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Landscape dynamics of Abies and Fagus in the southern Pyrenees during thelast 2200 years as a result of anthropogenic impacts

Albert Pelachs, Ramon Perez-Obiol, Miquel Ninyerola, Jordi Nadal

PII: S0034-6667(09)00048-7DOI: doi: 10.1016/j.revpalbo.2009.04.005Reference: PALBO 3026

To appear in: Review of Palaeobotany and Palynology

Received date: 26 September 2008Revised date: 24 March 2009Accepted date: 1 April 2009

Please cite this article as: Pelachs, Albert, Perez-Obiol, Ramon, Ninyerola, Miquel,Nadal, Jordi, Landscape dynamics of Abies and Fagus in the southern Pyrenees duringthe last 2200 years as a result of anthropogenic impacts, Review of Palaeobotany andPalynology (2009), doi: 10.1016/j.revpalbo.2009.04.005

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LANDSCAPE DYNAMICS OF ABIES AND FAGUS IN THE SOUTHERN

PYRENEES DURING THE LAST 2200 YEARS AS A RESULT OF

ANTHROPOGENIC IMPACTS

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Albert Pèlachs a,*, Ramon Pérez-Obiol b, Miquel Ninyerola b, Jordi Nadal a a GRAMP, Departament de Geografia, Universitat Autònoma de Barcelona. 08193 Bellaterra (Cerdanyola del Vallès). Spain. b Unitat de Botànica, Facultat de Biociències, Universitat Autònoma de Barcelona, 08193 Bellaterra (Cerdanyola del Vallès). Spain. * corresponding author. Tel.: +34 93 5868057; fax: +34 93 5812001. E-mail address: albert.pelachs@uab.cat (A. Pèlachs).

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LANDSCAPE DYNAMICS OF ABIES AND FAGUS IN THE SOUTHERN

PYRENEES DURING THE LAST 2200 YEARS AS A RESULT OF

ANTHROPOGENIC IMPACTS

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Abstract

The vegetation landscape dynamic is derived from the relationship established between

a society and its environment through time, and the current landscape has never been

seen in the previous 2000 years. The pollen study of a core from a peat bog in València

d'Àneu (Lleida, NE Iberian Peninsula) shows a maximum extension of Abies alba forest

about 2200-2000 cal. yr BP. Later on, there is evidence of selective actions affecting

this forest and the expansion of Fagus sylvatica at about 2000-1300 cal. yr BP.

Beginning in 1300 cal. yr BP, deforestation due to agricultural activities expanded and

beech definitively disappeared at 800 cal. yr BP. Natural and human disturbances

affected the dynamics of Abies alba and Fagus sylvatica from their first appearance to

the current vegetation landscape. Human impact on the silver fir forest, which reached

its maximum in the last millennium, favoured the beech population. Pollen data from

this region support our finding that human impact, not climate, is the most important

influential factor in the development of beech forests.

Keywords: Pyrenees, Holocene, palynology, GIS suitability mapping, Abies alba,

Fagus sylvatica.

Introduction (A)

The current discussion concerning the dynamics of the vegetation landscape is rooted in

the reasons for change over time and in the weighting of natural and human factors in

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its evolution (Galop and Jalut, 1994; Esteban et al., 2003; Riera et al., 2004; Beaulieu et

al., 2005; Riera et al., 2006; Pèlachs et al., 2007). Although climatic factors have a very

important role in the development of vegetation, palaeobotanic studies have

demonstrated the importance of taking into account the role played by human society.

Therefore, the primary objective of this study is to determine the extent to which the

human imprint has affected the current vegetation landscape, focussing on the dynamics

of Abies and Fagus forests in the Pyrenees.

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Colonization of Abies alba and Fagus sylvatica: the current state of affairs (B)

An explanation of the plant colonization of the Pyrenees from the beginning of the

Holocene can be undertaken on the basis of pollen analyses available from the Pyrenees

mountain range (Jalut et al., 1998). It is impossible to interpret which factors affect this

evolution without taking into account at least three variables: the location of refuge

zones, the development of climatic factors and the edaphic dynamics of the soils

(Pèlachs, 2005).

In recent years, the study of Abies alba dynamics in Europe (Terhürne-Berson et al.,

2004; Liepelt et al., 2009) has been associated with other species, such as Fagus

sylvatica (Tinner and Lotter, 2006). This area of study has developed from a series of

interpretations based on the study of climate change, migratory change, unequal growth

of species, and the effects of human disturbances and forest fires (Tinner and Lotter,

2006).

In this sense, phylogenetic studies reveal how the Abies populations in the Pyrenees

were isolated from the rest of Europe (Konnert and Bergmann, 1995). This argument

was definitive in defending the proximity of the Pyrenees to Abies alba refuge zones,

based on plant macroremains and pollen data (Terhürne-Berson et al., 2004; Liepelt et

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al., 2009). The hypotheses about distribution from the glacial refuges based on

isoenzyme studies and other genetic markers (El Mousadik and Petit, 1996) seem to

substantiate the existence of five areas of Abies alba refuge and recolonization: the

Pyrenees, central and eastern France, central Italy and the southern Balkans. Pollen and

genetic data indicate clearly that the Abies alba and Fagus sylvatica refuges in the

Pyrenees have suffered the “bottleneck” phenomenon during their history and that

recolonization was not produced exclusively from refuge populations. This theory is

well supported because of the low allelic levels, which can be correlated to the current

distribution of silver fir in the Pyrenees, with populations that are not extensive in

comparison with the rest of Europe.

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Palaeobotanical and genetic data for Fagus sylvatica (Magri et al., 2006) have been

used to evaluate the genetic consequences in Europe of long-term survival in refuge

areas and postglacial spread. The largely complementary palaeobotanical and genetic

data indicate that Fagus sylvatica survived the last glacial period in multiple refuge

areas. The central European refuges were separated from the Mediterranean refuges,

which did not contribute to the colonization of central and northern Europe. Likewise,

some populations expanded considerably during the postglacial period (Magri, 2008),

while others experienced only limited expansion. According to Ninyerola et al. (2007a),

inferences from more than a few studies lend credibility to the presence in the

Mediterranean of deciduous taxa such as Fagus during the early and mid-Holocene. The

climatic suitability of Fagus during the early Holocene has been shown by Lozano et al.

(2002), who identified Fagus and dated it at c. 17,895 cal. yr BP in Urdaibai (Basque

County) or López-Merino et al. (2008) in Sierra de Neila at c. 15,600-13,700 cal. yr BP.

This led them to suggest the northern Iberian Peninsula as a possible refuge zone

(Hewitt, 1999). In the Balearic Islands, the available data (Ninyerola et al., 2007a;

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Pérez-Obiol and Sadori, 2007) seem to indicate that Fagus had refuge in some concave

areas during the upper Pleistocene and the Holocene. The presence of small stands of

Fagus in Majorca, before the colonization from the Pyrenees took place, makes this a

credible hypothesis. Similarly, examining the Iberian Peninsula, Pott (2000) indicates

that over the last 9000 years Fagus has colonized northern areas from diverse

Pleistocene Mediterranean refuges.

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In the Iberian Peninsula, evidence exists (Costa et al., 1998) of the presence of Fagus

sylvatica in the Basque Country (Saldropo) and Tramacastilla more than 4000 and 7000

years ago, respectively, which would confirm the presence of various refuge zones in

the southern slope of the Pyrenees (Montserrat, 1992). This pattern of colonization is

supported by pollen records from the northeast Iberian Peninsula (Pérez-Obiol, 1988),

showing that Fagus colonization began between 8800 and 7850 cal. yr BP.

The difficulty comes from site differences that enormously complicate the interpretation

of local pollen and charcoal records, as at Burg Lake in the Pyrenees, close to the study

area, where Fagus sylvatica does not appear until 3000 cal. yr BP (1050 BC) (Pèlachs,

2005).

On the other hand, regional data support the introduction of Fagus sylvatica at Redó

Lake at about 4900 cal. yr BP (Catalan et al., 2001), and a little later at Redon Lake

(Catalan and Pla, 1998), where it arrives in about 4500 cal. yr BP, probably as a

consequence of the difference in altitude (Esteban et al., 2003). Miras et al. (2007)

implicate both anthropic participation and onset of new climate conditions (lower

summer temperatures and higher annual precipitation) in the timing of the first regular

observations of Fagus sylvatica in the Andorran valley of Madriu, at about 4800 cal. yr

BP.

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Similarly, much farther west of the Pyrenees, Montserrat (1992) explains that, although

beech appears intermittently at Ibon de Tramacastilla after 7859 cal. yr BP, its curve

does not become continuous until c. 5760 – 4476 cal. yr BP, making its appearance

contemporaneous with the other Pyrenees sites.

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No one disputes that the Abies alba dynamics in the Pyrenees during the Holocene

indicate colonization followed by expansion from east to west (Jalut et al., 1998;

Esteban et al., 2003; Pèlachs, 2005; Le Flao, 2005), which would confirm the existence

of refuges located in the Mediterranean basin. In fact, analysis of the current western

boundaries of Abies alba in the Pyrenees shows a progressive lag between the western

and eastern half of the mountain chain, which could be attributed to the progressive

distancing of this conifer from its refuge areas (Reille and Andrieu, 1991). Similarly,

some authors have reported that this species first developed on the north slope of the

Mediterranean Pyrenees at 11,224 cal. yr BP, specifically in the area of Nohèdes (Jalut,

1974; Reille and Lowe, 1993); this is consistent with the very first appearance of Abies

alba in the Garrotxa at about 10,204 cal. yr BP (Pérez-Obiol, 1988). Other registries of

long-term silver fir presence in the eastern Mediterranean also concur, e.g., Pla de

l’Estany (Burjachs, 1994), Banyoles (Pérez-Obiol and Julià, 1994), and Abric Romaní

(Burjachs and Julià, 1994), confirming the presence of refuges in coastal zones and in

intramountain valleys of the Iberian Peninsula (Carrión-García et al., 2000). Therefore,

colonization of Abies alba and Fagus sylvatica in the meridional slope of the Pyrenees

could be due, in part, to refuge zones located to the south and east of the Pyrenees (Fig.

1).

[FIGURE 1]

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Present day distribution of Fagus and Abies in the Iberian Peninsula related to

anthropogenically forced landscape changes (B)

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Although climate has been regarded as the determining factor in the development of

Fagus sylvatica forests at c. 4500 cal. yr BP (Jalut, 1974; Giesecke et al., 2007), it has

also been demonstrated that human influence may be responsible for its strong

expansion at that time (Kenla and Jalut 1979; Jalut 1984; López-Merino et al., 2008).

According to Tinner and Lotter (2006), beech survived human pressure, while other

deciduous trees (e.g. Tilia, Ulmus, Fraxinus excelsior) and silver fir (Abies alba) were

strongly disadvantaged. The authors hypothesize that in the absence of human impact,

silver fir would have expanded to areas in Europe where the species is absent today.

According to Peñalba (1994), the western and southernmost parts of the peninsula have

not been colonized by Fagus. The absence of Fagus in northwestern Spain is striking,

given the importance of this genus in similar climatic conditions in the other Cantabrian

regions. It is unlikely that the spread of Fagus was stopped in Galicia by natural causes

at 1390 cal. yr BP. At that time, humans exerted strong influence on the vegetation in

this region; their presence there is recorded since 5760 cal. yr BP. Anthropogenic

disturbance has proved responsible for the final, abrupt decline of Fagus populations in

the Cantabrian region. It is likely that severe anthropic pressure on populations of Fagus

at their range limit stopped the spread to the west. A similar situation could be inferred

for Abies, confined today to the eastern part of the Pyrenees although it had a wider

distribution in the Iberian Peninsula during previous interglacial periods. Two facts

must be considered: first, man favoured Fagus to the detriment of Abies at the

beginning of its extension to the northern side of the Pyrenees (Jalut 1984), and second,

Abies grows today in Italy under climatic conditions also found in Spain (Terhürne-

Berson et al., 2004; Liepelt et al., 2009), suggesting that the spread of the species into

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the Iberian Peninsula could have been stopped by human interference in the Pyrenees.

Nevertheless, climate forcing in the Post-Bronze Iberian Roman Humid Period (2600-

1600 cal. yr BP) could be a consideration, as proposed by Martín-Puertas et al. (2008).

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Fig. 2 shows the clear decline of Abies alba beginning in the medieval period. When the

human impact was too strong, silver fir totally disappeared (Pérez and Roure, 1990;

Pèlachs, 2005; Tantinyà, 2007).

The potential distribution of Abies alba in the northwest Iberian Peninsula proposed by

Rivas (1987) would result in a much larger region with a much more suitable surface if

we consider numerous biotic and abiotic factors that exist at present. To enhance the

potential distribution of these two taxa, a combined spatial suitability surface has been

developed through GIS and multivariate statistical methods. This map allows us to

understand the spatial behaviour of Fagus and Abies at regional scale, complementing

the palaeopalynological results.

[FIGURE 2]

Study Area (A)

The Prats de Vila peat bog (longitude 1° 6’ 13” E and latitude 42° 38’ 17” N) is found at

1,150 masl and has an estimated area of 2.8 hectares. The lithological substrate

corresponds to Cambro-Ordovician slates, even though during the fieldwork we found

important granite deposits of glacial remains.

The climatic conditions surrounding the peat bog (within a 1 km radius) are humid

(Thornthwaite humidity index) with an Autumn-Spring-Summer-Winter precipitation

pattern and mean annual values ranging between 652 mm and 887 mm (=736 mm). The

mean annual temperature ranges between 6.5 ºC and 10.5 ºC (=8.9 ºC), decreasing in

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winter to a mean minimum temperature of around -3.8 ºC. Potential evapotranspiration

(computed following the Hargreaves method) shows annual values ranging between 574

mm and 850 mm (=716). These values are close to precipitation values, meaning that

this area is free of hydric stress. All the climate data have been extracted from the

Digital Climatic Atlas of the Iberian Peninsula (Ninyerola et al., 2007b and 2007c).

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The present vegetation on the peat bog is Subalpine-Montane mesophilous and siliceous

meadows with Agrostis capillaris, Festuca nigrescens, Anthoxanthum odoratum,

Galium verum, and Genistella sagittalis. Vegetation surrounding the peat bog in shady

places includes some deciduous Quercus together with Corylus, Betula and Pinus,

which in many cases occupy formerly cultivated fields and give way to the most

extensive Abies alba stands of the Pyrenees: la Mata de València d’Àneu. In northern

Spain, distribution of Quercus petraea (the dominant oak near the study zone) is

typically fragmented. Taking into account its minimal presence in the pollen diagram, it

appears that its distribution area in the study zone has not been of great importance

during the last millennia. At the same time, in sunny places, the deciduous Quercus

share their protagonism with Q. Ilex subsp rotundifolia.

Materials and Methods (A)

The study methodology was based on a combination of pollen data extracted from a

peat bog in València d’Àneu (Axial Pyrenees) and fieldwork to identify the main plant

communities in the zone.

Three core samples were taken with a mechanical sampler and the one with the most

consolidated peat was selected for analysis. Two large, clearly differentiated

sedimentary units have been described in the register of the peat bog studied (Fig. 3):

the upper unit, characterized by the abundance and continuity of the bog, and the lower

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level, characterized by a granite conglomerate with very compacted gravel and some

pebbles at the transition between the two units. Two samples were selected for dating

using

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14C-AMS (Beta Analytic Inc.), based on a piece of wood at 59-60 cm depth and a

peat fragment at 165-166 cm depth (Table 1). The resulting sedimentation rate for the

peat section was 0.72 mm/year for the first 60 cm and 0.83 mm/year for the rest. Age

was calibrated to calendar age using the INTCAL04 program (Talma and Vogel, 1993).

For the pollen analysis, we selected only the first two meters of peat from one of the

cores (named VAL-III), down to the transition to gravel conglomerate. Chemical

treatment of the samples was carried out according to the protocol described by Goeury

and Beaulieu (1979).

[FIGURE 3]

[TABLE 1]

Suitability mapping (B)

The suitability vegetation maps for Fagus and Abies were developed using presence-

absence models adjusted with logistic linking in a General Linear Model (GLM).

Presence data were obtained by choosing plots where these species are dominant from

the third National Forest Inventory (a project administered by the Spanish state). The

resulting distribution of both species is shown in figures 7-8. This forest inventory

regularly samples the territory with a grid density of 1 km. This type of sampling is

very interesting because it covers a large area but especially because regular sampling

avoids the sampling bias that exists in many other types of chorological data. We would

also note that we have access to plots in which the absence of the species studied is

ensured, mitigating the problem of pseudo-absences (Chefaoui and Lobo, 2008). To

obtain an absence sample, we randomly chose a number of sites equal to the presence

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sites. In addition, plots that were absence sites for the species we considered were

avoided if they were within a 5 km radius of the presence plots and therefore had very

similar topoclimatic conditions.

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With respect to predictor variables, we incorporated geoclimatic variables obtained by

spatial interpolation methods (Ninyerola et al., 2000), based on a Digital Elevation

Model with 200-m resolution and data from Spain’s National Institute of Meteorology

(INM), which provided readings from 1346 temperature stations and 2519 for

precipitation. We would emphasize here that having access to a plot that was

georeferenced with a high level of precision allowed us to capture the climatology at

toposcale, minimizing methodological errors. Five variables were analysed: maximum

mean temperature for the warmest month, mean annual temperature, minimum mean

temperature for the coldest month, accumulated precipitation by season and potential

solar radiation by season. Table 2 shows the ranges for Abies alba and Fagus sylvatica.

We then enriched the databases using vector point files (presence-absence distribution)

with the corresponding values from the geoclimatic variables. This enriched database

was submitted to statistical analysis using a GLM with logistic linking, as in other

studies (Felicísimo et al., 2002) of the suitability of forest species. For the process of

adjusting the model we used 60% of the plots and saved 40% for validation and to be

able to quantify in this way the quality of the resulting maps.

Finally, mapping algebra was used to obtain the suitability maps by species using the

completed analysis. The regression equations, adjusted by statistical analysis, were

reproduced using GIS, replacing each variable with the corresponding topoclimatic

map.

[TABLE 2]

Results and discussion (A)

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Pollen diagram from the València d’Àneu peat bog (B) 264

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The pollen diagram from the València d’Àneu peat bog permitted us to reconstruct the

vegetation changes in the studied zone over the last two millennia (Fig. 4). The diagram

is described using pollen assemblage zones (PAZ).

[FIGURE 4]

VAL-III / I (2200-2000 cal. yr BP; 250 BC –50 BC): the decline of the

“original”Abies alba forest (C)

At the beginning of this time period, Abies frequency of more than 10% with a peak at

approximately 22% can only be explained by the Abies alba in situ occupying a much

larger land area than at present. The drop in Abies at the end of this period may be due

to selective human intervention with respect to this species, favouring other species

such as Corylus, which would colonize the space left by silver fir. We must take into

account the fact that wood forms part of the Roman social and economic system and is

an indispensable element (Conedera et al., 2004).

Mining is another sector related to exploitation of forest resources. We noted an

increase in lead in the sediment of Redon Lake (also in the axial Pyrenees) during the

Roman era and a high point in about AD 600 (Catalan and Pla, 1998). The dating of five

charcoal kiln sites between the 3rd and 4th VI centuries AD and the identification of

charcoals (Pinus and Abies) allows us to relate this first metallurgy with selective acts

related to the forest (Pèlachs and Soriano, 2003).

VAL-III / II (2000-1300 cal. yr BP; 50 BC – AD 650): Abies alba forest with Fagus

sylvatica (C)

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The intervention of the prior phase on the Abies alba forest opens up land that is

occupied first by some species that are typical of meadows and clearings (Poaceae,

Plantago sp., Asteraceae, etc.) and permit the expansion of plant populations that

compete with the silver fir for space, such as Corylus in the lowest areas and Fagus,

Pinus and Betula in the same zones as the Abies alba.

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The occasional presence of Juglans, Juniperus and Artemisia and the start of Cerealia

and Castanea curves denote human management of the landscape, mostly related to

grazing and agricultural activities. Pseudoschizaea (an algal remain indicative of

erosive processes) appears. The first occurrences of Juglans are well dated at Ariege

(2000±107 cal. yr BP in Jalut el al. 1982; 1792±59 cal. yr BP, Galop, unpublished).

There are regular records from the 10th to the 13th centuries, though the dates may vary

with area and altitude (1048±79 cal. yr BP and 706±28 cal. yr BP, Galop, unpublished;

near 643±61 cal. yr BP, Planchais 1985). This cultivated tree is an excellent marker of

the Greco-Roman times. It was introduced in western Mediterranean regions as early as

c. 1952 cal. yr BP (Bottema, 1980) by Greek and Roman settlers. According to the

curve of pollen concentration (pollen grains/g), the arboreal biomass does not suffer a

significant decline (Fig. 5). However, forest activities are evident.

In any case, the plant dynamics indicate a human pressure that shifts the permanent

character of the land. Without technical resources to minimize labour expenditure, mid-

slope soils are preferred for agricultural uses (Esteban et al., 2003). This is the reason

for disturbances of mid and lower slopes of the forest that affect the dynamics of the

silver fir-beech forest.

[FIGURE 5]

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VAL-III / III (1300-650 cal. yr BP; AD 650 – AD 1300): the explosion of human

activities (C)

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This entire phase is characterized by a declining AP percentage and an absolute increase

of herbaceous plants (Fig. 5). The massive forest clearance during this period is shown

by the fall in AP values and the greater Poaceae abundance in the studied area. The

decline of Abies and the noticeable extension of Fagus are probable evidence of this

deforestation.

The pollen diagram shows certain peculiarities that led to splitting the zone into three

subzones:

VAL-III / IIIa (1300-1100 cal. yr BP; AD 650 – AD 850): global disturbance (C)

At the same time that Pinus recedes below 20% and Abies falls below 5%, Fagus,

Betula and Corylus take advantage of this by increasing their presence even though,

later on, they will decline just as the rest of the tree population did.

The large increase in Artemisia, Poaceae, Rumex and Polygonum can be explained by

the increase in grazing. The strong increment of Cerealia (mostly Secale) and Fabaceae

also indicate the implementation of agricultural practices. This evidence permits us to

assume that opening up the landscape led to the arrival of Olea pollen. In that era, olive

tree cultivation is documented in the domains of a nearby monastery (Esteban et al.,

2003). These facts are clearly evidenced by the drop in pollen concentration. The impact

of human disturbance is more noticeable from the Late Medieval period onward.

VAL-III / IIIb (1100-800 cal. yr BP; AD 850 – AD 1150): management of the peat

bog (C)

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This zone is characterized by the notable presence of Alnus, which together with the

dynamics of Cyperaceae and Typha-Sparganium pollen type allows us to connect this

period with an increase in the groundwater level of the peat bog and possibly with its

expansion.

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Sparganium sp. has great colonizing abilities and may cause a rapid silting in shallow

waters. At this time, its development coincides with the establishment of Alnus. Before

this colonization, Pediastrum was already present, indicating a rise in water level. These

percentage increases in taxa are related to a major sedimentary stability (Andrieu et al.,

2000). Late Medieval period documents explain that during this period it was common

to plant crops in muddy zones along river banks, which flooded periodically and were

called “insules” (Esteban et al., 2003); consequently, it would seem reasonable that a

hygrophilous environment was favoured, controlling the flow and the hydric resources

of the area.

The rapidly invading Abies would out-compete Fagus, or substantially slow down its

recruitment rate until canopy disturbance created light openings large enough for

successful establishment and growth. According to Doležal et al. (2004), the higher

mortality of Fagus in denser Abies patches and the resulting spatial segregation of the

species reflect asymmetric interspecies competition.

VAL-III / IIIc (800-650 cal. yr BP; AD 1150 – AD 1300): disappearance of the Abies

alba-Fagus sylvatica forest (C)

The beginning of this phase is characterized by high percentages of Poaceae and

Cerealia and the disappearance of Fagus from the study area, a disappearance attributed

to the strong human impact on the landscape. From this point on, there will never again

be a beech forest or a small mixed Abies alba-Fagus sylvatica forest in the zone. This

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drastic change in the forest landscape is also evidenced by the decline in Alnus, Abies

and Pinus, which at the end of the sequence permits the return of Corylus.

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VAL-III / IV (650-350 cal. yr BP; AD 1300 – AD 1600): recovery of the Abies alba

forest (C)

Since 650 cal. yr BP (AD 1300) we have observed a certain recovery of the arboreal

cover, led by the presence of three primary species of trees that are distributed and

combined in various stages and habitats: Corylus, Abies and Pinus; Betula is added to

the list at the end of this time period. Human pressure on the environment is moderate.

Therefore, it doesn’t seem that the repercussions of the Little Ice Age were sufficiently

important to affect the economic activities of the dominant classes, primarily herders.

All the same, documents report major declines in the Pyrenees in some of the species

grown (such as grapevines), which leads us to assume the existence of local differences.

VAL-III / V (350-150 cal. yr BP; AD 1600 – AD 1800): a new increase in human

pressure (C)

The slight percentage oscillations in various tree taxa, such as Abies, Corylus, Betula

and Pinus, are accompanied by a large increase in Poaceae and Juglans; this denotes a

new and different landscape management with the existence of pastures and plantations

of trees. It is worth noting that oil was extracted from the walnut trees and had a high

food and therapeutic value, equal or superior to that of olive oil, and therefore at

particular times could have offered an alternative to the cultivation of olive trees

(Esteban et al., 2003). In addition, the appearance of Ericaceae could indicate an

increase in ruderal species, given the use of a road network, and that of Glomus would

explain the more edaphic conditions of the peat bog.

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VAL-III / VI (150 cal. yr BP--present; AD 1800 – present): the preamble to the

current landscape (C)

The final episode puts the vegetation landscape at the doorstep of the current landscape,

with the percentages of Abies at about 5%, while Pinus recedes significantly and

heliophilous colonizers increase progressively in formerly cultivated zones and open

forest areas, including Corylus –especially at the end of the sequence – or plastic

species such as Betula. This occurred in other areas as well.

This denotes a decrease in the groundwater of the peat bog as indicated by the curve for

Cyperaceae and Glomus and suggests the definitive disappearance of Alnus around the

bog studied here. Chlamydospores of Glomus cf. fasciculatum would be evidence of

erosive phenomena (Van Geel et al., 1989) related to anthropogenic activity and drought

(López-Sáez et al., 2000).

[FIGURE 5]

Vegetation dynamics and suitability (B)

It is clear from the palynological data presented here that human impact became

stronger and reaches its maximum in this last millennium. This stage of the Pyrenean

forest history saw the final shaping of the present-day landscape (Kenla and Jalut 1979;

Galop, 1998).

The pollen diagram is comparable to numerous diagrams of the southern and central

Alps, central France and the Pyrenees themselves (Beaulieu, 1978; Clerc, 1988; David,

1993; Nakagawa, 1998; Tinner et al., 2005; Finsinger and Tinner, 2006; Pèlachs et al.,

2007). In the central Alps, Nakagawa et al. (2000) found a sequence that is

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chronologically similar and has three stages of impact, each of which is followed by a

different pattern of forest restoration. The first deforestation occurs at about 2060 cal. yr

BP, during the Roman era, and a selective exploitation of Abies alba forest is evidenced.

The silver fir forests formed part of a very active economy near the Rhine river. Küster

(1994) compiles various pollen diagrams for the Rhine, Elbe, and Danube and

demonstrates that the use during Roman times was not totally destructive. Various

zones of silver fir forest were left untouched. The author concludes that the concept and

practice of “forest management” was common in Roman times. The second

deforestation, around 1520 cal. yr BP (or during the 5

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th and 6th centuries), denotes

substantial evidence of agricultural activity. The third, around 810 cal. yr BP or right in

the middle of the 12th century, is similar to its predecessor but much longer and not at

all selective, so that the forest had no chance to recover.

These facts coincide quite well with the changes in percentages and pollen

concentration for Abies (Fig. 5). This dynamic also coincides with those found in other

localities close to the studied zone (Esteban et al., 2003; Pèlachs et al., 2007). The peat

bog studied demonstrates much more clearly a possible selective action involving Abies

alba forests during the Roman era and confirms the indices that explain how some

dynamics began in the medieval period, continued during the Modern Age and the 20th

century, and brought us to the current landscape.

In other areas of the Pyrenees, Abies alba was the primary species of trees between

6200 and 2800 cal. yr BP (4250-850 BC), a time when the stable Abies alba presence

gave way to red pine forest in the subalpine stage. At the same time this was happening,

a mixture of oak (Quercus sp., Tilia sp., Ulmus sp. etc.) also experienced a sharp

decrease. This strong disturbance of the subalpine and mountain area would permit the

pine forest to expand as a rapid colonizer and populate the space that had been occupied

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by silver fir; this was the product of accumulating circumstances where climate change

and human actions intersected (Pèlachs et al., 2007). In the studied zone this didn’t

happen and the silver fir, despite the strong disturbance they suffered, recovered again

and again, even though the population would never reach the levels of 2000 years

earlier (Fig. 4). Abies forests remained important during a large part of the Holocene,

which could be explained by the topography of the valley and slopes.

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The current pollen spectrum had never been seen in the previous 2000 years. This fact

led us to deduce that models such as Modern Analogue Technique (MAT) could be

difficult to apply in this zone of the Pyrenees, at least during the last 2000 years.

Establishing detailed comparisons, we observe notable differences between two data

groups of interest: pollen and vegetation cover; this means that we must explore models

that work for mountain regions in particular. Calibrating the mountain vegetation and

pollen spectra is key to this type of research if we are to understand certain evolutionary

patterns. The hypotheses of authors such as Muller et al. (2005), which postulate that

there is an increase in regional and distant pollen in sediment at high altitude, is only

valid for certain taxa. In sedimentary samples of lake surfaces, we see that the presence

in the pollen spectra of taxa such as Tilia, Abies, Ulmus and Fagus almost always

represents a local or nearby presence in mountain regions. Many calibrations have used

correction factors or R-values (the ratio between the pollen group and the vegetation

community it represents). At present, different models are grouped within the Extended

R-value (ERV). With respect to Abies, a taxon that has had a strong impact on the

evolution of the vegetation landscape in this zone during the last 2000 years, it must be

said that it is very sensitive to the described method of weighing distance. For example,

according to Eisenhut (1961) Abies alba presents a falling speed of 0.12m.s-1, while

other similar plants such as Pinus sylvestris have values of 0.056 m.s-1. We must always

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think in terms of intertaxonomic differences if we are able to properly interpret pollen

dispersal and deposition patterns.

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The pollen analysis presented advocates the possibility of an anthropogenic trigger for

Fagus sylvatica expansion. Many other studies suggest that human disturbance

facilitated the expansion of this tree where climatic conditions were favourable (Küster,

1997). This hypothesis has its origin in northern and north-western Europe (e.g.

Andersen, 1973; Iversen, 1973), where Fagus sylvatica expanded only after the

beginning of the Neolithic (Lang, 1994). According to Tinner and Lotter (2006: 541):

“human activities as one (if not the most important) cause for the invasion of Fagus

sylvatica into Central Europe (e.g., Küster, 1997, 1999; Ralska-Jasiewiczowa et al.,

2003) has repeatedly been questioned and is still debated (e.g., Huntley et al., 1989;

Lang, 1994; Huntley, 1996; Gardner and Willis, 1999; Pott, 2000)”. In locations where

Fagus is found forming monospecific communities, it is because in the middle of its

distribution area young beech has behaved like an eurioic species with a broad

ecological valence, capable of shaping itself to edaphic and climatic conditions that are

relatively diverse (Costa et al., 1998), which gives a certain advantage in confronting

Abies alba and other colonizers. From this point on it seems logical to think that Fagus

sylvatica was occupying the lower part of the Abies alba forest, exactly in the place that

was cut and burned to convert the land to cultivated fields. For this reason it did not

repopulate and was replaced by hazelnut. This process could only begin in the Middle

Ages, with the availability of technologies to occupy the valley floor, the experience

necessary to manage the drainage of peat bogs, and the consolidation of fluvial

boundaries, in addition to the political capacity to carry out the appropriation of these

spaces. In this moment in the history of “slash and burn” agriculture, which means that

itinerant agriculture was replaced by the permanent roturation of valley floors, a fact

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that required limiting the diversity of resources available to peasants, who had to

specialize in specific products selected not for their productivity but rather for their

adaptability to feudal uses (Esteban et al., 2003).

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According to the ecological literature, Tinner and Lotter (2006) affirm that Fagus

sylvatica and Abies alba have similar environmental requirements. These authors have

put on record that 1) today, Abies alba is more competitive than Fagus sylvatica where

summer precipitation is higher and temperature is lower (Ellenberg, 1996) and 2)

palaeobotanical evidence suggests that high summer precipitation is more important for

Abies alba than low temperatures. If we analyse the distribution of Abies alba in the

Spanish National Forest Inventory, we see how the silver fir on the Iberian Peninsula

today live with a mean annual precipitation of about 1100 mm/year and an estimated

mean annual temperature between 3.5ºC and 10.5ºC (Ninyerola, 2001). In the Iberian

Peninsula, young beech stands are found in zones in which the monthly average

temperatures fluctuate very little between the coldest and warmest month. Normally this

change does not exceed 15 ºC, although it might reach 25 ºC in the middle of the

peninsula. Young beech has great resistance to cold during the fallow times.

The present-day suitability maps of Abies, Fagus and Abies-Fagus mixed forest can be

observed in Fig. 6 and Fig. 7. If we focus on the area closest to the studied peat bog, we

find low (<0.3) and intermediate (0.3-0.7) Fagus suitability values. The closest nucleus

with high suitability (>0.7) is found about 10 km east of the bog. In contrast, with

respect to Abies we can see that cells with intermediate values dominate and, most of

all, less than 2 km away we find abundant areas that are highly appropriate for this

species. This makes one think that the studied area, and nearby zones, have topoclimatic

characteristics that are more favourable to the development of Abies. This situation is in

accord with the interpretation of the pollen diagram (Fig. 5), which makes us think that

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Abies recovers more readily when topoclimatic factors outweigh anthropic ones. From

the point of view of plant suitability, we can consider the València d’Àneu peat bog as

located in an area where the influence of ideal zones for Abies is clearly higher than for

Fagus. Statistical details of the model (adjustment and validation) underlying this

cartography can be found in table 3.

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[FIGURE 6]

[FIGURE 7]

[TABLE 3]

5. Conclusions (A)

The València d’Àneu peat bog has been shown to be a good palaeoenvironmental

record, giving us an image of the short-term changes that make possible a study of the

abrupt anthropic effects. The pollen analysis has made evident, in no uncertain terms, a

possible selective action affecting Abies alba forest in the Roman period and confirmed

the indicators that explain how during the medieval period some dynamics began that

would evolve during the Modern Age and the 20th century to produce the current

landscape in this area. The current vegetation landscape of this region of the Pyrenees

has never before existed over the course of the last 2000 years and the climatic frame is

not well represented due to human disturbance of the landscape during this period.

The surroundings of the peat bog provided good conditions for human settlement and

pastures by removing forest. The palynological data support that human impact became

stronger and reached its maximum in the last millennium.

A direct climatic inference cannot be made. It is not possible to isolate the human

presence from the plant dynamics and therefore there can be no clear correlation during

this period between climate and original vegetation.

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Silver fir shows a decline in this area due to factors much more related to human

intervention than to climate. Likewise, Abies recovers with a certain ease, in contrast to

what happens in other parts of the Pyrenees and pre-Pyrenees; a higher suitability with

respect to its current habitat is evidenced.

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During the first millennium of our era, we note the presence of beech woods, most

likely coexisting with Abies alba as a product of continual and selective actions in the

forest. Fagus sylvatica acts as a colonizer of open space and can be? directly related

with human activity, especially since the Middle Ages, provoking a change in the

altitude limits of forest and other ecotonic zones. In the same way, the maximum levels

of Corylus avellana currently present are due to the colonization of humid lowlands

previously used for crops and pasture.

In summary, then, plant succession over the past two millennia in the studied area can

be described as a maximum extension of Abies alba forest (2200-2000 cal. yr BP);

selective actions affecting the silver fir forest and arrival of beech (2000-1300 cal. yr

BP); deforestation as the agricultural zone expanded, with a reduction in the upper

altitude limit of the forest and definitive disappearance of Fagus sylvatica (1300-800

cal. yr BP); total Abies alba deforestation (800-650 cal. yr BP) and the recovery of

silver fir forest (without Fagus sylvatica presence) that, with various fluctuations,

persists into the present.

Acknowledgments (A)

This research would not have been possible without the support received from those

responsible for the High Pyrenees Natural Park; we especially want to acknowledge

Agustí Esteban Amat for his sensitivity to environmental research and his knowledge of

the area. Sampling of the peat bog was possible thanks to the efforts of Aureli Carnicer

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and also of COPCISA, which authorized access under the supervision of María Álvarez,

to whom we are especially grateful for the assistance she provided. We also wish to

gratefully acknowledge the unselfish collaboration in the field that we received from

Riker Yll and Jordi Llorens. Finally, the authors thank Elaine Lilly, Ph.D., of Writer’s

First Aid for English translation and revision.

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Fig. 1. Location of the València d’Àneu (VAL-III) peatbog (star) and fir forest (in grey)

in the Pyrenees.

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Fig. 2. Current distribution of Abies alba (white dots) in the Pyrenees and first

occurrences and dynamics during the Holocene (Pérez-Obiol, 1988; Pèlachs et al.,

2007).

Fig. 3. Lithologic column and sediment structure of the peat bog.

Fig. 4. Main taxa pollen diagram and calibrated dates from the València d’Àneu (VAL-

III).

Fig. 5.

Left: Non Arboreal Pollen Concentration vs. Arboreal Pollen Concentration (pol/g).

Right: Arboreal Pollen Concentration of Abies alba and Fagus sylvatica. Peat bog of

València d’Àneu (VAL-III)

Fig. 6. Suitability maps of Abies alba (a) and Fagus sylvatica (b). Dots represent the

present observed distribution (National Forest Inventory). High, medium and low

suitability are denoted by black, grey and white tones, respectively.

Fig. 7. Suitability map of mixed Abies-Fagus. This map is based on the layered

combination of suitability maps of each species. Black colours represent areas where

both species have high suitability, grey tones where only one has high suitability and

white colour where there is no suitability.

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819 Fig 1

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833

Sample (cm)

Laboratory Code

Material Conventional dating BP

Dating calibrated to 2σ (95%

probability)

Intercept calibration curve

59-60 Beta-240388

Wood 780±40 cal BP (730-680) cal BP 690

165-166

Beta-240387

Peat 1990±50 cal BP (1990-1880) cal BP 1940

834 835 Table 1. 14C dating of peat bog VAL-III

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836 MX_HOT MT_AN MN_COL PR_WIN PR_SPR PR_SUM PR_AUT RAD_WIN RAD_SPR RAD_SUM RAD_AUT N

Fagus sylvatica 20.6-27.6 6.0-12.9 -5.9-3.1 121-509 166-449 94-330 139-436 338-1310 1978-2804 2656-3137 909-1935 3681

Abies alba 18.2- 26.4 3.5-10.4 -8.1, -1.1 139-385 212-395 186- 369 201-380 159-1345 1734-2799 2457-3133 652- 1968 614

837 838 839 840

Table 2. Ranging values from the predictors used in the GLM suitability models. The values presented avoid the lowest and highest 2.5% of values.

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841 Predicted Species R2

Nagelkerke Cut-off point Observed absence presence

CCR (a+d)/N

fpos b/(b+d)

fneg c((a+c)

absence 669 (a) 65 (b) Fagus sylvatica 0.84 0.30 presence 53 (c) 685 (d) 92% 0.09% 0.07%

0.91 absence 105 (a) 12 (b) Abies alba 0.55 presence 7 (c) 121 (d) 92% 0.09% 0.06% 842

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Table 3. Fitting and validation results from the Abies and Fagus suitability GLM models. CCR

(Correct classification rate) represents the general performance of the model. Fpos (False

positive rate) shows the percentage of suitability areas that do not match with present-day

distribution. This cannot be considered an error because exists suitability for both species. Fneg

(false negative rate) is the measure that can be considered as the error.