History of vegetation during the Holocene in the Courel and Queixa Sierras, Galicia, northwest Iberian Peninsula

JOURNAL OF QUATERNARY SCIENCE (2000) 15 (6) 621–632Copyright 2000 John Wiley & Sons, Ltd.

History of vegetation during the Holocene inthe Courel and Queixa Sierras, Galicia,northwest Iberian PeninsulaLUISA SANTOS1,*, JUAN RAMON VIDAL ROMANI1 and GUY JALUT2

1Facultad de Ciencias, Universidade da Coruna, 15071 A Coruna, Spain2Lab. d’Ecologie Terrestre, CNRS/UPS, 39, allees Jules Guesde, 31062 Toulouse, France

Santos, L., Romani, J. R. V. and Jalut, G. 2000. History of vegetation during the Holocene in the Courel and Queixa Sierras, Galicia, northwest IberianPeninsula. J. Quaternary Sci., Vol. 15, pp. 621–632. ISSN 0267–8179.

Received 5 August 1998; revised 1 October 1999; accepted 4 October 1999

ABSTRACT: Palynological studies undertaken in the mountainous regions of the northwest ofthe Iberian Peninsula are few in number and have been concerned largely with the second halfof the Holocene. New pollen data from two Galician sierras, the Courel and Queixa Sierras,provide a 10000-yr record of vegetation and climate change. In the Courel Sierra before 9750yr BP, Laguna Lucenza (1420 m a.s.l.) reflects a period of open landscape covered by Poaceaeand heliophilous plants, which may be correlated with the Younger Dryas. The onset of theHolocene is characterised by the expansion of oak woodland, prior to 9300 yr BP, following ashort phase of birch along with the gradual decline of pine. The oak values reach a peak at8350 6 80 yr BP. Towards 8800 yr BP Corylus begins to expand, followed by Alnus (7500 yrBP) and Ulmus. During this period, the Fraga pollen assemblage (Queixa Sierra, 1360 m a.s.l.)indicates Betula woodland surrounding the site, masking the regional predominance of oak.After 5000 yr BP there is a gradual decrease in arboreal pollen values in both Sierras. Castaneaappears in Laguna Lucenza (Courel Sierra) at 4075 6 75 yr BP. There is widespread deforestationduring the last 4000 yr. During this period the presence of large quantities of microcharcoalparticles points to the occurrence of fire. The reduction in forest is associated with the arrivalof cultivation at 4000 yr BP at low altitudes in the Queixa Sierra. At higher altitudes the firstagricultural activity is dated at later than 2000 yr BP. This coincides with the first record ofcereal cultivation at high altitude in the Courel Sierra. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: vegetation history; holocene; pollen; charcoal; human impact; northwest Iberian Peninsula.

Introduction

The northwest of the Iberian Peninsula is in a key geographi-cal location, where climate is likely to be sensitive tochanges in North Atlantic surface conditions. Pollen analyti-cal studies of sediments from lacustrine basins have thepotential to reveal the nature of past vegetation and climate.These data could indicate the extent to which climate andvegetation were influenced by sea-surface conditions (Allenet al., 1996).

Although a considerable number of palynological diagramshave been published from the area (Salas, 1995), the vastmajority begin at around the middle of the Holocene andlack a reliable chronology; very few studies covering theLate-glacial and early Holocene have been undertaken.Consequently, vegetation and climate changes in the region

* Correspondence: L. Santos, Facultad de Ciencias, Universidade da Coruna,Campus da Zapateira, 15071 A Coruna, Spain.Email: xesantosKudc.es

are poorly understood, and the few data available for theLate-glacial are somewhat controversial. Moreover, they havenot been precise enough to establish the chronology of theprincipal changes that have occurred in the composition ofthe forest during these periods. Although recent works haveutilised longer sequences and contain a number of absolutedates (e.g. Van Mourik, 1986; Watts, 1986; Turner andHannon, 1988; Maldonado Ruiz, 1994; Allen et al., 1996;Fig. 1), there is still insufficient information to provide a fullaccount of the vegetation history.

Our first objective was to obtain new radiocarbon-datedsequences from the area, covering the entire Holocene and,where possible, the Late-glacial, in order to provide a com-plete pollen chronology. The second was to describe inmore detail the Holocene history of the vegetation andclimate for a relatively unknown mountainous region of thenorthwestern Iberian Peninsula.

Pollen and charcoal from the sediments of five 14C datedprofiles from this area were examined (Fig. 2). The LagunaLucenza sequence (Courel Sierra), based on eight radio-carbon dates, covers the last 10000 yr. In the Queixa Sierra,a palynological study was carried out on four sequences

622 JOURNAL OF QUATERNARY SCIENCE

Figure 1 Pollen sequences mentioned in the text. SQ: QueixaSierra (Las Lamas, 1360 m a.s.l. and Prada, 1100 m a.s.l.;Maldonado Ruiz, 1994; and Cheira de Piedrahita 1320 m a.s.l.and Malonga, 1300 m a.s.l.; Menendez Amor, 1971); LU: LagunaLucenza (1440 m a.s.l.; Aira Rodriguez, 1986); SX: Xistral Sierra(Pena Vella, 700 m a.s.l., Chan do Lamoso, 1039 m a.s.l., PenaViera, 620 m a.s.l., Prado do Inferno, 520 m a.s.l. and Chan daCruz, 800 m a.s.l.; Ramil Rego, 1992); LM: La Mata (1500 ma.s.l.; Belet, 1993, Belet et al., 1996); VI: Villaseca (1320 m a.s.l.;Jalut, unpublished results); LS: Laguna de las Sanguijuelas orSanabria Marsh (1050 m a.s.l.; Turner and Hannon, 1988); LR:Laguna de la Roya (1600 m a.s.l.; Allen et al., 1996); PU: Puertosde Riofrio (1700m a.s.l.; Menendez Amor and Florschutz, 1963);VN: Valle de la Nava (,1000 m a.s.l.; Menendez Amor, 1968);LP: Las Pardillas (1850 m a.s.l.; Sanchez Goni and Hannon,1999); TP: Padul (785 m a.s.l.; Pons and Reille, 1988); LC: LagoaComprida (1640 m a.s.l.; Janssen and Woldringh, 1981); QS:Quintanar de la Sierra (1470 m a.s.l.; Penalba, 1989, 1994); F:Ferreira (680 m a.s.l.; Van Mourik, 1986); M. Mougas (0 m a.s.l.;Saa Otero, 1985); LA: Lago de Ajo (1570 m a.s.l.; Watts, 1986);EB: Bidasoa estuary (0 m a.s.l.; Sanchez Goni, 1996).

(Fraga, Castelo Cerveira, H profile and As Aguilladas), whichare supported by four radiocarbon dates, covering approxi-mately the last 8000 yr. In this paper we present only datafrom the Laguna Lucenza and Fraga sequences, given thatthey contain the most complete record for the Holocene.These records will be compared with both more Atlanticand more continental sequences, as well as with those fromhigh-altitude zones in the Iberian northwest.

In addition to reporting new pollen records and the palaeocli-matic reconstructions based upon them, we provide new infor-mation focusing on the chronology of expansion of specifictaxa (e.g. Corylus, Alnus, Castanea, Fagus) as well as on thecharacteristics and timing of anthropogenic deforestation.

Environmental setting

The areas studied are located in two sierras of the northwestof the Iberian Peninsula, both of which were affected byglacial activity during the Quaternary (Santos Fidalgo, 1996):the Courel Sierra and the Queixa Sierra (Fig. 2).

The Courel Sierra is located in the southeast of the prov-ince of Lugo (Fig. 2). The climate of this sierra is complex.

Copyright 2000 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 15(6) 621–632 (2000)

The upper slopes are open to the effects of oceanic influ-ences, whereas, in the shelter of the valleys, summerdroughts reflect the distance from the coast. Mean annualprecipitation is around 2500 mm, and the mean annualtemperature oscillates between 7°C and 12°C (Aira Rodrıg-uez, 1986). The flora displays various phytogeographicalcharacteristics, reflecting the transitional position of thissierra between Eurosiberian and Mediterranean regions (IzcoSevillano et al., 1982). It is possible to identify Eurosiberian-type deciduous forests belonging to the phytosociologicalclass Querco-Fagetea (beech forest, Quercus pyrenaica for-est, Alnus forest, Betula forest), as well as Mediterranean-type perennial vegetation composed mainly of Quercus ilex.However, the present-day vegetation of the sierra is basicallyscrubland and pasture (Amigo Vazquez, 1985; GuitianRivera, 1985). The most abundant scrub includes heather(Calluna and Erica) and Atlantic and Mediterranean–Iberoatlantic species of Ulex.

The Queixa Sierra is located in the northern central regionof the province of Ourense (Fig. 2). Despite the overallcontinental character of the sierra, a complex relief givesrise to several distinct microclimates. Mean annual precipi-tation is around 2000 mm, and mean annual temperatureis less than 8°C (Aira Rodrıguez, 1986). As in Courel, thetransitional position of this sierra is responsible for the pres-ence of a mixed flora, in which the Eurosiberian element ispredominant. According to Izco Sevillano et al. (1982), theregional climax is an oak Quercus pyrenaica forest withQuercus robur and Vacinium myrtillus at high altitudes.Corylus, Fraxinus and Betula occupy higher or coolerlocations, or those with greater precipitation. Very few ofthese natural forests have remained. The repeated burningof forest to obtain pastureland is responsible for the extensionand dominance of the scrubland, which is presently themost widespread form of vegetation cover.

Description of sites

Several corings were carried out at selected sites within thetwo sierras: one in the Courel Sierra (Laguna Lucenza; Fig.2a) and four in the Queixa Sierra (Fraga, Castelo Cerveira,H profile and As Aguilladas; Fig. 2b). In this paper wepresent detailed data from only the Laguna Lucenza andFraga sequences. Laguna Lucenza (Courel Sierra) is a smalllake (0.5 ha; Fig. 2a) whereas Fraga (Queixa Sierra) is locatedin an infilled depression (0.1 ha; Fig. 2b).

The dominant vegetation of the surrounding areas is basi-cally scrubland and pasture. A small stand of Salix sp. islocated near Laguna Lucenza and a few specimens of Betulasp. are scattered around both sites on the upper slopes.

Laguna Lucenza was cored previously by Aira Rodrıguez(1986). She recovered 540 cm of sediment and obtained aradiocarbon date of 8540 6 100 yr BP (Ny-1065) at 450 cm.

Materials and methods

Two types of corer were used: the Russian corer (Jowsey,1966) for the Laguna Lucenza sequence and the Eijkelkampcorer (Vergne, 1987) for the Fraga sequence. Conventionalradiocarbon dating was carried out in the Laboratorio deIsotopos Ambientais (ICEN) in Sacavem (Portugal), and theTandem Laboratory (Ua) at the University of Uppsala

623VEGETATION HISTORY IN NORTHWEST IBERIA

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Figure 2 Location of the Sierras studied. (a) Detailed map of the Courel Sierra showing the location of the Laguna Lucenza sequence. (b)Detailed map of the Quiexa Sierra showing the location of the Fraga, Castelo Cerveira, H profile and As Aguilladas sequences.

(Sweden) provided the AMS dates (Table 1). A calibratedage range within two sigma confidence limits was obtainedusing the Calibration Program of Stuiver and Reimer (1993,Mac Test Version 7).

A total pollen count of between 300 and 400 grains persample was carried out, and the pollen concentration wascalculated according to Cour’s Method (Cour, 1974). Thepollen diagrams were divided into ‘pollen assemblage zones’(Birks and Birks, 1980). The pollen sum (T) includes pollenof trees, shrubs and herbaceous taxa except Pteridophytaand Cyperaceae, the latter being considered as representingmainly hydrophytic species. In addition to the pollen analy-sis, the slides were scanned thoroughly for rare pollen typesand microcharcoal particles. Although no quantitativemeasurements of these particles were made, relative abun-dance was estimated on a five-point scale.

The interpretation of the pollen diagrams from these smallbasins takes into account previous work on recruitment ofpollen in basins of different sizes (Jacobson and Bradshaw,1981; Prentice, 1985; Sugita, 1993, 1994; Jackson, 1994).These studies suggest that small lakes between 0.1 and0.5 ha in size may recruit pollen mainly from local veg-


etation (up to several hundred metres from the edge of thebasin), although there may be some representation of themore well-dispersed and abundant regional taxa.

Results

Lithology

At Laguna Lucenza (Courel Sierra), coring reached a depthof 490 cm, and the sediment is relatively uniform. A layerof bluish-grey compacted clay occurs at the base of thesequence. This is overlain by a layer of highly organic siltand clay with various changes in colour and texture, anorganic silty layer containing some slender roots, and ahighly organic clay layer with abundant roots at the top. Inthe Fraga sequence (Queixa Sierra), coring reached a depthof 175 cm. The lithology consists of a highly organic clayeysilt alternating with more detritic sediment units. A detailed


Table 1 Radiocarbon dates from the pollen profiles. The calibrated age range (Stuiver and Reimer, 1993) was calculated with a probabilityof 95.4% (two sigma). LUC, Laguna Lucenza; FRAG, Fraga; AGUI, As Aguilladas; CC, Castelo Cerveira. LS, lacustrine sediment; MLS, massivelacustrine sediment; P, peat

Depth Material Reference Age Calibrated Calibrated age d13CSite (cm) dated laboratory (yr BP) age (yr BP) (AD(+); BC(−)) (‰)

LUC 30 LS Ua-10832 2235 6 70 2351–2052 −401 −102 −26.77LUC 111 LS Ua-10833 4075 6 75 4735–4409 −2785 −2459 −27.64LUC 150 LS Ua-10141 4110 6 60 4734–4503 −2784 −2553 −29.10LUC 182 LS Ua-10834 5310 6 65 6209–5935 −4259 −3985 −28.74LUC 300–313 LS ICEN-1253 7180 6 140 8195–7661 −6245 −5711 −30.35LUC 400 LS Ua-10142 8350 6 80 9485–9185 −7535 −7235 −30.32LUC 440 LS Ua-10143 8950 6 85 10047–9807 −8097 −7857 −29.12LUC 460–470 MLS ICEN-1252 8990 6 400 10975–9204 −9025 −7254 −29.28FRAG 155–170 LS ICEN-932 8030 6 80 9047–8571 −7097 −6621 −26.77AGUI 30–40 P ICEN-1026 Modern −25.80AGUI 95–105 P ICEN-1025 550 6 130 717–304 +1233 +1646 −25AGUI 168–178 P ICEN-1024 2020 6 70 2141–1818 −191 +132 −26.69CC 175 P ICEN-1043 2720 6 90 3075–2710 −1125 −760 −26.30

sediment lithology of the Laguna Lucenza and Fragasequences is shown in Figs 3 and 4.

Radiocarbon dating

The results of radiocarbon dating are displayed in Table 1.Eight 14C dates were obtained on bulk samples of sedimentfor Laguna Lucenza (Courel Sierra). Figure 5 illustrates therelationship between uncalibrated 14C age and depth. In orderto avoid contamination problems due to the thickness of sedi-ment required for conventional 14C age determinations, it wasdecided that it would be more accurate to estimate the agesof the pollen samples by means of linear interpolations betweenthe six samples dated using AMS (Fig. 5). The relative homogen-eity of the sediment leads us to conclude that no great changeshave occurred either in the hydrodynamic conditions of thelake or, consequently, in the rates of sedimentation. We haveassigned an age 0 yr BP to the top of the sequence. Ages forpollen samples from below the oldest AMS dated sample wereestimated by extrapolation of the sediment accumulation ratebetween the two oldest dated samples; this provided an esti-mated age for the bottom of the sequence of ca. 9800 yr BP.The overall sedimentation rate is of 0.5 cm yr−1. The ageestimates in Fraga (Queixa Sierra) show a linear pattern ofsedimentation between the date obtained (8030 6 80 yr BP)and the uppermost level, the mean rate of sedimentation being0.02 cm yr−1.

Pollen sequences

Laguna Lucenza (Courel Sierra, 1420 m a.s.l.)

The pollen record was divided into 13 pollen assemblagezones (PAZs). These are numbered upwards from the baseof the sediments and referred to by the prefix LUC- (Fig. 3).A brief description of the pollen assemblage zones is givenin Table 2.


Fraga (Queixa Sierra, 1360 m a.s.l.)

The diagram was divided into five pollen assemblage zones(PAZs) (Fig. 4 and Table 3). These were numbered upwardsfrom the base of the sediments and given the prefix FRAG-.According to the 14C dates, this peat bog developed at least8000 yr ago (8030 6 80 yr BP ICEN-932). The most strikingcharacteristic of this pollen diagram is the great abundanceof Betula for most of the sequence. The removal of theBetula values from the pollen sum did not have a significanteffect on the counts for the other pollen types.

Vegetation and climate history in theCourel and Quiexa Sierras

The end of the Late-glacial

Before 9750 yr BP Laguna Lucenza (1420 m a.s.l.) reflects anopen landscape composed of Poaceae and heliophilous taxaat high altitudes in the Courel Sierra. This short period corre-sponds to a layer of low organic content (LUC-1) and may becorrelated with the end of the Younger Dryas event. Thereduced steppe conditions during this phase around LagunaLucenza (represented only by the dominance of Poaceae andlow percentages of Artemisia and Chenopodiaceae), comparedwith other Mediterranean zones, suggest that oceanic con-ditions prevailed in this area during the Younger Dryas. RamilRego et al. (1996) date the onset of organic sediment accumu-lation in Laguna Lucenza at 10 200 yr BP.

In the Queixa Sierra, the Fraga sequence (1360 m a.s.l.)does not record this period, whereas in the As Lamassequence (Maldonado Ruiz, 1994), the Younger Dryas ischaracterised by an increase in herbaceous taxa and arelative increase in Pinus. In absolute terms, however, allmajor tree taxa (Pinus, Betula, Quercus) decrease in abun-dance.

Evidence of palynological changes related to the YoungerDryas phase has been found at sites throughout northwestIberia. The existence of this pre-Holocene cold phase also


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Figure 5 Chronological model (age–depth relationship) for theLaguna Lucenza sequence showing straight-line interpolationsbetween AMS radiocarbon age determinations.

has been confirmed by other sequences from Galicia (PenaVella; Ramil Rego, 1992) and Cantabria (La Mata; Belet,1993; Belet et al., 1996; and Villaseca; Jalut, unpublishedresults) (Fig. 1). In contrast, in Sanabria Marsh (1085 ma.s.l., Sierra de Cabrera; Fig. 1) there is no evidence ofYounger Dryas conditions (Allen et al., 1996).

Early and mid-Holocene

Between 9750 and 9000 yr BP there is a local dominanceof Betula and Quercus woodland at 1420 m in the CourelSierra, along with a reduction in regional Pinus forest. Theappearance of more favourable conditions is supported bythe development of the lake ecosystem. After 9300 yr BP,the expansion of a Betula–Quercus forest at high altitude inthe Courel Sierra (1420 m a.s.l.) coincides with that at asimilar altitude (1360 m a.s.l.) in the Queixa Sierra, asshown by the As Lamas sequence (Maldonado Ruiz, 1994).At a higher altitude, Laguna de la Roya (1600 m a.s.l.) (Fig.1) reflects an earlier expansion (around 10000 yr BP) ofthe Quercetum mixtum in the Sierra de Cabrera (Allenet al., 1996).

Between 9000 and 7500 yr BP, the improvement in cli-mate becomes more evident. The Laguna Lucenza sequence(Courel Sierra) records the substitution of birch by denseoak forest at this altitude, reaching a maximum at 8350 6 80yr BP. Quercus competed with Betula, although the poorsubstratum favours the latter. This reflects the transitory,pioneering role of Betula. Pine and birch would haveexpanded at a higher altitude or remained present furtherinland. The local expansion of Corylus begins at around8800 yr BP in the Courel Sierra. This early expansion islikely to have been in areas of more oceanic climatic con-ditions (Sanchez Goni and Hannon, 1999).

In the Queixa Sierra, prior to 8030 6 80 yr BP, the Fragarecord (1360 m a.s.l.) indicates that the local vegetation was


dominated by Poaceae and Betula, the latter colonising thepeat bog itself as well as the surrounding area. Corylusappears to be an important component of the regional veg-etation. The weak representation of oak in Fraga contrastswith its abundance in the As Lamas basin (Maldonado Ruiz,1994), located at the same altitude and in the same sierra.This difference could be explained by the regional recruit-ment of pollen in As Lamas ($0.5 ha) compared with thelocal recruitment of Betula pollen in the Fraga peat bog (0.1ha). Betula also must have been present at the site itself.Turner and Hannon (1988) in Laguna de las Sanguijuelas(Sanabria Marsh; 1050 m a.s.l., Sierra de Cabrera, Fig. 1)also point to the local development of Betula, as indicatedby macroremains, the pollen of which would mask therecord of the regional Quercus dominant forest. Thus, in theQueixa Sierra, the evolution of the vegetation seems tofollow the same pattern as in the Courel Sierra, whereLaguna Lucenza ($0.5 ha) records a Quercus-dominant for-est. The greater initial development of Betula over that ofQuercus, as recorded by the Fraga sequence during thisinitial phase of the Holocene, is reflected in other pollendiagrams such as those from Puertos de Riofrıo (MenendezAmor and Florschutz, 1963) and Valle de la Nava(Menendez Amor, 1968) (Fig. 1). However, the authorsexplain the dominance of Betula in those sites as being theresult of higher altitude or greater continentality.

In the Queixa Sierra, the expansion of Corylus has beendated to at least 7630 6 80 yr BP (Menendez Amor, 1971),whereas more recent work places this expansion at ca. 8300yr BP (Maldonado Ruiz, 1994). Our data confirm the pres-ence of Corylus at least before 8030 6 80 yr BP. Accordingto Aira Rodrıguez (1986), the expansion of Corylus in theCourel Sierra occurred around 8540 6 100 yr BP. A newdate from Laguna Lucenza (Santos Fidalgo, 1996) puts itsregional development at earlier than ca. 8800 yr BP, whereasthe Laguna Lucenza diagram published by Ramil Rego et al.(1996) indicates Corylus expansion perhaps as early as ca.10000 yr BP. However, the absence of adequate informationon this chronology makes confirmation difficult. In any case,the data show that the expansion of Corylus in the moreoceanic Courel Sierra (ca. 8800 yr BP) occurred earlier thanin the more continental Queixa Sierra (8300 yr BP). Thiscontrasts with the hypothesis of Aira Rodrıguez (1986),which proposed an expansion of Corylus in the QueixaSierra earlier than in the Courel Sierra.

According to Penalba (1994), in the greater part of theIberian Peninsula the development of Corylus would havetaken place at around 8000 yr BP, i.e. 1000 yr later thanon the northern slopes of the Pyrenees. The author explainsthis difference in chronology by an eastwest migration ofthis taxon throughout the Pyrenees. This hypothesis does nottake into account the presence of charcoal particles in cavesof the Cantabrian Chain and the French Basque Country,which demonstrate the persistence of Corylus in refugiumzones (Uzquiano, 1992). In northern Spanish sequences,such as Laguna Lucenza and those from the southern slopesof the Pyrenees (Jalut et al., 1992; Montserrat Martı, 1992)(all located in the Eurosiberian region), Corylus expandsduring the very early Holocene. This contrasts with the lateexpansion of the taxon in the north of the Iberian Peninsulaproposed by Penalba (1994). Sanchez Goni and Hannon(1999) also point to an early expansion of Corylus (LasPardillas, 1850 m a.s.l.; Sierra de Neila; Fig. 1), probablydue to the existence of more oceanic conditions in the highaltitudes of the northwestern part of the Iberian MountainChain.

The retreat of Quercus at ca. 8300 yr BP in the Courel


Table 2 Description of the pollen assemblage zones in the Laguna Lucenza sequence (Courel Sierra)

Age (yr BP)PAZ Depth (cm) Description

LUC-12 and LUC-13 2200–0 The presence of substantial microcharcoal, followed by reduced forest cover30–0 involving all the tress, ca. 2000 BP (Roman times), combined with the elevated

values of Poaceae and Ericaceae reflect the opening up of the landscape,although Poaceae may indicate its local presence in the lake. The abundance ofmicrocharcoal particles suggests the occurrence of regional fires. Deforestation isevident and cultivated plants appear (Secale, Juglans, etc.). Human activity is notvery apparent in this sierra and only in the top few centimetres do pollen typesappear which typically indicate anthropogenic activity after 2000 yr BP

LUC-10 and LUC-11 4000–2200 Arboreal pollen values continue to fall, except for Salix, which increases locally.110–30 Poaceae and Ericaeae expand. A gradual increase in the occurrence of

microcharcoal particles indicates an increase in the frequency of fires due toanthropogenic influence and/or natural causes. There is an important increase inCyperaceae and Ranunculaceae pollen, perhaps associated with a sedimentarychange, after which peat is recorded in the sequence

LUC-6 to LUC-9 7500–4000 LUC-6 reflects maximum Corylus values and the appearance of continuous curves340–110 of Alnus, Ulmus, and Salix. Development of the Quercetum mixtum around the

site may have occurred during the climatic optimum of the Holocene. Oak, andto a lesser extent hazel, dominate. No evidence of an expansion of otherthermophilous taxa (Tilia), which are characteristic of this period exists. A slightincrease in concentrations of Betula and Ericaceae without any change insedimentation is observed at around 6200 yr BP (zone LUC-7). The consistentlyhigh concentrations of pollen are maintained practically up to the end of LUC-8(4110 yr BP), where there is a slight general decrease in arboreal pollen values,together with a regular presence of Cistus, and the local colonisation of Salix. Theappearance of Castanea also characterises this zone. The vegetation compositionappears to have remained stable for approximately 3500 yr. In LUC-9 there is agradual decrease in tree cover. Around 4500 yr BP, herbaceous plants (Poaceae,Rumex and Apiaceae) re-expand, suggesting a reduced forest cover, possibly ofanthropogenic origin

LUC-5 8350–7500 A decrease in both Quercus pollen percentages and total pollen concentrations400–340 coincides with a maximum in the occurrence of microcharcoal particles. This is

accompanied by a slight increase in Poaceae and Pinus. A local fire may beinferred from these data. The aquatic pollen suggests the development of asublittoral zone in which pollen of Cyperaceae, Potamogeton and Ranunculaceae(Batrachium group) are recorded

LUC-4 9100–8350 Values of Quercus continue to increase, indicating that the regional ecological450–400 conditions continue to favour expansion. The beginning of the continuous curve

of Corylus before 8800 yr BP dates the expansion of the local population of thistaxon. Subsequently, the expansion of Corylus is added to that of Quercus.Development of Corylus, associated with the regular presence of Alnus and Salix,reflects an increase in humidity. The herbaceous cover was poor and the lake isthought to have been surrounded by denser woodland than in the previous zone

LUC-3 .9300–9100 A general development of arboreal vegetation is evident. Thermo-mesophilous taxa465–450 expand from this point onwards. Betula pollen values display a brief maximum,

and Quercus begins its expansion here. At the same time, the previouslysignificant herbaceous taxa diminish or virtually disappear from the area

Sierra is also recorded in various pollen diagrams from theIberian Peninsula: Sanabria Marsh (Menendez Amor andFlorschutz, 1961; Turner and Hannon, 1988), Padul (Ponsand Reille, 1988), Lagoa Comprida (Janssen and Woldringh,1981) (Fig. 1), and generally has been interpreted asreflecting climatic deterioration. The expansion of Corylus,however, argues against the onset of colder conditions.Furthermore, in Laguna Lucenza, this period coincides withan increase in microcharcoal particles, suggesting that agreater frequency of fires could have caused a reduction ofQuercus woodland and the subsequent recolonisation by


Corylus. Moreover, recent diagrams from the Iberian Penin-sula (e.g. Quintanar de la Sierra, Penalba, 1994; Las Pard-illas, Sanchez Goni and Hannon, 1999; Fig. 1) do not recordthis climatic deterioration.

At ca. 7500 yr BP the higher altitudes of the Courel Sierrawere colonised by the Quercetum mixtum along with afurther expansion of Corylus. At the same time, the higheraltitudes of the Queixa Sierra were also colonised by thistype of forest, as indicated by the As Lamas diagram(Maldonado Ruiz, 1994). The Fraga diagram, however, sug-gests the local development of Betula woodland in marsh-


Table 2 Continued

Age (yr BP)PAZ Depth (cm) Description

LUC-2 .9300 Represents the onset of the Holocene. Local development of Cyperaceae487.5–465 appears to indicate a transition to a more nutrient-rich environment and the

beginning of the colonisation of the lake. Quercus and Betula forests beginto develop. Corylus, Alnus and Ulmus were all present, indicating pocketsof relatively mesic woodland. Pinus, Poaceae and heliophilous taxa pollendecrease within this zone. Some patches of herbaceous and dwarf shrubvegetation (Artemisia, Ericaceae, Rumex and Apiaceae) are also present.High concentrations of microcharcoal particles are recorded throughoutLUC-1 and LUC-2, indicating a persistent regime of natural fires

LUC-1 .9750 Represents approximately the final period of the Younger Dryas chronozone490–487.5 and corresponds to a layer of reduced organic content. Characterised by

low pollen concentrations and reflects an open vegetation. Relatively highvalues of Poaceae and the significant presence of other heliophilous taxa(Artemisia, Caryophyllaceae, Chenopodiaceae, Plantago, Rumex, Apiaceaeand Asteraceae) suggest an open environment and perhaps somewhat coolerconditions. Pinus pollen percentages make up 10–30% of the pollenrecruited to the site, which could be interpreted as isolated specimensgrowing in the area during this time period and a regional pollen transport

Table 3 Description of the pollen assemblage zones in the Fraga sequence (Queixa Sierra)

Age (yr BP)PAZ Depth (cm) Decription

FRAG-5 2000–0 The decrease in the AP values involves all the trees. The increase in pollen concentrations40–0 is accompanied by an increase in herbaceous and shrub colonisation (Poaceae and

Calluna) and a considerably higher content of microcharcoal particles in the sediment,indicating a possible increase in the frequency of fire. In fact, there is close correlationbetween Calluna pollen and burning, with or without evidence of human intervention. Thepresence of Castanea and cereal-type pollen grains is recorded for the first time

FRAG-4 4000–2000 The decrease in both arboreal pollen percentages and palynological concentrations may80–40 indicate a reduction in arboreal cover. Herbaceous plants (Poaceae and Ericaceae) re-

expand, coinciding with an increase in microcharcoal particles in the sediment. Thisimplies local fires that caused an erosional event, as observed by an increase in the inputof inorganic material in a 10-cm-thick sandy layer (between 60 and 50 cm). All theseobservations are possibly fire-related and may be of anthropogenic origin

FRAG-3 7000–4000 The pollen assemblages indicate that the surroundings at 1360 m a.s.l. were colonised by140–80 Betula forest. Birch was the dominant local taxon, coinciding with the progressive decline

of oak continuing until at least 5000 yr ago. Pollen values of Pinus gradually decrease.The increase in values of Corylus and Salix and the appearance of a continuous curve ofAlnus contribute to a regional mixed woodland. At around 6000 yr BP, the values ofarboreal taxa decrease, whereas Poaceae, Ericaceae and Calluna increase with no evidenceof sedimentary change. All this, combined with an increase in microcharcoal particles,may indicate the presence of fire, whether from natural or anthropogenic causes

FRAG-2 8000–7000 An increase in forest biomass is observed. The development of more stable conditions is160–140 also indicated by the sedimentation. The evolution of the vegetation cover, characterised

mainly by the development of Betula and Quercus, combined with the decrease in NAP(Poaceae, Apiaceae and Asteroideae), suggests that an open forest may have been presentlocally. A regional deciduous Quercus forest may have occupied the lowlands, Betulabeing present in the area surrounding the site

FRAG-1 .8000 Open vegetation initially surrounding the site, composed mainly of Poaceae, Apiaceae and175–160 Asteroideae. Arboreal cover was very poor with the exception of Betula (30%) and Pinus

(15%), the latter being interpreted as representative of Pinus forest growing at loweraltitudes. Isolated specimens of Quercus, Corylus and Salix may have been present nearby.The low organic content at the beginning of the zone is consistent with the existence ofunforested slopes surrounding the peat at this time



lands at these altitudes. The increase in Corylus and Salixand the appearance of the continuous curve for Alnus sug-gests further diversification of this mixed woodland, whichprobably was better developed at lower altitudes. Themaximum development of Quercetum mixtum in the Queixaand Courel Sierras at ca. 7500 yr BP occurred during theclimatic optimum of the Holocene.

In the Courel Sierra the expansion of Alnus occurred atabout 7500 yr BP. In the As Lamas (Maldonado Ruiz, 1994)and Fraga sequences in the Queixa Sierra, this occurs laterthan 7700 yr BP, and therefore was contemporaneous in bothSierras. Other authors (Menendez Amor, 1971) date the firstoccurrence of Alnus in the diagram of Cheira de Piedrahıta(1320 m a.s.l.) and Malonga (1300 m a.s.l.; Queixa Sierra) atlater than 7700 yr BP.

In the northernmost Galician sierra, the Buio Sierra (Ferreira,680 m a.s.l.; Fig. 1), the expansion of Alnus takes place muchlater, at around 4800 yr BP (Van Mourik, 1986). In contrast,Mougas (Saa Otero, 1985), on the Galician coast (Fig. 1),displays high percentages of Alnus earlier than 9800 yr BP.The beginning of the Alnus curve in the Mougas sequence asearly as 9800 yr BP may be explained by the fact that thegreater part of the coastal series studied in Galicia werereworked by solifluction (Santos Fidalgo, 1996; Costa Casaiset al., 1996). Penalba (1994) explains the early first appearanceof Alnus in Puertos de Riofrıo at 8800 yr BP as being dueto long-distance transport. It therefore would seem that thedevelopment of Alnus in the northwest of the Iberian Peninsulaoccurred between 8000 and 7000 yr BP. This early expansionalso contrasts with the later increase of this taxon in theeasternmost sequences, which is dated to between 6000 and4500 yr BP (Sanchez Goni, 1993). There is therefore a signifi-cant time difference in the timing of Alnus expansion betweenthe western and eastern parts of the northern Iberian Peninsula.

At ca. 6000 yr BP the Laguna Lucenza record reflects theslight expansion of Betula and moorland in the high altitudeof the Courel Sierra. Such a development is clearer at highaltitudes of the Queixa Sierra in the As Lamas diagram(Maldonado Ruiz, 1994). The author explains the phenomenonby suggesting a slight cooling of the climate from 6000 yr BPonwards. On the basis of quantitative climatic reconstructions(Guiot, 1987) such a cooling also can be detected in otherwestern European pollen diagrams. Astronomical calculationsalso predict this cooling from 6000 yr BP (Berger and Tricot,1986).

The late Holocene and human impacts

Between 5000 and 2500 yr BP there is a contraction in areasof mixed-forest in both the Queixa and Courel Sierras, asindicated by the Laguna Lucenza (Courel Sierra) and Fraga(Queixa Sierra) sequences, which probably may be linked toan increase in the frequency of fires. The reduction of theforest before the appearance of the first signs of agriculturalactivity also may be explained by a climatic change and/orhuman impact. The reduction in the arboreal cover is recordedin other pollen sequences (e. g. Watts, 1986; Turner andHannon, 1988; Penalba, 1989). Aira Rodrıguez (1986) hasinterpreted this deforestation as the result of climatic cooling,but it is difficult to disentangle the effect of climate from thatof human impact on vegetation. Maldonado Ruiz (1994) pointsout that in the As Lamas diagram, extensive deforestation wouldconceal the effects of climatic change.

The spread of moorland in the Queixa Sierra has been datedat 2700 yr BP in the Castelo Cerveira diagram (Santos Fidalgo,


1996; Fig. 2b). Maldonado Ruiz (1994) dates this event atbetween 2550 6 50 yr BP (As Lamas, 1360 m a.s.l.) and 2000yr BP (Prada, 1100 m a.s.l.) (Fig. 1). In the Courel Sierra theclearing of the forest resulted in an increase in Betula, althoughsites left by clearing of the oak were mostly occupied by grassesand moorland plants. The weak development of Ericaceae inthe Courel Sierra, however, may be explained by the base-rich nature of the soil in that area. Widespread forest clearancealso has been recognised in other southern and easternmostSierras, such as the Sanabria Marsh at ca. 1000 yr BP (1085m a.s.l., Allen et al., 1996) and Lago de Ajo at ca. 2000 yrBP (1570 m a.s.l., Allen et al., 1996) (Fig. 1). In Sanabria Marsh(Menendez Amor and Florschutz, 1961; Hannon, 1985; Turnerand Hannon, 1988; Allen et al., 1996), however, oak woodlandis still dominant with stands of Pinus at higher elevations. Oakmaintains its dominance in Lago de Ajo (Watts, 1986; Allenet al., 1996) with a decline in the values of Corylus and Ulmusand an increase in grass and moorland.

Castanea colonises the Courel Sierra from 4000 yr BP(4075 6 75 yr BP) although attaining its maximum represen-tation only in the last 2000 yr. In the Queixa Sierra, the AsLamas diagram records isolated pollen grains of Castanea asearly as 7000 yr BP. A continuous presence of this pollen isrecorded from 4000 yr BP in both this sequence and the Pradasequence and, from 2000 yr BP, in the As Aguilladas sequence(Santos Fidalgo, 1996; Fig. 2b). In all three cases the maximumrepresentation is attained at 1000 yr BP. The history of Castaneain the Iberian Peninsula throughout the glacial and interglacialperiods of the Pleistocene has been a matter of considerablediscussion (Uzquiano, 1992; Sanchez Goni, 1993). In parti-cular, several authors have associated the presence of Castaneawith the Roman period (Dıaz-Fierros Viqueira et al., 1979;Torras Troncoso et al., 1980; Penalba, 1989; Janssen, 1994),assuming that the Romans brought this tree to the IberianPeninsula. Our data refute the idea that its appearance inGalicia is linked to Romanisation, although certainly this taxonbecame more widespread as the result of Roman cultivation.

From 2500 yr BP up to the present, deforestation is progress-ive and accompanied by the development of Cerealia in bothSierras. At ca. 2700 yr BP the Castelo Cerveira diagram (QueixaSierra; Santos Fidalgo, 1996; Fig. 2b) shows scant arborealcover, in the form of Betula, and a dominance of grasses andEricaceae. The Laguna Lucenza and Fraga diagrams record,along with an increase in the values of Cerealia pollen, anintensification of fires associated with the abrupt increase in thevalues of grass pollen. This would seem to indicate extensiveanthropogenic activity in the region.

Whereas in Laguna Lucenza (1420 m a.s.l., Courel Sierra)isolated pollen grains of Cerealia are recorded at ca. 1500 yrBP, in As Aguilladas (1580 m a.s.l., Queixa Sierra; SantosFidalgo, 1996) a cereal curve (values at around 3%) appearsto reflect cereal cultivation at high altitudes. In Prada(Maldonado Ruiz, 1994), a significant presence of Cerealiapollen at lower altitudes of the Queixa Sierra (1100 m a.s.l.)is evident from 4000 yr BP. In other Galician low-altitudesequences, the first appearances of Cerealia pollen occur earl-ier; in Tremoal da Pena Veira (620 m a.s.l., Xistral Sierra) from5490 6 90 yr BP onwards (Ramil Rego, 1992), and before4740 6 40 yr BP in Ferreira (680 m a.s.l., Montes del Buio)(Van Mourik, 1986). In Prado do Inferno (520 m a.s.l., TerraCha), Cerealia has been identified at 4140 6 120 yr BP (RamilRego and Aira Rodrıguez, 1993).

Until the most recent millennium, as is indicated by the AsAguilladas sequence (1580 m a.s.l.; Santos Fidalgo, 1996),agricultural activity did not reach the high and inaccessiblemountain regions of the northwest of the Iberian Peninsula.


Allen et al. (1996) reached a similar conclusion based onevidence from the Laguna de la Roya sequence (1608 m a.s.l.).

At around 2000 yr BP isolated pollen grains of Fagus aredetected in the Queixa Sierra (As Aguilladas and H profile;Santos Fidalgo, 1996; Fig. 2b), whereas in the Courel Sierra,Laguna Lucenza records the first Fagus pollen much later (from1000 yr BP). According to Bennett (1985), a small number ofFagus trees will probably remain undetected by pollen analysis.Hence, these isolated records of Fagus pollen probably reflectthe existence of pockets of Fagus forest in the Queixa andCourel Sierras at around 2000 and 1000 yr BP, respectively.In fact, a beech forest close to the Laguna Lucenza (at ca. 3km) is recorded in the topmost level of the sequence by onlyone Fagus pollen grain. This contrasts with the idea proposedby Ramil Rego (1993) that a record of isolated pollen grainsindicates the occurrence of only a few individuals.

In other Galician sequences, the first appearance of Fagusis recorded at ca. 3500 yr BP (3680 yr BP in Ferreira, VanMourik, 1986; 3405 yr BP in Chan do Lamoso and 3350 yrBP in Chan da Cruz, Ramil Rego, 1992; Fig. 1), although theexpansion of this taxon in northwest Spain is later, at about1510 yr BP (Van Mourik, 1986). Penalba (1994) suggests thepresence of isolated occurrences of Fagus before 3000–4000yr BP. Fagus pollen has been recorded in central northernSpain prior to 6000 yr BP in the estuary of the Bidasoa(Sanchez Goni, 1996) and prior to 7000 yr BP in the LasPardillas sequence (Sanchez Goni and Hannon, 1999) (Fig. 1).Charcoal identifications from numerous archeological sites alsoindicate the presence of Fagus trees in northern Spain fromthe beginning of the Holocene (Uzquiano, 1995).

The later record of Fagus in Galicia may be due to themajor expansion of this tree in northern Spain dated at around3000 yr BP. The present-day western limit of this taxon is innorthwest Spain (Courel Sierra).

The most recent botanical event, as reflected in the increasein Pinus in the As Aguilladas diagram (Queixa Sierra; SantosFidalgo, 1996), is the establishment of pine forests at altitudesabove 1700 m within the past 200–300 yr (Dıaz-FierrosViqueira et al., 1982). The Late-glacial Pinus forest disappearedwith the onset of the Holocene, and was replaced by anAtlantic deciduous forest. The relatively low Galician Sierrashave since been dominated by oak forest, leaving no spacefor Pinus until the above-mentioned recent reforestation.

Conclusions

Pollen, charcoal and radiocarbon evidence from the sedimentsof five sequences from the mountain areas of northwesternIberia provide a new record of vegetation and climate changefrom the end of the Younger Dryas up to the present.

In the Courel Sierra before 9750 yr BP, Laguna Lucenza(1420 m a.s.l.) reflects a period of open landscape which maybe correlated with the Younger Dryas event. The YoungerDryas is characterised by reduced steppe elements owing tothe oceanic character of the Courel Sierra. Quercus forestdominates the first part of the Holocene in both Sierras, withBetula woodland colonising marshland areas, and Pinus gradu-ally declines as the climate becomes more oceanic. In contrastto previous results, the spread of Corylus has been dated atca. 8800 yr BP in the Courel Sierra and before 8300 yr BP inthe Queixa Sierra, the more oceanic climate of the CourelSierra favours the early expansion of this taxon. After 5000 yrBP there is a gradual decrease in arboreal pollen in recordsfrom both sierras. The presence of Castanea from 4075 yr BP


in the Courel Sierra confirms its indigenous character. A generaldeforestation is evident over the last 4000 yr, and the presenceof large quantities of microcharcoal particles points to thewidespread occurrence of fire. The reduction of the forest isassociated with the arrival of agriculture at 4000 yr BP at lowaltitudes of the Queixa Sierra. At higher altitudes the firstagricultural activity is dated to after 2000 yr BP, and coincideswith the first record of cereal cultivation at high altitude in theCourel Sierra.

Acknowledgements We wish to thank C. Monge, A. Grandal, R.Bao, M. Dedoubat and B. Jalut for their assistance in the field whencoring in 1991 and to M. Leira in 1994. We gratefully acknowledgethe assistance of Miguel Lopez Caeiro with the laboratory work.One of the authors (L. S.) wishes to acknowledge the contributionof Marıa Fernanda Sanchez Goni in offering valuable suggestions,and stimulating discussion during the course of this work. We thankProfessor C. R. Janssen and an anonymous referee for their valuablesuggestions, which improved the manuscript. Special thanks are dueto Tim Quinn who translated the text from Spanish and to AnaEiroa for revising the final version. This paper is a contribution toProject XUGA10307B93 (Xunta de Galicia).

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Documents

History of vegetation during the Holocene in the Courel and Queixa Sierras, Galicia, northwest Iberian Peninsula