Proxy climate and vegetation changes during the last five millennia in NW Iberia: Pollen and non-pollen palynomorph data from two ombrotrophic peat bogs in the North Western Iberian Peninsula

Review of Palaeobotany and Palynology 141 (2006) 203 – 223 www.elsevier.com/locate/revpalbo

Proxy climate and vegetation changes during the last five millennia in NW Iberia: Pollen and non-pollen palynomorph data from two ombrotrophic peat bogs in the North Western Iberian Peninsula T.M. Mighall a,⁎, A. Martínez Cortizas b , H. Biester c , S.E. Turner d a

Department of Geography and Environment, University of Aberdeen, Elphinstone Road, Aberdeen, AB24 3UF, UK b Edafología y Química Agrícola, Fac. Biología, Campus Sur, Santiago de Compostela, E-15782, Spain c Institute of Environmental Geochemistry, University of Heidelberg, INF 236 69120, Heidelberg, Germany d Geography, Coventry University, Priory Street, Coventry, CV1 5FB, UK Received 4 January 2005; accepted 20 March 2006 Available online 12 June 2006

Abstract Pollen and non-pollen palynomorph data are presented from two radiocarbon-dated ombrotrophic peat bogs from the Xistral Mountains in the North Western Iberian Peninsula. The results suggest that vegetation changes over the last five millennia are the result of human disturbance and climate change. Four major periods of forest disturbance are recorded: during the Late Neolithic, Metal Ages, Roman period and culminating in the permanent decline of deciduous forests since the Middle Ages, as agriculture and metallurgy intensified. Records of non-pollen palynomorphs, particularly those derived from fungi, proved to be useful indicators of climate change and human activity. Discriminant and cluster analysis suggest that trends in certain pollen and NPP reflect changes in humidity and to a lesser extent temperature. Cyperaceae and Types 18 and 18b increase during more humid, wet phases, whilst Type 306 increases during drier phases. Various ascospores, derived from coprophilous fungi, complement changes in pollen taxa to infer human activity. © 2006 Elsevier B.V. All rights reserved. Keywords: ombrotrophic bog; proxy climate record; pollen; non-pollen palynomorphs; NW Iberia

1. Introduction It is now well accepted that peatlands, in particular ombrotrophic bogs, are important archives of Holocene climate change (Chambers and Charman, 2004). Peat humification as well as quantitative macro- and microfossil analyses, including plant macrofossils, testae amoebae, pollen and, to a lesser extent, other non-pollen palynomorphs (NPPs), have been used to ⁎ Corresponding author. E-mail address: [email protected] (T.M. Mighall). 0034-6667/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2006.03.013

reconstruct Holocene proxy climatic changes in NW Europe (e.g. Barber, 1981; Blackford and Chambers, 1991; Chambers et al., 1997; Charman et al., 1999; Hughes et al., 2000). Although little is known about many of the NPPs often recorded in peat bogs, previous studies have shown that they can provide useful palaeoenvironmental information as part of a multi-disciplinary study including evidence for human activity and climate (van Geel, 1986; Blackford, 1993; Innes and Blackford, 2003; van Geel et al., 2003). van Geel (1972, p. 269) suggests that ‘there are conceivably species among fungal remains whose occurrence

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gives information about certain climatic factors, such as temperature and humidity, prevailing at the time of deposition of their spores in the bog.’ Changes in macro- or microfossil composition, including NPPs, of consecutive samples can be used to infer past changes in local moisture conditions and therefore about changes in effective precipitation (Blaauw et al., 2004). For example, van Geel's T.10 and 12 have often been used to indicate relatively dry conditions on peat bog surfaces (e.g. van Geel, 1986; Blaauw et al., 2004). NPPs have also been successfully used to support evidence for vegetation changes associated with agriculture and settlement (e.g. van Geel et al., 1982/1983, 1986a,b). Ascospores produced by coprophilous species, such as the Sordariaceae, Podospora-type (T.368) and Tripterospora-type (T.169), have been used to provide evidence of dung and grazing animals in forest openings exploited by Mesolithic peoples (e.g. Innes and Blackford, 2003) and following the introduction of herbivores during historical times in the western United States (Davis, 1987), whilst spores such as those produced by Neurospora (Type 55C) indicate local bog fires (van Geel, 1986). This paper aims to reconstruct vegetation changes during the mid- to late Holocene using pollen and NPP data from two ombrotrophic peat bogs in NW Spain and to examine the extent to which (1) human activity has been responsible for vegetational changes in the region during the last five millennia and (2) changes in the mire pollen taxa and NPPs can be used to infer changes in past climate. 2. Sites and methods 2.1. Site characteristics and sampling The Pena da Cadela (PDC) and Borralleiras da Cal Grande (BLL) ombrotrophic peat bogs are situated in the Xistral Mountains and Montes do Cabaleiros in Northwestern Spain at elevations of 970 and 600 m a.s. l., and at 25 and 15 km south of the coast, respectively (Fig. 1). The PDC bog is a saddle mire of 0.5 ha, slightly domed at its centre, immersed in a large blanket macrotope that covers some 800 ha whilst BLL is a blanket mire (watershed mire, after Lindsay, 1995). The microtopography of both bogs is very smooth. The mean annual temperature at the elevation where the bogs are located is 7.5 °C and 11.5 °C and mean annual precipitation is some 1800 mm and 1400 mm, respectively (Martínez Cortizas and Pérez Alberti, 1999). Present-day vegetation on these mires is dominated by

Fig. 1. Location of study area and sample sites. PVO = Penido Vello (after Martínez Cortizas et al., 1999).

sedges (Carex durieui, Carex vulgaris, Carex panicea, and Eleocharis multicaulis) and grasses (Agrostis curtisii, Agrostis hesperica, Molinia caerulea, and Deschampsia flexuosa) but heathers are also present (Erica mackaiana and Erica cinerea) (Fraga Vila et al., 2001). The ombrotrophic nature of this bog is corroborated by the low ash contents, pH and bulk density, as well as other geochemical parameters as discussed by Martínez Cortizas et al. (2002). Peat monoliths were obtained in October 1998 and August 2000 by direct sampling new, freshly exposed sections of recently open ditches by cutting sections of 25 × 25 × 25 cm3 down to 185 cm at PDC (the depth of the water table at that moment) and 240 cm at BLL. The maximum depth of the PDC bog at the point the monolith was sampled is 2.5 m, but in the centre of the formation is up to 5 m; while the BLL monolith was taken until the mineral base. The sections were wrapped in plastic bags, then aluminium foil, and brought to the laboratory. The fresh monoliths were immediately sliced into 2-cm sections. 2.2. Pollen and NPP record Pollen, NPP and microscopic charcoal analyses were undertaken using standard techniques. Plugs of 1 cm in diameter from each peat slice were prepared for pollen

T.M. Mighall et al. / Review of Palaeobotany and Palynology 141 (2006) 203–223 Table 1 Bulk Radiocarbon and calibrated ages for different depths at the Pena da Cadela and Borralleiras da Cal Grande peat bogs (PDC radiocarbon age dates after Martínez Cortizas et al., 2002) Depth (cm)

Radiocarbon age (14C yr BP)

Calibrated age (cal yr)

Laboratory code

PDC 10 30 50 70 100 110 130 184

150 ± 60 450 ± 60 770 ± 60 1230 ± 60 1980 ± 60 2310 ± 70 2630 ± 70 4620 ± 80

1665–1890 AD 1425–1460 AD 1220–1285 AD 700–800 AD 45 BC–80 AD 405–365 BC 830–785 BC 3630–3565 BC

B-127477 B-127478 B-127480 B-127481 B-127482 B-127483 B-127484 B-127486

BLL 10 30 60 90 120 150 180 210 230

160 ± 60 1030 ± 60 1560 ± 60 1960 ± 60 2370 ± 60 2900 ± 60 3360 ± 60 3850 ± 70 4660 ± 70

1660–1900 AD 890–1075 AD 390–625 AD 95 BC – 185 AD 600–360 BC 1260–920 BC 1775–1515 BC 2475–2130 BC 3640–3330 BC

B-145560 B-145561 B-145562 B-145563 B-145564 B-145565 B-145566 B-145567 B-145568

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using the point-count estimation method described by Clark (1982). Selected peat samples were sent to Beta Analytic Inc (Miami, USA) for radiocarbon dating. The radiocarbon dates are calibrated using CALIB 4.1 radiocarbon calibration program and IntCal98 (Stuiver and Reimer, 1993; Stuiver et al., 1998). The ages of the base of the monoliths indicate that they represent more than 5000 calibrated years of peat accumulation (Table 1). Age/ Depth models were obtained by fitting non-linear polynomial equations to calibrated ages (Martínez Cortizas et al., 2002) and thus all dates mentioned in this paper are given as calibrated years (AD or BC). 2.3. Statistical methods

and NPP analysis using the procedure described by Barber (1976). Pollen, NPP and charcoal were analysed at 2-cm intervals. At least 500 land pollen grains were counted for each sub-sample and for most levels at least 100 NPP were also recorded. One Lycopodium clavatum tablet was added to each sub-sample (Stockmarr, 1971) in order to calculate charcoal concentrations. Pollen identification was made using the identification keys from Faegri et al. (1989) and Moore et al. (1991) and a pollen type slide collection housed in the Geography Subject Area at Coventry University. Grass pollen grains over 40 μm were either classed as cereal-type (based on cereal pollen morphology as described in Faegri et al. (1989) or as Poaceae (> 40), when the pollen morphology was inconsistent with those characteristics used to differentiate cereal pollen grains and therefore they are considered to be wild grasses. NPPs were identified using the type system devised by van Geel. Type 10b was separated from T.10 as the basal cells were not as wide or wider than the other cells. The pollen data are calculated as percentages of total land pollen (TLP), excluding spores and aquatics from the total sum. Spores and aquatics are also expressed as percentages of TLP, whilst NPP are expressed as the combined sum of TLP and NPP. The pollen diagrams were constructed using Tilia and Tilia.graph (Grimm, 1991–1993). Zones were delineated using CONISS. Microscopic charcoal is expressed as a concentration

Cluster and discriminant analysis were performed using the SPSS software package. Since the objective of this analysis is to establish the affinity between pollen and NPP types, cluster analysis was performed in the variable mode and normalising the data populations using Z scores to avoid the size effect in the classification. A two-step procedure was followed in order to check first the relations among NPPs and then of the other pollen types to the NPPs. The first step only included the NPP records and in the second the other pollen types were incorporated. To examine the relationship between the behaviour of pollen and NPP with climate, in the discriminant analysis we used a simple classification of samples between wet and dry periods, based on the description given, and supported by the HHg/MHg ratio, as a grouping variable (see Martínez Cortizas et al., 1999). Thus, the analysis tries to look for both the minimum set of variables (abundances of NPPs and pollen types), which supports the classification of wet/dry periods and also measures the degree of agreement between the proxies. 3. Results Radiocarbon dates for PDC and BLL are presented in Table 1. Pollen and NPP data are shown in Figs. 2 and 3. A description of the major changes recorded in each pollen diagram is presented in Tables 2 and 3. 3.1. Changes in vegetation and land use 3.1.1. Pollen and NPP records The PDC and BLL pollen records show that the area contained a mixed deciduous forest dominated by oak and hazel (Quercus and Corylus avellana-type), while the total percentage of tree pollen reflects several major

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Fig. 2. Percentage pollen diagram for Pena da Cadela.

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Fig. 2 (continued).

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Fig. 2 (continued).

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209

Fig. 3. Percentage pollen diagram for Borralleiras da Cal Grande.

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Fig. 3 (continued).

210

211

Fig. 3 (continued).

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Fig. 3 (continued).

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Table 2 Zone characteristics for Pena da Cadela (TLP = total land pollen; AP = arboreal pollen; NAP = non-arboreal pollen) Zone

Depth (cm)

Age (BP)

Characteristics

1

Base–163

c.4620–3845

2

163–145

3845–3185

3

145–101

3185–1980

4

101–75

1980–1355

5

75–35

1355–515

6

35–21

515–315

7

21–0

315–present

Total AP is greater than 50%, characterised by high Quercus and Corylus avellana-type. Alnus and Betula are also well represented. Notwithstanding the occasional reversal, Cyperaceae increase throughout the zone whilst Poaceae decrease. Cereal pollen, Plantago lanceolata, Asteraceae (Lactuceae) and Potentilla-type are recorded. Pteridium increases gradually. Type 18 increases throughout the zone whilst T.18b is well represented. Type 16A peaks in mid-zone. Charcoal concentrations are high but decrease mid-zone. Total AP % reach a mid-Holocene maximum, c. 60–65 TLP % and is characterised by high Quercus and Corylus avellana-type. Alnus and Ulmus increase mid-zone. Cyperaceae percentages fluctuate before declining at the end of the zone whilst Poaceae decrease across the zone boundary and continue to fall until mid-zone. Cereal-type pollen is present in the first part of the zone and Plantago lanceolata, Brassicaceae and Artemisia-type are recorded in low percentages. Pteridium percentages peak in mid-zone. Types 10, 10b, 16A, 18b, 28 and 55A are all recorded whilst T.18 peaks in mid-zone. Charcoal concentrations fall dramatically across the zone boundary and remain low. Total AP % decreases across the zone boundary. It recovers by the middle of the zone only to fall again. Quercus generally follows this pattern, but remains the dominant taxon, with Corylus avellana-type, which increases in mid-zone but declines in the later stages. Alnus, Betula, Pinus and Ulmus form part of the woodland whilst Fagus makes its first appearance. NAP % increase at the start of the zone, with Poaceae and Cyperaceae both well represented. Cyperaceae decline mid-zone as AP % recover but Poaceae increase after an initial mid-zone drop. Cereal pollen is recorded sporadically but Plantago lanceolata curve is fairly continuous. Other NAP taxa such as Apiaceae, Artemisia-type and Brassicaceae are recorded in low percentages, whilst Anthemis-type peaks at the end of the zone. Charcoal concentrations increase from mid-zone but fall close to the upper zone boundary. Types 16A, 18 and 18b are well represented throughout the zone but achieve mid-zone maxima. Type 28 is regularly recorded whilst T.306 features in the initial stages of zone 3. Total AP percentages continue to fall across the zone boundary until the middle of zone 4, when they recover before falling once again by the end of the zone. Quercus and Corylus avellana-type are most affected although both Pinus and Ulmus are also recorded in lower percentages. Despite the late fall in total AP %, Castanea is first recorded at the end of this zone. Charcoal concentrations peak at the zone 3–4 boundary before decreasing too much lower values. Poaceae initially increase across the zone boundary but fall to a mid-zone minimum before increasing sharply at the end of the zone when cerealtype pollen is also recorded. Plantago lanceolata, Apiaceae, Artemisia-type are all regularly recorded throughout zone 4 whilst Filipendula peaks. Types 18 and 18b increase in the first half of the zone, whilst T.16A and T.28 are regularly recorded. Type 72 is recorded for the first time at the end of the zone when T.306 percentages rise sharply as T.16A, 18 and 18b all fall. Total AP percentages are much lower, especially Quercus, in this zone and continue to decrease until 55 cm when Quercus and Corylus avellana-type suffer another fall in values. Microscopic charcoal values are higher during this time. From there, they stabilise. Castanea is regularly recorded and Juglans is more common by the end of the zone. Calluna increases and Ericaceae are more common throughout this zone. Poaceae increase substantially across the zone boundary and Cyperaceae increase until the latter stages of zone 5. Plantago lanceolata, Apiaceae and Artemisia-type are amongst many NAP taxa that are regularly present in this zone. Cereal-type, including Hordeum and Triticum-type are more common by the end of the zone. Type 306 is well represented for the first half of the zone but is eventually replaced by T.18 and 18b. Types 10/10b and 28 are consistently recorded whilst T.55A is poorly represented mid-zone. Total AP percentages increase over the 5/6 zone boundary, especially Castanea and Corylus avellanatype. However, both decrease mid-zone. Pinus becomes more prominent. Cyperaceae increase throughout the zone, whilst Poaceae and Calluna percentages are lower compared to the previous zone. Plantago lanceolata, Apiaceae and Artemisia-type are still well represented and Cereal-type and Triticum-type increase in the early part of the zone. Types 18 and 18b decrease whilst T.55A and T.306 increase. T.28 is regularly recorded as are T.169, T.16 and T.6. Total AP percentages increase as Pinus values rise throughout the zone. Castanea percentages decrease by the end of the zone but Oleaceae are well represented. Cyperaceae percentages fall as Calluna, Ericaceae and Poaceae are consistently present. Cereal-type pollen and numerous NAP taxa are also recorded, especially Plantago lanceolata, Apiaceae, Potentilla-type and Rumex spp. Types 10 and T.306 increase whilst T.18 and 18b fade. Types 6, 55A, 169 and 368 are all well represented.

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Table 3 Zone characteristics for Borralleiras da Cal Grande (TLP = total land pollen; AP = arboreal pollen; NAP = non-arboreal pollen) Zone

Depth (cm)

Age (BP)

Characteristics

1

Base–219

?–4215

2

219–181

4215–3360

3

181–169

3360–3190

4

169–123

3190–2425

5

123–99

2425–2095

6

99–73

2095–1705

7

73–53

1705–1435

8

53–15

1435–380

Total AP percentages increase, in particular Quercus and Corylus avellana-type, for the first half of the zone. They then decrease as Betula and Alnus are regularly recorded. Calluna initially decreases but recovers mid-zone. Poaceae increase rapidly mid-zone as Pteropsida and Polypodium values fall. Other NAP taxa such as Asteraceae, Asteraceae (Lactuceae), Brassicaceae and Cirsium are well represented during the latter stages. One cereal-type pollen is recorded. The occurrence of NPP is limited to T.303 and T.18, 18b and T.52. Notwithstanding the occasional reversal, total AP percentages gradually fall in the early stages. There is a mid-stage recovery than they fall again. Corylus avellana-type mainly follows this trend whilst Quercus increases. Betula and Alnus are well represented. Calluna occurs throughout the zone but suffers two declines in the first part of the zone. Cyperaceae increase then values fall to a mid-zone minimum whilst Poaceae and cereal-type peak. Cyperaceae recover and increase to the zone boundary. A range of NAP occur in this zone, in particular Brassicaceae, Potentilla-type, Plantago lanceolata, Pedicularis-type and Asteraceae, whilst Pteridium increases mid-zone. Types 18 and 18b proliferate whilst T.55A, Sordariaceae, T.16 and 16C are well represented. Total AP percentages fall, especially Quercus, Corylus avellana-type, Pinus and Betula, until mid zone then they all recover apart from Pinus. Cyperaceae is the main beneficiary, peaking mid-zone, whilst Poaceae also fall. No other NAP taxa peak but Ericaceae, Pedicularis-type, and Pteropsida values fall across the zone boundary. Types 16A, 18, 18b, 28 and 55A are all well represented. Total AP percentages continue to rise for the first half of this zone, with Pinus, Quercus and Corylus avellana-type and Betula all increasing. This AP expansion stops mid-zone with a sudden decrease and total AP percentages then fluctuate for the remainder of the zone. Calluna is well represented and Poaceae increase throughout the zone, although Cyperaceae are the dominant NAP taxa. Other NAP taxa, such as cereal-type, Plantago lanceolata, Apiaceae, Brassicaceae and Chenopodiaceae, are regularly recorded. Narthecium, and Sphagnum are more prominent in this zone whilst Pteridium exceeds 3% TLP. Types 16A, 18, 18b, 28 and 55A fluctuate throughout the zone and T.303 increases sharply at the upper zone boundary. Total AP percentages increase across the zone boundary then fall in two stages before gradually recovering in the second part of the zone. The initial peak is due to an increase in Corylus avellana-type which then declines whilst Quercus percentages increase throughout the zone. Taxus peaks mid-zone whilst Alnus, Betula and Pinus are all well represented. Calluna increases in representation in zone 5 and Empetrum and Ericaceae are consistently recorded. Poaceae percentages remain stable whilst Cyperaceae peak in the lower and upper part of the zone, the latter with Sphagnum. Plantago lanceolata and Brassicaceae increase in the upper part of zone 5 whilst Artemisia-type and Rumex acetosa/ acetosella-type are recorded in low percentages. Type 18 increases during zone 5 whilst 18b, 28, 35, 55A and Sporormiella-type are regularly recorded. Type 16 peaks early on, whilst T.10 is present in the latter stages. Apart from the occasional short-lived reversal, total AP percentages remain around 50% TLP, dominated by Quercus, which decreases gradually until the end of the zone, and Corylus avellana-type. Castanea pollen is recorded for the first time. Calluna and Ericaceae pollen peak mid-zone. Poaceae gradually increase to peak towards the end of the zone, whilst Cyperaceae fall mid-zone and do not recover to their pre-zone values. Sphagnum spores initially increase then fall and peak against the end of the zone. Cereal type pollen is recorded at the start of the zone whilst Artemisia-type, Rumex spp., Brassicaceae, Apiaceae and Plantago lanceolata are regularly recorded. Types 16A and 16C peak early on, whilst 18 and 18b, 35 and 55A are consistently recorded. Total AP percentages gradually decrease, especially Quercus and Corylus avellana-type, towards the end of the zone. Ericaceae type pollen falls rapidly at the start of the zone whilst Calluna increases to exceed values recorded the previous zone. Cyperaceae and Poaceae both increase but other NAP taxa are only recorded in low, sporadic percentages. Cereal type pollen is present by the end of the zone. Types 10 and 10b are well represented in mid-zone whilst T.18 values fade. Type 55A is regularly recorded until the latter stages of the zone. Type 303 peaks at the upper zone boundary. Total AP pollen increases across the zone boundary and maintains higher percentages until the middle of the zone. Both Quercus and Corylus avellana-type decrease whilst Castanea increases in representation. Despite the occasional reversal Calluna and Ericaceae increase. Poaceae and Cyperaceae are well represented but decrease in value in the latter part of the zone. Pteridium and a suite of NAP taxa, including Artemisia-type, Cereal-type, Asteraceae and Plantago lanceolata are consistently recorded whilst Brassicaceae peak in the upper part of the zone. Types 18 and 18b increase dramatically in the top of the zone whilst T.28, 35, 55A and 368 are regularly recorded. Type 303 has two peaks in the lower part of the zone but it is not recorded in great numbers in the top part of the zone.

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Table 3 (continued) Zone

Depth (cm)

Age (BP)

Characteristics

9

15–0

380–Present

Total AP percentages initially decrease, especially deciduous taxa, but recover at the end of the zone as Pinus increases sharply. Calluna increases and Ericaceae are well represented whilst Cyperaceae and Poaceae do not achieve the values recorded in zone 8. Cereal-type is consistently present along with Plantago lanceolata and a suite of other NAP taxa are recorded in low, sporadic percentages. Types 10, 10b, 46, 55A, 169, 368, 733 and Sordariaceae dominate the NPP.

phases of expansion and regression. However, evidence for human activity is present throughout both records (Figs. 2 and 3). Four main phases of the deciduous forest decline at PDC and BLL coincide with known cultural periods in NW Spain: the Middle Neolithic period (beginning by 4000–3500 BC), the Late Bronze Age and Iron Age (1200–100 BC), the Roman period (100 BC–400 AD) and since the Germanic period (Visigoth Kingdom) in the very early Middle Ages (7th century AD). Quercus, C. avellana-type, Alnus and Betula show most of the major changes in forest composition. Martínez Cortizas et al. (2005) have also reconstructed the chemical record preserved in the PDC peat profile and compared the results with changes in forest composition. The analysis of the composition of the deposited dust based on lithogenic elements contained within the peat (K, Ca, Ti, Rb, Sr, Y, Zr) suggests the dominance of more local sources for the Middle–Late Neolithic, the Metal Ages and the Middle Ages, and regional sources for the Roman period and the Industrial Revolution. 3.1.1.1. Middle–Late Neolithic. A decrease in Quercus is responsible for the relatively low arboreal pollen (AP) percentages at the base of the PDC core (below 178 cm, > 2980 BC). Quercus pollen recovers by 174 cm as total AP percentages gradually increase throughout the remainder of zone PDC 1 (Fig. 2). This local phase of forest disturbance is reflected by the presence of relatively few non-arboreal (NAP) taxa normally associated with human activity: only Poaceae, cereal-type, Plantago lanceolata and Potentilla-type pollen are recorded and they do not occur in percentages that exceed those during periods of forest growth and stability suggesting that this early possible phase of human activity is of low intensity. This interpretation is supported by the lack of NPP that are characteristic of grazing, fire or dung. Although the forest expands in the latter stages of zone PDC 1 and during PDC 2, human activity continues to be recorded albeit in a sporadic and suppressed form. Several NAP taxa continue to be recorded either intermittently in low percentages or as an occasional short-lived peak. Their presence is possibly the result of minor fluctuations in specific

tree taxa that created small openings in the forest canopy, such as the reduction in Quercus pollen at the start of zone PDC 2, which were exploited for grazing and/or small-scale cereal production. Evidence for grazing and/or decaying wood is supported by the presence of coprophilous spores of Sordaria-type (Type 55A) and Tripterospora-type (Type 169, 169b) (Lundqvist, 1972; van Geel et al., 1980/1981). High microscopic charcoal values also occur between 2980 and 2240 BC during a phase of high tree pollen percentages. Fire does not appear to have been influential in changing forest composition, although the charcoal may be deposited in the bog due to atmospheric transport of fine particles after forest fires, but it can also be generated in situ. At BLL the first clear decline in total AP percentages occurs mid-way through zone 1 with a sizeable decrease in Quercus (Fig. 3). However, this may be the result of differential preservation rather than human activity as the mineral content of the peat is high, and therefore must be treated with caution. Total AP percentages do decrease at the start of zone BLL 2 from 220 cm to 192 cm. It is caused by a decrease in Corylus avellana-type. Although other tree taxa such as Quercus benefit, the loss of hazel coincides with the first recording of Plantago lanceolata, a rapid increase in Poaceae and the presence of cereal-type pollen (c. 196 cm). Pteridium percentages also increase whilst Potentilla-type, Brassicaceae, Artemisia-type, Asteraceae (Lactuceae) and Cirsium are all regularly recorded. It is also noticeable that Coniochaeta xylariispora (T.6), Chaetomium sp. (T.7), Sordaria-type (T.55A), Sordariaceae, Tripterospora-type (T.169) and Coniochaeta cf. ligniaria (T.172) occur, suggesting the presence of decaying wood and/or dung from ungulates (Innes and Blackford, 2003; van Geel et al., 2003). Arboreal pollen percentages recover briefly, as C. avellana-type recovers and Quercus continues to increase in representation, before decreasing again by the end of zone BLL 2. NAP taxa, often associated with human activity, continue to be recorded (e.g. Artemisia-type, Asteraceae (Lactuceae), P. lanceolata) and certain NPPs (including the coprophilous Podospora-type and Tripterosporatype). This decline culminates with a total AP

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percentage minimum in the middle of zone BLL 3. This is primarily caused by a sizeable increase in Cyperaceae. Because few NAP taxa or NPPs are recorded and other mire taxa (such as Sphagnum and Pedicularis-type) also decline, this change is considered to be the result of the local expansion of Cyperaceae on the mire surface rather than a disturbance by humans during the Late–Middle Neolithic. 3.1.1.2. The Metal Ages. Another phase of forest clearance occurred during the Metal Ages (1330– 450 BC). Total tree pollen shows relatively low percentages (50–52% TLP) between 146 and 118 cm during the first half of zone PDC 3 (Fig. 2). Quercus and Alnus are most adversely affected. NAP taxa often associated with agricultural activities are regularly recorded and include Poaceae, Plantago lanceolata, Asteraceae (Lactuceae), Anthemis-type, Rumex acetosella, Rumex acetosa-group, Brassicaceae, Chenopodiaceae, the occasional cereal-type pollen and taxa such as Oleaceae and Vitis. The evidence for increased grazing is also suggested in the NPP record. Coprophilous spores of Sordaria-type, Tripterospora-type and Podospora-type are all recorded during this phase of forest disturbance. All of these fungal spores have been found in deposits from archaeological settlements (van Geel et al., 1980/1981; Willemsen et al., 1996; van Geel et al., 2003). This episode of forest disturbance is nonpermanent and total AP percentages begin to recover, especially Quercus from 120 cm to the latter stages of zone PDC 3, although coprophilous fungi and NAP taxa with cultural affinities are still present in the palaeoecological record. At BLL, total AP percentages increase over the zone 3/4 boundary as Cyperaceae return to their pre-BLL 3 zone values (Fig. 3). Evidence for possible human disturbance at the start of zone BLL 4 is suggested by the slight decline in total AP coincident with a shortlived peak in Plantago lanceolata and the presence of Potentilla-type, Chenopodiaceae and Artemisia-type. Sordaria-type, Sordariaceae, Tripterospora-type, Sporormiella-type and Podospora-type are also recorded supporting the pollen inference for land being grazed. Total AP percentages fluctuate during the remainder of zones 4 and 5 at BLL. In particular, lower percentages of Quercus and Betula characterise the second half of zone 4 whilst Corylus avellana-type pollen gradually decreases throughout zone 5. A phase of permanent forest clearance occurs in the middle of zone 5 as total AP percentages do not recover to their earlier percentages. Higher percentages of Poaceae are recorded during the latter part of zone 4 with increased Pteridium

and Artemisia-type, Plantago lanceolata, Brassicaeae, Potentilla-type are regularly recorded. The occasional cereal-type pollen is also present. Many of the fungi indicative of grazing and/or decaying wood are present including Cercophora-type, Sporormiella-type and Sordaria-type indicating that a mixed arable/pastoral economy existed during the Metal Ages. 3.1.1.3. The Roman period. A significant decrease in total AP percentages is observed during the Roman period at PDC (106 cm to 90 cm; AD 25–340), reaching minimum percentages of 38% TLP (at 92 cm) (Fig. 2). This decline begins towards the end of zone PDC 3 when first Corylus avellana-type and then Quercus percentages fall. The analysis of atmospheric dust suggests that human disturbance occurred on a regional scale (Martínez Cortizas et al., 2005). Concomitant with this decline is the increase in abundance and/or regularity in which cultivated trees, NAP taxa and coprophilous fungi are recorded. In particular, Poaceae, Plantago lanceolata, Artemisia-type, Chenopodiaceae, Rumex acetosella-type and Ranunculaceae are present whilst Oleaceae and Vitis, also occur. Sordaria-type and Tripterospora-type represent the coprophilous fungi but they are only recorded in three separate levels and their values do not reflect the regional scale of Roman impact on this region of NW Iberia. Whilst total AP percentages fluctuate, Quercus and Corylus avellana-type values gradually fall during zone 6 at BLL. Poaceae percentages increase to peak in the latter stages of the zone and Artemisia-type, Brassicaceae, Plantago lanceolata and Apiaceae peak in midzone or towards the end of the zone. Other NAP taxa, such as cereal-type, Rumex acetosa/acetosella-type, Chenopodiaceae, Potentilla-type and Asteraceae, are all regularly recorded. Sordaria-type and Podosporatype are present at the start and end of the zone whilst other NPP associated with dung are only recorded sporadically. Trees, such as Oleaceae and Vitis, are also recorded during the Roman period at BLL. Although these taxa occur in low percentages, they may be cultivated. The expansion of peatland is also suggested by the increased amounts of Calluna and Ericaceae pollen. 3.1.1.4. Post Roman times. The most dramatic and permanent decrease in total tree pollen is recorded at 74–80 cm (AD 610–74), where percentages fall from 60% to 35% TLP at PDC (Fig. 2). From 74 cm to 28 cm total AP percentages decrease slightly in two steps (AD 740–1040 and AD 1080–1570), above 28 cm it recovers slightly (until ca. AD 1840) and then more

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quickly in the top 10 cm. Quercus, Pinus, Ulmus and Corylus avellana-type are all affected, although the cultivated tree Castanea appears for the first time at 84 cm (ca. AD 525) and is always present until the top of the core. The NPPs support the suggestion that humans were responsible for these changes. Sordaria-type is recorded throughout the sequence but only achieves regular and sustained percentages across the zone PDC 4/5 boundary and from the later stages of zone PDC 5 to the top of the peat profile. The increase in Sordaria-type across the PDC 4/5 zone boundary coincides with the permanent decline of forest. The presence of cereal pollen, increased Poaceae and the occurrence of a suite of NAP normally associated with anthropogenic activity such as Plantago lanceolata, Apiaceae, Urticaceae, Chenopodiaceae, Brassicaceae and Asteraceae (Lactuceae) are all present. Other fungi associated with dung or decaying wood such as Cercophora-type (T.112), which is present for the first time, and Podospora-type are also recorded (van Geel et al., 2003). The highest percentages of many of the coprophilous fungi occur in the uppermost zones (PDC 6 and 7) and coincide with the permanent loss of deciduous forest cover, the expansion of grassland and peat (as indicated by higher percentages of Poaceae, Calluna and Ericaceae). Tripterospora-type and Podospora-type also are regularly recorded. It is likely that plenty of decaying wood was available following the phases of forest clearance that occurred halfway through zone PDC 6. Alternatively the increase in cereal pollen and other indicators of agriculture, such as Plantago lanceolata, Rumex acetosella and acetosa–group, Artemisia-type and Potentilla-type, suggest that the uplands of NW Spain have been used for pasture and hence the increased amount of dung fungi. An examination of the lithogenic elements suggested that the forest disturbance from the 7th century was localised until historical times (Martínez Cortizas et al., 2005). The response of the fungal spores is limited until the uppermost zones and during the more intense phase of human disturbance. The modest recovery of forests during the last two centuries does not depend on the recovery of the deciduous taxa but mainly on pine. A long, gradual and permanent decline in forest cover commences during late Roman times and continues into Germanic period during the Middle Ages at BLL (Fig. 3). After a slight resurgence at the zone 6/7 boundary Quercus, Betula and Alnus decrease followed by Corylus avellana-type midway through zone 7. A second phase of forest decline occurs in the middle of zone 8 and more dramatically at the start of

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zone 9. Peatlands replace forests as Calluna and Ericaceae pollen increase in stages. An expansion in open agricultural land is also supported by the regular record of cereal-type from the end of zone BLL 7 onwards and a suite of NAP taxa and NPP with cultural affinities (e.g. Plantago lanceolata, Artemisia-type, Asteraceae, Brassicaceae, Potentilla-type; Coniochaeta xylariispora, Sordaria-type, Neurospora, Sporormiellatype, Podospora-type). Cultivated trees also feature in post-Roman times. Castanea is first recorded at the end of zone 6 and is common in zones 8 and 9, whilst Vitis and Oleaceae are also recorded regularly in zone 9. 3.2. Cluster analysis No substantial differences were found in NPP relationships in the two-step cluster analysis and only a few pollen types showed similarities with NPPs. This analysis revealed a number of common groupings of NPPs for BLL and PDC. Three main groups of affinities were identified and include: First main group: – At both sites Sordaria-type (T.55A) and Podosporatype (T.368) are very close related; both link to Sordariaceae undiff. and Tripterospora-type (T.169). This group of coprophilous fungi shows a good relation to Ericaceae-type abundance, except perhaps after 1000 AD at BLL and Type 10 at PDC. – Type 10b and Type 733 are closely related, and link to Meliola cf. niessleana (T.14) at BLL whilst Type 733 and Type 306 appear to be closely related. – Pleospora sp. (Type 3B) is linked to Sporormiella (T.113) at BLL. – Spermatophores of Copepoda (T.28) at both sites and Type 35 at BLL are closely related and link to Coniochaeta xylariispora (T.6). Together with Pleospora sp. and Sporormiella, this group is related to Sphagnum abundance. Type 10 and Type 52 are closely related and link to Type 8E. Second main group: – Types 18 and 18b are very closely related at both sites, and link to Chaetomium sp. (T.7) and Type 90 at BLL and Byssothecium circinans (T.16A; van Geel and Aptroot, 2006) at PDC. Third group: – Cercophora-type (T.112) and Type 303 do not link to any other NPP at BLL.

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3.3. Evidence for changes in past climate To establish the use of pollen and NPP as proxies of climate change in NW Iberia the data were compared against independent proxies for temperature and humidity. Martínez Cortizas et al. (1999) reconstructed a 4000year proxy record of climate change using atmospheric mercury deposited in an ombrotrophic peat bog (Penido Vello, PVO), which is located within 5 km of PDC and BLL (Fig. 1). They found that enhanced accumulation of Hg with low thermal stability (LHg) occurred during periods of cold climate whilst warm climates were characterised by lower accumulation of mercury (Hg) but with a higher thermal stability (MHg and HHg). Based on the Hg record, which compares favourably with changes in temperature from other proxies, Martínez Cortizas et al. (1999) developed both a temperature and a

humidity index. The temperature index identified two major cooling phases during the last 4000 years (the Neoglacial period ca. 5000 to ca. 3000 years BP and the Little Ice Age) and two major warm periods (the Roman and Medieval warm periods). The humidity index (HI) was established using the shift of Hg from less stable to more stable forms of mercury. Martínez Cortizas et al. (1999) showed that during warm and wet climates both HHg and MHg are abundant and that the variations of MHg/HHg ratio (moderate/high thermal stability ratio) fit with the humidity variations described for Spain. A discriminant analysis of the Hg data against samples derived from known climate classes showed that the first two canonical functions (temperature, humidity) explained 96.5% of the total variance in the Hg data. This humidity record is represented in Fig. 4. Six major periods were

Fig. 4. Humidity index (after Martínez Cortizas et al., 1999), scores of the discriminant canonical functions and T18 and 18b data for the study sites. Shaded areas indicate phases of high values of the proxies for wetness at both bogs, suggesting increased effective precipitation in NW Spain. The figure also shows that the scores of the discriminant analysis, obtained by assigning a climate class, are dominated by the T.18 signal for each sample.

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identified: at ca. 850 cal BC, ca. 560–250 cal BC, ca. cal AD 240–770 (with two probably severe droughts at cal AD 280–300 and cal AD 560–580), ca. cal AD 1100– 1130, ca. cal AD 1330–1460 and ca. cal AD 1810– 1860. No wet periods are detected for sections older than ca. 1000 cal BC. Martínez Cortizas et al. (1999) suggested that the humidity index based on Hg thermal lability may not be sensitive to wetness variations in cold climates, since this phase coincides with the Neoglaciation period and the index also shows low values during the Little Ice Age; but it may also be influenced by the fact that the resolution of the record in this sections is relatively poor due to the fact that it was sectioned each 5 cm and some wet events may have been averaged out. However, the importance of humidity rather than temperature as a climatic parameter reflects the strong oceanic nature of the sub-coastal mountains, which are not only close to the sea but also subject to a continuous advance of cyclonic fronts (Ramil-Rego et al., 1998). The record of Type 18 shows good similarity to the variations of the HI (Fig. 4), suggesting that its abundance is related to variations in bog wetness. The discriminant analysis also corroborates this fact and reveals some significant climatic trends in the data. For both bogs the discriminant canonical function resulted in 93% of corrected classified cases i.e. the agreement between the HI index and the combination of NPPs and pollen types is highly significant. The minimum set of significant variables at BLL is composed by T.18, T.3B, T.10b, T.90 and Poaceae, whilst at PDC it is composed by T.18, T.18b, T.306, T.10 and Ericaceae. The discriminant score is dominated by the T.18 signal (Fig. 4) and it can be seen that it is quite similar for both bogs and share many common phases with the PVO humidity index. The most remarkable difference is the presence of a wet period between ca. 2200 BC and ca. 1590 BC, which is not recorded by HI supporting the idea that Hg lability may not be a precise proxy for humidity in cold periods – i.e. the wetness signal is obscured by the effect of low temperature on Hg lability. It is also worth mentioning that this period is longer at BLL than at PDC. At BLL its initiation coincides with the transition between the minerogenic and ombrogenic peat, so it can reflect variations in the local water table not strictly related to changes in effective precipitation. There are also slight variations between the two records. Some other minor differences between bogs may also arise from local changes in the bog microtopography, changes in the abundance of possible host plants or short-lived changes in peat accumulation rates between the bogs.

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A discriminant analysis using temperature classes (cold, mild, warm and very warms, as in Martínez Cortizas et al., 1999) failed to find consistent relationships with NPP abundance. For both bogs only 60% correct classification was attained and the variations in discriminant scores seem to be biased by humidity. This would indicate that humidity plays a more relevant role in NPP abundances than temperature. 4. Discussion The statistical analysis suggests that T.18 and 18b ascospores may be responding to changes in moisture on the PDC bog surface caused by external forcing of climate. Types 18 and 18b are commonly recorded throughout the PDC and the BLL peat profiles. Both types follow a similar pattern with higher percentages occurring during the middle of zones PDC 2, a large proportion of zone PDC 3, the second half of zone PDC 5 and the early part of zone PDC 6 (Fig. 2). Types 18 and 18b are most abundant during zones BLL 2–6 and zone 8, whilst the lowest values in zones 9 and 1 are mirrored by low Cyperaceae percentages (Fig. 3). However, van Geel (1978) suggests that T18 and 18b could be connected with the occurrence of Eriophorum vaginatum in the peat and it is possible that the fungi grew on the leaves or rootlets of this species. The data here lend some support to this association as T.18 and 18b are strongly correlated to Cyperaceae pollen. There are common periods of high abundance of Types 18 and 18b in both BLL and PDC but there are also periods of elevated T.18 at BLL that are not represented in PDC. Whilst discriminant analysis shows that PDC and PVO are quite similar in terms of humidity reconstruction (> 90% of similarity determined), it seems that the peat bog records have been influenced by other local factors in parts of the core. For example, minerogenic peat found at the base of BLL could possibly account for the lack of a response in the data at the base of the profile and before the initiation of the ombrotrophic peat. Local changes in water table could also influence the data and there may be also an altitudinal effect. Types 18/18b also appear to be associated with other NPPs, including Chaetomium (T.7) and Type 90. Because of the relatively low number of some of the NPPs in the core such associations must be treated with caution and there is the possibility that the close relation has only occurred by chance. Irrespective of other influences, both bogs record similar wet and dry phases during the mid and late Holocene (Fig. 4, shaded areas). These phases coincide, or lag slightly, with wet and dry shifts recorded in other

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peat bogs in north-west Europe. For example, RoosBarraclough et al. (2004) record wet shifts based on lower peat humification records from two Swiss peat cores between c. 2500–1350 cal BC and c. 1050 to 550 Cal BC. The Bronze Age/ Iron Age transition (Homeric Minimum) 850 cal BC wet shift commonly identified across Europe by Killian et al. (1995, 2000), van Geel et al. (1996), Speranza et al. (2002) and Blaauw et al. (2004) is also recorded in the PDC and BLL peat bogs. Blaauw et al. (2004) also records wet shifts and changes in Δ14C c. 415 and 345 cal BC. Chiverrell (2001) and Roos-Barraclough et al. (2004) also record a wet shift from cal AD 550 onwards. Wet periods recorded in the Spanish peat bogs, between 1110 and 1210 cal AD, 1345 and 1475 cal AD and from 1770 to 1910 cal AD, also occur in two ombrotrophic peat bogs (Walton Moss and Lille Vildmose) by Mauquoy et al. (2002a,b) and the latter two at May Moss, Northern England (Chiverrell, 2001). Variations also exist, for example, the Dark Age wet shift detected by Blackford and Chambers (1991) and Chiverrell (2001) is not recorded in the Spanish peat bogs, and these are likely to be the result of local environmental controls including internal hydrology and/or regional climatic variability (Blaauw et al., 2004). Type 306 is one example of a large number of fungal spores and other non-pollen palynomorphs (NPP) recorded in peat bogs produced by unknown fungi, so data concerning their ecology cannot be obtained (van Geel, 1972). Results from a Holocene sequence at De Borchert in The Netherlands suggested that T.306 correlated with no other fossils and showed no preference to special conditions (van Geel et al., 1980/ 1981). However, its pattern at PDC suggests that it may be a useful indicator of mire surface dryness as it behaves inversely to Cyperaceae and T.18 and 18b. Type 306 increases across the PDC 2/3 boundary and attains percentages of up to 5%, then decreases by mid zone and is only recorded once until the end of zone PDC 4. It peaks at the start of zone PDC 5 and is well represented until the latter stages when it declines sharply too much lower percentages. T.306 percentages remain low until the second part of zone PDC 6 when they increase and remain high throughout zone PDC 7. Unfortunately T.306 is not recorded in the BLL peat core. The exact reason for its absence is unknown but a likely candidate is the absence of its unknown host plant. Other NPPs also follow a similar pattern that may be explained in terms of moisture differences on the bog surface at PDC. van Geel (1978) found that T.10 occurs with high frequencies with Calluna vulgaris

rootlets and indicates locally dry conditions. Despite occurring in relatively low percentages at PDC, T10/ 10b does appear to have a close association with the Calluna pollen curve. Both show noticeable increases from 160 to 155 cm, with Calluna sustained higher values and T.10 being recorded more frequently (Fig. 2). Another increase is observed at 132 cm but then both fall to lower percentages until Calluna begins to increase from 75 cm at the start of zone PDC 5, which is followed by a sustained increase in T.10/10b until a decline in zone PDC 6. Types 10/10b percentages are then recorded in relatively high percentages in zone PDC 7 and this is matched by a small increase in Calluna pollen across the zone PDC 6/7 boundary. The increase in T.10 does not coincide with the increase in T.306 in zone PDC 3. There is good correspondence during PDC 5 although higher percentages of T.10 occur after the rise of T.306 and the decline of T.10 is more protracted than T.306 at the end of this zone. Both T.10/10b and 306 have high percentages during PDC 7. A different host plant or in the sensitivity of the fungi to changes in moisture may account for these differences. The relationship between T.10 and Calluna and Ericaceae is also observed at BLL (Fig. 3). The highest percentages of T.10 and T.10b occur in zones BLL 7 and BLL 9, which coincide with lower or falling values of T.18. One exception is at the top of zone BLL 5 when relatively high T.10 values correspond with high T.18 but Calluna is also well represented and this may reflect a mosaic of dry and wet parts across the mire. The cluster analysis also suggests that T.8E, Meliola cf. niessleana (T.14), T.52 and T.733 are associated with T.10/10b. These associations are consistent with previous research. van Geel (1978) suggests that Meliola cf. niessleana is associated with dry conditions. It is a parasite on Calluna in ombrotrophic bogs. The fungi that produce Types 8E and 52 spores are unidentified but their presence is probably linked to the ombrotrophic nature of the bog. Byssothecium circinans (T.16A) is regularly recorded but in higher percentages during zones PDC 3 and 4 and to a lesser extent, zones 2 and 5. van Geel (1978) suggests that T.16A and 16C occurred in levels where Poaceae are present in the local vegetation such as Molinia caerulea. Such an association is unclear at PDC and BLL. Poaceae pollen percentages do increase from relatively low percentages from zone PDC 2 to zone PDC 4 but fewer T.16A and 16C are recorded (zones PDC 1, 5, 6 and 7) when Poaceae values are at their highest. At BLL higher percentages of T.16A do correspond to higher Poaceae in zones 8 and 6 but not

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in other zones. van Geel et al. (1980/1981, 1982/1983) suggest that T.16A seems to prefer mesotrophic conditions in Sphagnum peat, which also coincide with dry phases, whereas during relatively moist conditions and oligotrophic phases T.16A occurs less frequently or is absent. It is difficult to see a clear association between T.16A and dry phases at PDC or BLL. Indeed the cluster analysis suggests that T.16A may have some association with T.18. The rise of T.16A when zone PDC 3 commences coincides with the decrease in T.306 and it is regularly recorded in relatively high percentages with Cyperaceae and T.18/18b during zones PDC 3 and 4. T.16A then falls across the zone PDC 4/5 boundary when T.306 increases. However, this pattern is not observed in the uppermost zones. T.16A continues to be recorded throughout zone PDC 5, albeit at slightly lower percentages when compared to the preceding zones and does not increase with Cyperaceae and T.18b during the second half of this zone. A more consistent relationship occurs in the final zone as T.16A is sparse when T.10/10b and T.306 are present. There does not appear to be a strong association between Type 16A and Sphagnum in the PDC or BLL profiles. The cluster analysis did not show a significant association between T.16A and T.18/18b at BLL. However, more often than not higher values of T.16A occur when T.18/18b values are high or increasing and show little correspondence with T.10/10b. The cluster analysis also suggests that spermatophores of Copepoda (T.28) and Coniochaeta xylariispora (T.6) are linked at both bogs whilst T.35 also has an association with T.28 at BLL. Together with Pleospora sp. (T.3B) and Sporormiella (T.113), this group is also related to Sphagnum abundance. Type 28 is regularly recorded in low percentages from zones PDC 2 and BLL 3. Their absence from zone 1 might be from a higher mineral soil component at the base of the peat as their production indicates the presence of temporary open water (van Geel, 1978). Little is known about T.35 whilst Pleospora sp. (T.3B) and C. xylariispora (T.6) are regularly recorded in ombrotrophic peat. Given the unreliability of Sphagnum spore production and the fact that pools often develop on bogs, the presence of these taxa are more likely to be related to the ombrotrophic nature of the peat. The development of a mixed deciduous forest dominated by oak and hazel is characteristic of the sub-coastal mountains ranges of the NW Iberian peninsula (Ramil-Rego et al., 1998). Four phases of forest regression recorded at PDC and BLL occur during the Neolithic period (beginning by 4000–3500 BC), the Metal Ages (1200–100 BC), the Roman period (100

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BCE–AD 400) and since the Germanic period (Visigoth Kingdom) in the very early Middle Ages (7th century AD). They are consistent with changes in forest cover described by other researchers in northern Portugal and NW Spain (Törnqvist and Joosten, 1988; Törnqvist et al., 1989; Aira et al., 1992; Ramil et al., 1994; Janssen, 1994; van der Knaap and van Leeuwen, 1995) although the resolution of the records presented in this paper is much improved and the chronology more precise. Although there is some evidence for Mesolithic forest disturbance on the NW Iberia Peninsula, Ramil-Rego et al. (1998) suggest that human activity began to have a significant impact on vegetation during the sixth millennium although agricultural activities are not reflected in pollen diagrams at higher altitudes until approximately 2500 BP. Whilst there is evidence of human activity throughout both records, this observation is evident at PDC although taxa associated with agricultural activities, such as Plantago lanceolata, are more regularly recorded before 2500 BP at BLL, the site at lower altitude. The NPP data as a new proxy complements the pollen and charcoal data to interpret land use changes. A suite of NPPs has been identified as indicators of decaying wood, dung (and by implication the presence of ungulates) and fire (van Geel, 1978; Blackford, 1998; Hoaen and Coles, 2000). These include coprophilous spores of Sordaria-type (T.55A), Tripterospora-type (T.169) and Podospora-type (T.368) but also Coniochaeta xylariispora (T.6), Chaetomium sp. (T.7A), Cercophora-type (T.112) and Sporormiellatype (T.113) (van Geel et al., 2003). The cluster analysis for PDC and BLL is consistent with the ecological interpretation of these fungi. In particular the coprophilous Sordaria-type, Tripterospora-type and Podospora-type are very closely related. All feature during periods of forest disturbance, particularly the more substantial phases of forest clearance from Roman times onwards and they coincide with the presence of pastoral and arable NAP taxa. Their association with Ericaceae is also consistent as it increases in representation along with Calluna vulgaris as arboreal pollen taxa decline. Whilst they do not feature prominently in the statistical analysis, other fungi with possible associations to human disturbance also occur at both sites. Coniochaeta cf. ligniaria (T.172) is recorded sporadically at BLL, Sporormiella-type is common from zone BLL 3 onwards whilst Cercophora-type is recorded during some short-lived phases of forest disturbance and following a permanent decline in deciduous woodlands (start of zone PDC 5 and BLL 8).

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5. Conclusions This paper set out to examine the relationship between changes in pollen and NPP taxa and human activity and climate change. Palaeoecological data from PDC and BLL suggests that the presence of Cyperaceae and the fungal spore Types 18 and 306 in ombrotrophic peat bogs reflect changes in moisture/ humidity and can be used as proxy record of past changes in moisture. Temperature is much less influential. The relationship is best observed at PDC where Cyperaceae and Type 18 compare favourably to periods of increased moisture whereas Type 306 corresponds to drier periods. Other factors can, however, have an influence on the presence and abundance of such taxa. Although the pattern is still evident at BLL, T.306 is not recorded and T.18 does not only coincide with regional wetter phases. Minerotrophic mire conditions, a lack of a specific host plant, and possibly altitude may account for these differences. Thus, this paper demonstrates the potential use of pollen and NPPs in reconstructing Holocene climate change, but further studies are needed to fully evaluate the findings presented in this paper. Various ascospores of coprophilous fungi were regularly recorded at both sites, especially in the uppermost levels. Their occurrence was consistent with changes in the pollen record to identify agricultural activities as a primary agent of vegetation change during the mid to late Holocene, demonstrating their value in palaeoenvironmental reconstruction. Acknowledgements Financial support for this study was obtained from the Spanish Ministerio de Ciencia y Tecnología (project REN2003-09228-C02-01). Didier Galop and Dmitri Mauquoy provided insightful comments, which improved the manuscript. References Aira, M.J., Saa, P., López, P., 1992. Cambios del paisaje durante el Holoceno: análisis de polen en turberas (Galicia, España). Revue de Paléobiologie 11, 243–254. Barber, K.E., 1976. History of vegetation. In: Chapman, S.B. (Ed.), Methods in Plant Ecology. Blackwell, Oxford, pp. 5–83. Barber, K.E., 1981. Peat Stratigraphy and Climatic Change. Balkema, Rotterdam. Blaauw, M., van Geel, B., van der Plicht, J., 2004. Solar forcing of climatic change during the mid-Holocene: indications from raised bogs in The Netherlands. The Holocene 14, 35–44. Blackford, J.J., 1993. Peat bogs as sources of proxy climate data: past approaches and future research. In: Chambers, F.M. (Ed.), Climate

Change and Human Impact on the Landscape. Chapman and Hall, London, pp. 47–56. Blackford, J.J., 1998. Fungal remains from Holywell Coombe. In: Preece, R. (Ed.), Environments Past and Present from Holywell Coombe, Kent. Chapman and Hall, London, pp. 117–126. Blackford, J.J., Chambers, F.M., 1991. Blanket peat humification: evidence for a Dark Age (1400 BP) climatic deterioration in the British Isles. The Holocene 1, 63–67. Chambers, F.M., Charman, D., 2004. Holocene environmental change: contributions from the peatland archive. The Holocene 14, 1–6. Chambers, F.M., Barber, K.E., Maddy, D., Brew, J., 1997. A 5500 year proxy-climate and vegetational record at Talla Moss, Borders, Scotland. The Holocene 7, 391–399. Charman, D., Hendon, D., Packman, S., 1999. Multi-proxy surface wetness records from replicate cores on an ombrotrophic mire: implications for Holocene palaeoclimatic records. Journal of Quaternary Science 14, 451–463. Chiverrell, R.C., 2001. A proxy record of late Holocene climate change from May Moss, northeast England. Journal of Quaternary Science 16, 9–29. Clark, R.L., 1982. Point count estimation of charcoal in pollen preparations and thin sections of sediments. Pollen et Spores 24, 523–535. Davis, O., 1987. Spores of the dung fungus Sporormiella: increased abundance in historic sediments and before Pleistocene megafaunal extinction. Quaternary Research 28, 290–294. Faegri, K., Kaland, P.E., Krzywinski, K., 1989. Textbook of Pollen Analysis, 4th edition. John Wiley and Sons, Chichester. Fraga Vila, M.I., Sahuquillo, E., García Tasende, M., 2001. Vegetación característica de las turberas de Galicia. In: Martínez Cortizas, A., García-Rodeja, E. (Eds.), Turberas de Montaña de Galicia. Consellería de Medio Ambiente, Xunta de Galicia, Santiago de Compostela, pp. 79–98. Grimm, E., 1991–1993. TILIA and TILIA.GRAPH. Illinois State Museum, Illinois. Hoaen, A., Coles, G., 2000. A preliminary investigation into the use of fungal spores as anthropogenic indicators on Shetland. In: Nicholson, R.A., O'Connor, T.P. (Eds.), People as An Agent of Environmental Change. Symposia of the Association for Environmental Archaeology. Oxbow Books, Oxford, pp. 30–36. Hughes, P.D.M., Mauquoy, D., Barber, K.E., Langdon, P.G., 2000. Mire-development pathways and palaeoclimatic records from a full Holocene peat archive at Walton moss, Cumbria, England. The Holocene 10, 465–479. Innes, J.B., Blackford, J.J., 2003. The ecology of Late Mesolithic woodland disturbances: model testing with fungal spore assemblage data. Journal of Archaeological Science 30, 185–194. Janssen, C.R., 1994. Palynological indications for the extent of the impact of man during Roman times in the western part of the Iberian Peninsula. In: Frenzel, B. (Ed.), Evaluation of Land Surface Cleared from Forests in the Mediterranean Region During The Time of the Roman Empire. Palaeoclimate Research, vol. 10, pp. 15–22. Killian, M.R., van der Plicht, J., van Geel, B., 1995. Dating raised bogs: new aspects of AMS 14C wiggle matching, a reservoir effect and climatic change. Quaternary Science Reviews 14, 959–966. Killian, M.R., van Geel, B., van der Plicht, J., 2000. 14C AMS wiggle matching of raised bog deposits and models of peat accumulation. Quaternary Science Reviews 19, 1011–1033. Lindsay, R., 1995. Bogs: The Ecology, Classification and Conservation of Ombrotrophic Mires. Scottish Natural Heritage, Edinburgh.

T.M. Mighall et al. / Review of Palaeobotany and Palynology 141 (2006) 203–223 Lundqvist, N., 1972. Nordic Sordariaceae. Symbolae Botanicae Upsalienses 20, 1–374. Martínez Cortizas, A., Pérez Alberti, A., 1999. Atlas Climático de Galicia. Consellería de Medio Ambiente, Xunta de Galicia, Santiago de Compostela. 250 pp. Martínez Cortizas, A., Pontevedra Pombal, X., García-Rodeja, E., Nóvoa Muñoz, J.C., Shotyk, W., 1999. Mercury in a Spanish peat bog: archive of climate change and atmospheric metal deposition. Science 284, 939–942. Martínez Cortizas, A., García-Rodeja, E., Pontevedra Pombal, E., Nóvoa Muñoz, J.C., Weiss, D., Cheburkin, A., 2002. Atmospheric Pb deposition in Spain during the last 4600 years recorded by two ombrotrophic peat bogs and implications for the use of peat as a geochemical archive. The Science of The Total Environment 292, 33–44. Martínez Cortizas, A., Mighall, T.M., Pontevedra Pombal, X., Nóvoa Muñoz, J.C., Peiteado Varela, E., Piñeiro Rebolo, R., 2005. Linking changes in atmospheric dust deposition, vegetation evolution and human activities in NW Spain during the last 5300 years. The Holocene 15, 698–706. Mauquoy, D., Engelkes, T., Groot, M.H.M., Markesteijn, F., Oudejans, M.G., van der Plicht, J., van Geel, B., 2002a. High-resolution records of late Holocene climate change and carbon accumulation in two north-west European ombrotrophic peat bogs. Palaeogeography, Palaeoclimatology, Palaeoecology 186, 275–310. Mauquoy, D., van Geel, B., Blaauw, M., van der Plicht, J., 2002b. Evidence from north-western European bogs shows ‘Little Ice Age’ climatic changes driven by changes in solar activity. The Holocene 12, 1–6. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, 2nd edition. Blackwell Scientific Publications, London. Ramil-Rego, P., Muñoz-Sobrino, C., Rodríguez-Guitián, M., GómezOrellana, M., 1998. Differences in the vegetation of the Northern Iberian Peninsula during the last 16,000 years. Plant Ecology 138, 41–62. Ramil, P., Aira, M.J., Taboada, M.T., 1994. Análisis polínico y sedimentológico de dos turberas en las sierras septentrionales de Galicia (NW de España). Revue de Paléobiologie 13, 9–28. Roos-Barraclough, F., van der Knaap, W.O., van Leeuwen, J.F.N., Shotyk, W., 2004. A late-glacial and Holocene record of climate change from a swiss peat humification profile. The Holocene 14, 7–20. Speranza, A., van Geel, B., van der Plicht, J., 2002. Evidence for solar forcing of climate change at ca. 850 cal BC from a Czech peat sequence. Global and Planetary Change 35, 51–65. Stockmarr, J., 1971. Tablets with spores used in absolute pollen analysis. Pollen et Spores 13, 615–621. Stuiver, M., Reimer, P.J., 1993. A computer program for radiocarbon age calibration. Radiocarbon 35, 215–230. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K. A., Kromer, B., McCormac, F.G., van der Plicht, J., Spurk, M., 1998. INTCAL98 radiocarbon age calibration 24,000–0 cal BP. Radiocarbon 40, 1041–1083.

223

Törnqvist, T.E., Joosten, J.H.J., 1988. On the origin and development of a Subatlantic man-made mire in Galicia (northwest Spain). Proceedings of the 8th International Peat Congress 1, 214–224. Törnqvist, T.E., Janssen, C.R., Pérez Alberti, A., 1989. Degradación de la vegetación en el noroeste de Galicia durante los últimos 2500 años. Cuadernos de Estudios Gallegos 37, 175–198. van der Knaap, W.O., van Leeuwen, J.F.N., 1995. Holocene vegetation succession and degradation as response to climatic change and human activity in the Serra da Estrela, Portugal. Review of Palaeobotany and Palynology 89, 153–211. van Geel, B., 1972. Palynology of a section from the raised peat bog “Wietmarscher Moor” with special reference to fungal remains. Acta Botanica Neerlandica 21 (3), 261–284. van Geel, B., 1978. A palaeoecological study of Holocene peat bog sections in Germany and the Netherlands. Review of Palaeobotany and Palynology 25, 1–120. van Geel, B., 1986. Application of fungi and algal remains and other microfossils in palynological analyses. In: Berglund, B.E. (Ed.), Handbook of Holocene Palaeoecology and Palaeohydrology. John Wiley and Sons, Chichester, pp. 497–505. van Geel, B., Aptroot, A., 2006. Fossil ascomycetes in Quaternary deposits. Nova Hedwigia 82, 313–329. van Geel, B., Bohncke, S.J.P., Dee, H., 1980/1981. A palaeoecological study of an upper late glacial and Holocene sequence from ‘De Borchert’, The Netherlands. Review of Palaeobotany and Palynology 31, 367–448. van Geel, B., Hallewas, D.P., Pals, J.P., 1982/1983. A Late Holocene deposit under the Westfriese Zeedijk near Enkhuizen (prov. of Noord-Holland, The Netherlands). Review of Palaeobotany and Palynology 38, 269–335. van Geel, B., Bos, J.M., Pals, J.P., 1986a. Archaeological and palaeoecological aspects of a medieval house terp in a reclaimed raised bog area in North Holland. Berichten van de Rijksdienst voor het Oudheidkundig Bodemonderzoek 33, 419–444. van Geel, B., Klink, A.G., Pals, J.P., Wiegers, J., 1986b. An Upper Eemian lake deposit from Twente, Eastern Netherlands. Review of Palaeobotany and Palynology 47, 31–61. van Geel, B., Buurman, J., Waterbolk, H.T., 1996. Archaeological and palaeoecological indications of an abrupt climate change in the Netherlands and evidence for climatological teleconnections around 2650 BP. Journal of Quaternary Science 11, 451–460. van Geel, B., Buurman, J., Brinkkemper, O., Schelvis, J., Aptroot, A., van Reenen, G., Hakbijl, T., 2003. Environmental reconstruction of a Roman period settlement site in Uitgeest (The Netherlands), with special reference to coprophilous fungi. Journal of Archaeological Science 30, 873–884. Willemsen, J., van 't Veer, R., van Geel, B., 1996. Environmental change during Medieval reclamation of the raised-bog area Waterland (The Netherlands): a palaeophytosociological approach. Review of Palaeobotany and Palynology 94, 75–100.