimates of ions sources in deciduous and conif-erous throughfall. Atmos Environ 1990;24:545–555.

Ragsdale HL, Lindberg SE, Lovett GM, Schaefer DA. Atmos-pheric deposition and throughfall fluxes of base cations. In:Johnson DW, Lindberg SE, editors. Atmospheric depositionand forest nutrient cycling. New York: Springer-Verlag,1992. p. 236–253.

Reuss JO, Johnson DW. Acid deposition and the acidificationof soils and waters. New York: Springer-Verlag, 1986. (119pp).

Rice WR. Analyzing tables of statistical tests. Evolution1989;43:223–225.

Roda F, Avila A, Bonilla D. Precipitation, throughfall, soil``solution and streamwater chemistry in a holm oak(Quercusilex) forest. J Hydrol 1990;116:167–183.

Roda F, Bellot J, Avila A, Escarre A, Pinol J, Terradas J.`` ´ ˜Saharan dust and the atmospheric inputs of elements andalkalinity to Mediterranean ecosystems. Water, Air, SoilPollut 1993;66:277–288.

Roda F, Avila A, Rodrigo A. Nitrogen deposition in Mediter-``ranean forests. Environ Pollut 2002;118:205–213.

Rodrigo, A (1998) Deposicio atmosferica en dos alzinars´ `(Quercus ilex, L.) del Montseny sotmesos a una exposicioćontrastada de contaminants de l’area barcelonina i valle-`sana. Ph.D. Thesis. Universitat Autonoma de Barcelona.ÙAB, 330 pp.

Rodrigo A, Avila A. Dry deposition to the forest canopy and`

surrogate surfaces in two Mediterranean holm oak forestsin Montseny (NE Spain). Water, Air, Soil Pollut2002;136:269–288.

Shanley JB. Field measurements of dry deposition to sprucefoliage and petri dishes in the black forest, F.R.G. AtmosEnviron 1989;23:403–414.

Sokal RR, Rohlf FJ. Biometry: the principles and practice ofstatistics in biological research. 3rd ed. New York: W.H.Freeman and Canopy, 1995. (1887 pp)

Tanner RL. Sources of acids, bases, and their precursors in theatmosphere. In: Lindberg SE, Page AL, Norton SA, editors.Acidic precipitation: sources, deposition and canopy inter-actions, and mitigation. New York: Springer-Verlag, 1990.

Terradas J. Holm oak and holm oak forest: an introduction.In: Roda F, Retana J, Gracia CA, Bellot J, editors. Ecologyòf Mediterranean evergreen oak forests. Berlin: Springer,1999. p. 3–14.

Tsusumi T, Nishitani Y. On the effects of soil fertility on thethroughfall chemicals in a forest. Jpn J Ecol 1984;34:321–330.

Wolaver, T.G. and Lieth, H.(1972) U.S. precipitation chem-istry. Theory and quantitative models. Univ. North Carolina,Division Ecol Res Chapel Hill.

the chemistry of precipitation, throughfall and stemflow in two holm oak (quercus ilex l.) forests...

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