Vegetation shifts,human impact and peat bog development in Bassa Nera pond (Central Pyrenees) during the last millennium

Abstract

High-mountain lakes are suitable ecosystems for studying local environmental shifts driven by large-scale climate changes, with potential applications to predict future scenarios. The precise features in the response of species assemblages are not fully understood, and human pressure may often hide climatic signals. To investigate the origin and impact of past environmental changes in high-mountain ecosystems and apply this palaeoecological knowledge to anticipate future changes, we performed a multi-proxy study of a sediment core from Bassa Nera, a pond located close to montane–subalpine ecotone in the southern central Pyrenees. Combining pollen and diatom analysis at multidecadal resolution, we inferred vegetation shifts and peat bog development during the past millennium. We introduced a montane pollen ratio as a new palaeoecological indicator of altitudinal shifts in vegetation. Our results emphasize the sensitivity of the montane ratio to detect upward migrations of deciduous forest and the presence of the montane belt close to Bassa Nera pond during the Medieval Climate Anomaly. Changes in aquatic taxa allowed to date the onset of the surrounding peat bog which appeared and infilled the coring site around AD 1565. Overall, our results suggest a low-intensity human pressure and changes in management of natural resources during the last millennium, where farming was the main activity from the Medieval Climate Anomaly until AD 1500. Afterwards, people turned to highland livestock raising coinciding with the ‘Little Ice Age’.

Keywords

diatoms montane–subalpine shifts palaeoecological indicators peat bog pollen Pyrenees

Introduction

Ecosystem reconstructions for the past millennium are crucial for understanding past environmental variability and predicting future changes. During this interval, different climatic phases are distinguished in the Northern Hemisphere (Mann et al., 2009): the relatively warm and arid Medieval Climate Anomaly (MCA), the cold Little Ice Age (LIA) with increased irregular rainfall and the current global warming (CGW), with an increase in temperatures caused by human activities (IPCC, 2007), starting with the Industrial Revolution (IR) (Seager et al., 2007). However, due to large spatial and temporal heterogeneities, the precise features of climatic variability during these periods and the responses of the ecosystems to this variability at regional or local levels are not yet fully understood. In many cases, strong human pressure greatly influenced the ecosystems, hiding climatic signals even when climate was the dominant driver (Bal et al., 2011). Reconstructing pre-industrial environmental conditions helps to discriminate anthropogenic mechanisms from natural forcings (Jansen et al., 2007) and thus to assess human impact and to predict how anthropized ecosystems will respond to current climate change. Lakes located in high-mountain ranges in temperate mid-latitude areas are especially suitable for assessing the potential consequences of climate fluctuations on mountain biota because they will most likely be among the first ecosystems to exhibit a response to current climate changes (Engler et al., 2011). Gottfried et al. (2012) have reported elevated replacement rates of cold-adapted plants by thermophilic species in several European mountain regions due to the current changes in the climate, and Thuiller et al. (2005) predicted high rates of vegetal species loss in the same areas. In this context, the Pyrenees are an interesting region to study, as the Mediterranean area is one of the most vulnerable regions on Earth to the CGW (Christensen et al., 2007). Several studies have furnished evidence that the Pyrenees have acted as a glacial refuge for forest species during past climate changes (Benito et al., 2008) and might contribute to buffer the effects of climate variability in the future (Alba-Sánchez et al., 2010). Although most of the currently described changes in the Pyrenees are related to land use (Améztegui et al., 2010), some Pyrenean forests have already experienced an enhancement of tree recruitment and growth during warm periods of the last century (Camarero et al., 2006; Camarero and Gutiérrez, 2004). The palaeoecological history of the northern Pyrenean slope throughout the Holocene has been intensively studied (e.g. Jalut et al., 1992; Reille and Lowe, 1993). Working on the southern slope of the Central Pyrenees, González-Sampériz et al. (2006) emphasized the occurrence of abrupt climate changes during the Holocene and the response of vegetation and lake systems to such changes, implying an efficient translation of climate variability from the North Atlantic to the mid-latitudes. Using a centennial resolution, Cunill et al. (2013) and Pérez-Sanz et al. (2013) were able to describe vegetation changes associated with the MCA and the ‘LIA’ in the same area. During the last millennium, human exploitation of mountainous resources has also played a substantial effect on landscape (Ejarque et al., 2010; Miras et al., 2010; Pèlachs et al., 2009).

The main purpose of our work is to integrate palaeoecological research from the Pyrenees into the ongoing efforts to estimate the future ecosystem dynamics of European high-mountain environments in the face of global warming by trying to unravel human from climatic influences. The site of our case study is located in the Central Pyrenees of Catalonia, where future scenarios predict a 5°C increase in temperature by 2100 (A2, IPCC, 2007), while precipitation will decrease and extreme hydric events will be more frequent (Barrera-Escoda and Cunillera, 2011; Brunet et al., 2009; López-Moreno and Beniston, 2009).

With this aim, we present a multi-proxy study combining high resolution (multidecadal) of palaeobotanical (pollen, stomata) and palaeolimnological data (diatoms, freshwater sponges and chrysophyte cysts) to disentangle the nature of ecological changes in a lacustrine system and its surrounding vegetation during the last millennium. We also aim to develop and validate specific palaeoecological indicators that are useful for measuring potential altitudinal shifts in vegetation. Bassa Nera Pond is a location well suited for this purpose in view of its relative proximity to the montane–subalpine ecotonal boundary, a feature that is highly sensitive to altitudinal vegetation shifts in response to climate-driven changes (Luckman and Kearney, 1986). Moreover, the Bassa Nera is located in Aiguamòg Valley, where potential indicator taxa have been described (Cañellas-Boltà et al., 2009; López-Vila et al., 2014), providing information essential for properly interpreting palaeoclimatic and palaeoecological records.

Study area

The Bassa Nera (42°38′18.5″N, 0°55′27.6″E, 1891 m) is a small lacustrine system located in the peripheral zone of Aigüestortes i Estany de Sant Maurici National Park. It is situated in the Aiguamòg Valley (Aran Valley), on the Northern slope of the Central Pyrenees (Figure 1). Nowadays, this pond is surrounded by a complex of marshes and a peat bog formed by Sphagnum spp. and Carex lasiocarpa Her., with abundant Molinia caerulea (L). Moench, Drosera longifolia L., Menyanthes trifoliata L. and Parnassia palustris L (Carrillo et al., 2008). A conifer forest of Pinus mugo ssp. uncinata (Ramon) Domin. and Abies alba Mill. with Rhododendron ferrugineum L. in the understory and some Poaceae meadows surrounds the catchment. The pond has an area of 2.01 ha and 5 m maximum depth, with a small outlet that drains into the Garonne River. The water receives mineral salts only from precipitation and runoff. The pond’s watershed bedrock is composed of Carboniferous–Permian granite rocks (Roca i Adrover et al., 2010). The annual average precipitation reaches 1152 mm and is evenly distributed over the seasons. The mean annual temperature is 4.25°C, with January being the coldest month (−3°C on average) and July the warmest month (14°C on average) (Ninyerola et al., 2003).

Figure 1.

Location of the study area: (a) map indicating the relative location of Bassa Nera Pond (red point) and other palaeoenvironmental sequences mentioned in the text (black points), (b) topographic map of the terrain surrounding Bassa Nera (red point) and (c) coring site (red star).

Biogeographically, Aiguamòg Valley lies within the Boreo-Alpine and Eurosiberian zones. Cañellas-Boltà et al. (2009) and López-Vila et al. (2014) described three altitudinal vegetation belts: the montane belt (<1600 m) is composed of deciduous oak forests of Quercus petraea (Mattuschka) Liebl. with Betula pendula Roth., riverine forests (Alnus glutinosa L., Fraxinus excelsior L. and Salix spp.), forests with Tilia platyphyllos Scop., Prunus avium L. and Corylus avellana L. and mixed forests of B. pendula with Pinus sylvestris L. The subalpine belt (1600–2250 m) is dominated by coniferous forests of A. alba and R. ferrugineum at the lowest altitudes and Pinus mugo ssp. uncinata with R. ferrugineum at higher altitudes. Wetlands are characterized by Scirpus cespitosus L. communities, assemblages of Juncus balticus Willd. ssp. pyrenaeus, Carex rostrata Stokes and Caltha palustris L. with Epilobium palustre L. and Sphagnum peat bogs. The alpine belt (>2250 m) is formed by the open and sparse vegetation of Nardus stricta L. and Festuca eskia Ramond ex DC. meadows with Carex spp. The main anthropogenic activities until the creation of the National Park in 1955 and the demarcation of a peripheral protection area by 1990 were extensive cattle husbandry, forest exploitation and hydroelectric power generation. Tourism has displaced those activities, although pasturing and hydroelectric exploitation are still authorized.

Methods

Coring, sampling, dating and sedimentology

A sediment core 706 cm long (PATAM12) was collected in 2007 using a ‘Russian’ corer (Jowsey, 1966) on the peat bog that surrounds Bassa Nera and was sliced every 3–5 cm. This study is focused on the uppermost 330 cm. In total, 10 radiocarbon dates (Table 1) were obtained from wood and seed macroremains along the entire core by the accelerator mass spectrometry (AMS) method at the Beta Analytic Radiocarbon Dating laboratory (Miami, FL, USA) or Keck Carbon Cycle AMS Laboratory (Irvine, CA, USA). Seven radiocarbon dates fall within the interval analysed here. Ages were calibrated with the IntCal13.14C curve (Reimer et al., 2013), and the age–depth model was obtained by using smoothing spline interpolation in R package Clam 2.2 (Blaauw, 2010). The sedimentological description was performed following Schnurrenberger et al. (2003).

Table 1.

Results of ¹⁴C radiocarbon dates for different depths at the Bassa Nera. Sample Beta-247298 was excluded of the age–depth model as stratigraphically incongruent (marked with an asterisk (*)).

Depth (cm)	Laboratory code	Dated material	AMS ¹⁴C years BP
97.5	Beta-247296	Wood	220 ± 40
127.5	Beta-251879	Wood	190 ± 40
192.5	Beta-247297	Wood	270 ± 40
222.5	Beta-251880	Wood	490 ± 40
261.5*	Beta-247298	Wood	250 ± 40
304.5	Beta-251881	Wood	880 ± 40
428.5	Beta-247300	Wood	2380 ± 40
517	Beta-247301	Seeds	3570 ± 40
604.5	Beta-251883	Wood	4530 ± 40
698.5	UCI-43704	Wood	6410 ± 20

Pollen analysis

A total of 51 samples were processed at the Catalan Institute of Human Paleoecology and Social Evolution, using standard palynological methods (Moore et al., 1991) with NaOH, HCl, HF and mineral separation in Thoulet solution (density 2.0 g/cm³). Microscopic slides were mounted in glycerine. Pollen grains were counted until diversity saturation (Rull, 1987) and identified according to Faegri et al. (1989), Reille (1992) and the reference pollen collection of IBB-CSIC. Given that most slides had superabundant Pinus, which could conceal the vegetation dynamics, counts were increased to obtain a representative sample (200–481 pollen grains without Pinus). Data are presented as a percentage of the pollen sum, excluding Pinus, Cyperaceae, aquatic plant pollen and spores. Diagrams were plotted using Psimpoll 4.27 software (Bennett, 2002), and statistically significant zones were based on changes in percentages of taxa showing abundances >1%. The method of Optimal Splitting by Information Content (Bennett, 1996) was used for this purpose. Stomata, Botryococcus algae and sedimentary charred particles (<100 µm; 100–500 µm) on the same pollen slides were also counted.

A new montane pollen ratio was calculated to infer past altitudinal variations in the montane–subalpine belt in the study site (see supplementary data, available online). To obtain this montane ratio, we used several pollen indicator types identified in the area by Cañellas-Boltà et al. (2009). Montane pollen types included Alnus, Betula, Buxus, Corylus, Fraxinus, deciduous Quercus, Tilia and Salix, while subalpine–alpine indicators included Asteraceae, Calluna, Campanula, Ericaceae, Plantago and Poaceae. These genera were selected as indicators because the local occurrence and abundance of both pollen and parent taxa show the same or similar altitudinal patterns. The percentages of the montane pollen were summed and divided by the sum of the percentages of subalpine pollen. This ratio was proved with 33 surface moss samples from the altitudinal transect studied by Cañellas-Boltà et al. (2009) in the Aiguamòg Valley. The usefulness of this ratio for discriminating both vegetation belts was assessed by calculating its values and their 95% confidence intervals for the 33 moss samples from the altitudinal transect studied by Cañellas-Boltà et al. (2009). According to the modern analogues, values of 2.5 indicate the close presence of the montane belt, while higher values imply the upward montane migration of the latter within Bassa Nera basin.

Diatom analysis

A total of 0.1 g of dry sediment from 35 samples was treated with H₂O₂, and the frustules were mounted in Naphrax (R.I. = 1.7). Valve concentrations (valves g sediment⁻¹) were estimated through the addition of a known number of latex microspheres (Battarbee, 1986). A minimum of 500 valves per sample were counted with a Polyvar light microscope at 1000× magnification and identified at the lowest taxonomic level according to Krammer and Lange-Bertalot (1999a, 1999b, 2004a, 2004b), Cejudo-Figueiras et al. (2011), Buczkó et al. (2010), Bey and Ector (2013) and Morales (2005). Observed chrysophycean stomatocysts and sponge spicules were also counted. The centric-to-pennate (Ce/Pe) ratio was calculated as an indicator of the relative abundance of planktonic to benthic habitat availability. The diatom dissolution index (DDI) was computed as the percentage of valves showing dissolution and/or breakage signals. The Shannon–Wiener diversity index (H′) was used (Shannon, 1963). The planktonic-to-fragilarioid (P/F) species ratio was calculated as an indicator of the duration and extent of ice cover (Douglas and Smol, 1999; Lotter and Bigler, 2000). The chrysophytes-to-diatoms (Cr/Di) ratio was calculated as a means for estimating trends in algal succession, nutrient content and the length of the growing season (Smol, 1985). Diatom diagrams were plotted using Psimpoll 4.27 software (Bennett, 2002), and zonation was based on changes in percentages of taxa showing abundances >3% according to the method of Optimal Splitting by Information Content (Bennett, 1996).

Results

Chronology and sedimentology

Seven radiocarbon dates were used to construct the age–depth model that covers the last 2500 cal. yr BP (Table 1; Figure 2). One date (Beta-247298) was rejected as stratigraphically incongruent, probably because the roots might have dragged down the wood macrorest where dating was performed. The x-ray radiograph of a core retrieved by other researchers working close to PATAM12 reveals that the depth of extracting our macrorest coincides with a root-rich level (Pelachs, 2015, personal communication), reinforcing this rejection. According to the age–depth model, the entire core extends from ca. 7464 cal. yr BP to the present, with an average confidence interval error of ca. 150 years. The age–depth pattern for the last millennium discussed here showed a sedimentation rate of 0.36 ± 0.15 cm yr⁻¹ (mean ± SD) from 0.09 to 0.7 cm yr⁻¹, with an average interval of 14–22 years between samples (Figure 2). Three sedimentary facies have been described (Figures 3 and 4): (1) massive brown sandy silt with medium-to-large granulometry and scarce vegetal organic matter (0–50 cm); (2) massive brown-red clay and abundant vegetal organic matter (50–195 cm); and (3) massive brown-dark clay with abundant vegetal organic matter (195–330 cm).

Figure 2.

Age–depth model for the last 2500 years based on radiocarbon dating of Bassa Nera pond and performed with Clam 2.2 software. The box marks the studied period, the last 1000 years. Sample in red was considered as outlier.

Figure 3.

Percentage diagram of sporomorph, including the total pollen (relative abundance ⩾1%), charcoal percentages and M/A ratio. Pinus pollen, wetland plants and fern spores were excluded from the pollen sum (ΣP). Pinus percentage was calculated with the pollen sum plus Pinus pollen vegetal associations: lowland (L), montane deciduous forest (MDF), subalpine deciduous forest (SDF), alpine meadows (AM), and human-related taxa (H). The continuous horizontal lines correspond to statistically significant zones (Bennett, 1996) and the dotted lines correspond to subzones. Percentages of the elements out of the pollen sum were calculated by dividing them by the pollen sum. Palaeonvironmental phases (see the ‘Discussion’ section) are indicated at the right side of the diagram. Poaceae** shows the combined frequencies of Poaceae and Cerealia-t pollen.

Figure 4.

Relative abundances (⩾3%) of diatom taxa throughout the Bassa Nera record. The continuous horizontal lines delimitate three statistically significant zones (Bennett, 1996) and the dotted lines arbitrarily defined subzones. Diatoms are arranged in order of appearance.

Pollen and charcoal record

The pollen diagram (Figure 3) is dominated by arboreal taxa, mainly Pinus and Abies, with a notable abundance of deciduous taxa. Several anthropogenic taxa and various Mediterranean species are present along the whole sequence, increasing from 180 cm to the top. Three pollen zones (PZ-1–PZ-3) have been identified.

PZ-1 (330–265 cm, 22 samples)

The PZ-1 zone is characterized by the maximum values of coniferous taxa (Pinus and Abies), with notable abundances of some deciduous taxa (deciduous Quercus, Alnus, Betula and Fagus). Shrub elements such as Corylus, Prunus and Ericaceae are well represented, while herbs show low percentages. This zone shows the highest montane pollen ratio of the studied sequence, with values of approximately 2.75. Charcoals are particularly abundant. Two subzones can be distinguished. The subzone PZ-1a has a clear Abies dominance (45%), with a high amount of deciduous Quercus and large percentages of Cyperaceae, Myriophyllum and ferns. In contrast, subzone PZ-1b is characterized by a decrease in conifers and deciduous Quercus and a marked rise of Prunus, Artemisia and Poaceae. Cyperaceae sharply decrease from 10% to 2%, while Myriophyllum and ferns are significantly reduced. In addition, Botryococcus rises by the middle of the subzone, coinciding with the appearance of conifer stomata and the decline of charcoal.

PZ-2 (265–197.5 cm, 13 samples)

This zone is distinguished by a drop of arboreal pollen from 60% to 35%, mainly caused by the decrease in conifers (Abies, Pinus). Deciduous taxa decline slightly, but a minor increase in some shrub and herbaceous elements (Prunus, Poaceae, Rumex, Artemisia, Apiaceae and Plantago) is observed. Myriophyllum dominates the aquatic taxa. Potamogeton shows a remarkable increase; this increase is followed by a Botryococcus peak. The montane ratio displays short periodic fluctuations ranging from 0.5 to 2.2. Charcoal notably diminishes by the top of this zone, coinciding with the disappearance of stomata. Two subzones can be distinguished according to cereal presence and changes in the wetland and aquatic communities. Subzone PZ-2a is characterized by high abundance of Secale cereale, Cerealia-t and wetland and aquatic plants (Cyperaceae, Ranunculaceae, Myriophyllum and Potamogeton). Botryococcus has the highest values of the sequence. The onset of subzone PZ-2b is marked by the disappearance of Cerealia-t and S. cereale as well as by a progressive decrease in Artemisia and montane taxa (Alnus, Corylus and Betula). In contrast, Olea increases markedly. Cyperaceae remain abundant, but the other aquatic plants virtually disappear. Botryococcus also declines. Charcoal decreases and stomata are absent.

PZ-3 (197.5–0 cm, 16 samples)

The PZ-3 zone shows a notable decrease in Alnus, Betula and Prunus. Although Artemisia diminishes, the herbaceous taxa spread. Olea reaches its maximum values, and there is a rise in broken and damaged pollen by the middle of the zone. Cyperaceae have the same low frequencies as in PZ-2b. Potamogeton becomes the only aquatic macrophyte, with very low abundances, while Botryococcus disappears. The montane ratio continues with oscillations ranging from 0.2 to 1.7 and charcoal increase by the middle of this zone. Two subzones can be differentiated. The PZ-3a subzone is characterized by an increase in human-related taxa (Urtica and Potentilla) and Parnassia. In contrast, Abies shows a slight decreasing trend. Poaceae follow an oscillating pattern ranging from 25% to 5%. The evergreen Quercus peaks, and Olea spreads. Additionally, charcoal is scarce. The PZ-3b subzone is marked by an abrupt decrease in Potentilla, Urtica and Parnassia, while Olea reaches its maximum values. Cyperaceae and charcoal increase markedly, with two charcoal peaks at 50 and 90 cm. At the top of the subzone, Ericaceae and Corylus increase.

Diatom record

Diatom assemblages are composed of 240 taxa distributed among 52 genera. A substantial portion of these taxa are benthic (Figure 4). The most significant features in the diatom stratigraphy are the high percentages of small Fragilarioid taxa, with only five tychoplanktonic species showing abundances >10% at any given time along the sequence (Achnanthidium minutissimum (Kützing) Czarnecki, Staurosirella pinnata (Ehrenberg) Williams and Round, Staurosira construens var. venter (Ehrenberg) Hamilton, Pseudostaurosira alvareziae (Cejudo-Figueiras, Morales and Ector) and Stauroforma exiguiformis (Lange-Bertalot) Flower, Jones and Round). According to changes in the diatom assemblage, three diatom zones are differentiated for the last millennium, which can be divided into subzones.

DZ-1 (330–207.5 cm, 18 samples)

In this zone, diatoms reach their maximum concentrations in the record, showing periodic fluctuations from 1.61 to 2.54 × 10⁹ valves g sed^-1. The Ce/Pe and P/F ratios also present their highest values in the record. H′ ranges from 2.8 to 3.7, and the DDI indicates that diatom frustules are well preserved. Assemblages are dominated by the periphytic S. construens var. venter, A. minutissimum and S. pinnata, while the abundances of Encyonopsis subminuta Krammer and Reichardt, Nitzschia fonticola Grunow in Cleve and Möller and Brachysira procera Lange-Bertalot and Moser fluctuate. Minor diatom assemblages formed by Kobayasiella sp., Psammothidium subatomoides (Hustedt) Bukhtiyarova and Round and Sellaphora cf. radiosa are found exclusively in this zone. Aulacoseira nivaloides (Camburn) English and Potapova and Aulacoseira valida (Grunow in Van Heurck) Krammer are abundant in the planktonic assemblage. Two subzones can be differentiated. The DZ-1a subzone is dominated by S. pinnata, A. minutissimum and S. venter. In the DZ-1b subzone, S. pinnata decreases and S. venter shows a dramatic drop at 252 cm. However, P. alvareziae and Staurosirella oldenburgiana (Hustedt) E. Morales show an increasing trend towards the top of this subzone. The species that characterize this zone prefer habitats with low nutrient and alkaline water bodies (Van Dam et al., 1994). The Cr/Di ratio is low. The presence of spicules of the sponge Ephydatia muelleri (Lieberkühn) (Økland and Økland, 1996) is noteworthy except between 222.5 and 152.5 cm.

DZ-2 (207.5–177.5 cm, three samples)

At the transition between the DZ-1 and DZ-2 zones, the diatom concentration decreases drastically to 9.43 × 10⁷ valves g sed^-1 and recovers shortly thereafter. The Ce/Pe and P/F ratios decrease, and H′ decreases markedly at 192.5 cm. This zone is characterized by the prominent peak of the freshwater species P. alvareziae, which becomes the dominant species, reaching 70% abundance at 192.5 cm. The previously dominant species S. pinnata declines to 5%. Furthermore, S. venter and the epiphytic A. minutissimum and E. subminuta virtually disappear for the first time in the entire record while S. oldenburgiana remains constant. In contrast, Eunotia arcus Ehrenberg, S. exiguiformis and the acidophilus Tabellaria flocculosa (Roth) Kützing increase slightly, peaking at approximately 203 cm. In the planktonic assemblage, A. nivaloides and A. valida are replaced by the acidophilus Aulacoseira tethera Haworth (10%). This latter taxon has been identified according to Krammer and Lange-Bertalot (2004a) and Bey and Ector (2013). The density and width of striae together with the rows of areolae and the dimensions of the cells seem to indicate that the observed species is indeed Aulacoseira, although it could be close to the morphotype described in Buczkó et al. (2010). The diatom assemblages of the DZ-2 zone are commonly present in habitats with a low-to-medium electrolyte content, circumneutral waters and moist places (Van Dam et al., 1994). Even the Cr/Di ratio remains low, reaching 0.048 at 202.5 cm. Meanwhile, E. muelleri spicules are present throughout the entire zone.

DZ-3 (177.5–0 cm, 14 samples)

This zone is marked by the abrupt drop of the diatom concentration to 1.81 · 10⁶ valves g sed^–1. The Ce/Pe value slightly increases, while H′ oscillates approximately 3.2 bits and decreases towards the top of the zone. The DDI reaches its highest values found in the record by the middle of the zone, and the P/F ratio remains low. P. alvareziae decreases significantly and is substituted by S. exiguiformis (25%), which peaks at 81.5 and 152.5 cm. S. pinnata and S. venter recover to their former abundances (DZ-1 zone). Indeed, S. venter has a prominent peak at 152.5 cm, reaching 30%. Further minor diatom assemblages formed by Gomphonema bohemicum Hustedt, Gomphonema exilissimum (Grunow) Lange-Bertalot and E. Reichardt, Encyonema vulgare Krammer, Encyonema neogracile Krammer and Nitzschia sp. show slight peaks. Four subzones can be identified. The DZ-3a subzone is characterized by a decrease in P. alvareziae and S. oldenburgiana and an increase in S. venter. Planktonic A. tethera decreases, to be replaced by Aulacoseira alpigena (Grunow) Krammer. The DZ-3b subzone is marked by two peaks of S. exiguiformis. At 122.5 cm, P. alvareziae reappears, with a peak that coincides with decreases in S. exiguiformis and S. venter. The DZ-3c subzone is distinguished by a decreasing trend of S. exiguiformis and a peak of P. alvareziae, A. minutissimum, S. pinnata and S. venter, while S. oldenburgiana shows an increasing tendency. The DZ-3d subzone is characterized by a peak of A. minutissimum at 33.5 cm, coinciding with a minimum of S. pinnata, S. venter and S. exiguiformis. At the end of this subzone, S. pinnata, S. venter, S. oldenburgiana, S. exiguiformis, P. elliptica and A. alpigena increase until the present. DZ-3 species are usually found in oligotrophic habitats with alkaline waters in freshwater bodies (Van Dam et al., 1994). E. muelleri disappears at 162.5 cm; however, chrysophytes increase, peaking at 132.5 and 112.5 cm.

Discussion

Vegetation and palaeoenvironmental reconstruction

Our reconstruction of the past environment and vegetation dynamics in the Bassa Nera catchment is based on pollen and diatom zonations (Figure 5). Our results suggest that aquatic habitats progressively shrank through time due to changes in hydrological conditions. In the record, temperature changes have led to shifts in vegetation ecotones. The study of the montane ratio in this highland ecosystem has helped to detect these replacements. The discussion will be structured into five main phases according to lacustrine, vegetation and montane ratio changes (Figures 3 and 4). Figure 6 shows the ecosystem dynamics and anthropogenic pressure on regional sequences located in the Central Pyrenees and pre-Pyrenees.

Figure 5.

Summary diagram grouping pollen according to vegetal associations, with additional information about aquatics, charcoal, diatom concentration and ratios. Medieval Climate Anomaly (MCA), ‘Little Ice Age (LIA)’ and Industrial Revolution (IR) periods are indicated on the right. ‘Local mowing’ includes the following taxa: Centaurea spp, Cerealia-t, Sanguisorba spp and Secale cereale; ‘Local grazing’: Galium-t, Juniperus sp, Plantago-t, Potentilla-t, Ranunculus-t, Rumex sp and Stellaria sp and ‘Human-related taxa’: Urtica sp, Asphodelus-t and Chenopodiaceae/Amaranthaceae.

Figure 6.

Overview of sediment results (diatom, pollen, algae, macrophyte remains) and resultant climate and environmental inferences of Bassa Nera and other lacustrine and peat bog sequences from the Central Pyrenees and pre-Pyrenees recording the last 1000 years. Dark Ages Cold Period (DACP), Medieval Climate Anomaly (MCA), ‘Little Ice Age (LIA)’ and Industrial Revolution (IR). Vegetal associations – Mixed forest: Pinus, Abies, Betula, Corylus and deciduous Quercus; Conifer forest: Pinus and Abies; Montane forest: Corylus and deciduous Quercus. Human pressure – Crops: Cerealia-t and Secale cereale; Grazing: Rumex, Chenopodiaceae, Urtica and Potentilla; Human-related taxa: Plantago, Asteraceae and Artemisia.

Phase I. 330–265 cm, AD 801–1297

The pollen record suggests that the surroundings of Bassa Nera were dominated by montane forest (deciduous Quercus, Betula, Corylus and Prunus) mixed with Pinus, Abies and Ericaceae. The conifer stomata demonstrate the close proximity of these taxa to the pond. The highest montane ratio values, which are close to the ones observed in the present montane belt samples, indicate the proximity of the montane vegetation. This upward shift of the montane boundary with respect to the current location suggests warmer conditions. In contrast, the increase in Artemisia and Poaceae after the charcoal peak points to forest clearance at approximately AD 1000. The S. cereale indicates the presence of local and regional crops by AD 1150. A change in water level occurred approximately AD 990, inferred from higher planktonic diatom percentages and the large decrease in Myriophyllum and Cyperaceae, usually associated with lake margins and shallow waters (0.4–4 m deep) (Grosjean et al., 2001). Some periphytic diatom species that live attached to this littoral vegetation, such as S. pinnata, also declined. The oscillations of diatom concentration, Ce/Pe and P/F might be related to periods of strong seasonality and hydric fluctuations. This environmental instability is also evidenced by the presence of tychoplanktonic and opportunistic small Fragilarioids (S. pinnata, S. venter, P. alvareziae) and A. minutissimum, which has been related to periods of increased mixing or turbidity (Axford et al., 2009; Corella et al., 2011; Scussolini et al., 2011). This phase is set in the context of the MCA (9th–14th centuries), characterized by an increment in temperatures and by relatively arid conditions in south-western Europe (Mann et al., 2009; Seager et al., 2007). The Iberian Peninsula (IP) experienced generally drier conditions during this period (Moreno et al., 2012). Indeed, some lakes of the Pyrenees such as Basa de la Mora and Burg (Figure 6) lowered their lake levels, with strong seasonality marked by higher summer/autumn temperatures (Catalan et al., 2009) and lower winter/spring temperatures (Pla and Catalan, 2005). Other nearby pre-Pyrenean lakes also showed a negative hydric balance (Catalan et al., 2009; Morellón et al., 2011a).

Phase II. 265–220 cm, AD 1297–1493

Between AD 1297 and 1493, the decrease in montane ratio suggests a downward shift of the montane vegetation belt. In regard to this finding, the pollen diagram shows a decrease in deciduous Quercus and Alnus. However, the high percentages of Betula, Corylus and Prunus indicate relatively close montane vegetation or mixed forest. This montane downward shift and the landscape opening, prompted by the increase in Poaceae and other herbs, suggest colder conditions during this period, which correspond to the MCA–LIA transition. In contrast, the maximum of Artemisia, together with an increase in some human-related taxa (Cerealia-t, Plantago, Potentilla and S. cereale) and small charcoal, evidence the use of regional fires for crops and grazing, as has been observed in nearby regions (Ejarque et al., 2010; Pèlachs et al., 2009). The small decrease in the Ce/Pe ratio jointly with the onset of Potamogeton and higher values of Myriophyllum suggests that the pond had relatively shallower water and poor nutrient status (Bornette and Puijalon, 2011). The decrease in P/F (Lotter and Bigler, 2000) and the increase in S. oldenburgiana might also be indicative of lower temperatures (Finkelstein and Gajewski, 2008). The decrease in S. venter and A. minutissimum could imply a substantial period of ice cover (Smol, 1988). Thus, the extreme fluctuations of diatom concentrations and the decrease in planktonic frequencies might be associated with hydrological fluctuations, which have also been observed in karstic Montcortès (1027 m a.s.l.) and Estanya Lakes (670 m a.s.l.) (Figure 1), between AD 1400 and 1460 (Morellón et al., 2009; Scussolini et al., 2011). In contrast, Bassa de la Mora lake presented higher lake levels (Pérez-Sanz et al., 2013) (Figure 6). Thus, unstable, cold and humid conditions were inferred for the MCA–LIA transition. In Europe, this period was humid with cold conditions (Pfister et al., 1998), whereas in the IP and the Pyrenees, it was distinguished by fluctuating moist conditions and cold temperatures (Morellón et al., 2011a).

Phase III. 220–177.5 cm, AD 1493–1618

The increase in Abies coinciding with lower montane ratio values, evidenced by the decline of Betula and Alnus, points to a maintained downward shift in the vegetation communities and a temperature decrease. A conspicuous increase in Poaceae suggests a continuity in the openness of the vegetation. The expansion of Olea and Quercus ilex could reflect a high upward flow of regional pollen (Cañellas-Boltà et al., 2009), most likely due to the increasing development of agricultural practices in the lowlands and favoured by the landscape opening (Pérez-Sanz et al., 2013). The drastic decrease in aquatic taxa, the Ce/Pe decrease and the change from brown-dark to brown-red fibrous peat moss sediment suggest a lower rate of decay of material and the presence of the peat bog at the sampling site (Clymo, 1984). The decrease in submerged vegetation restricted the amount of habitat suitable for benthic and epiphytic diatoms (A. minutissimum, A. valida and E. subminuta (Rivera Rondón, 2013)) and caused a decrease in H′. These conditions lead to the replacement of A. minutissimum by a large amount of P. alvareziae, together with S. oldenburgiana and other Fragilarioids, suggesting colder conditions and, possibly, longer periods with ice cover (Lotter and Bigler, 2000). Indeed, a sequence extracted close to PATAM12 coring point also evidenced peat bog presence for the last millennium (Pèlachs et al., 2015). The presence of Aulacoseira, the main planktonic genus found in this study, could also indicate longer ice cover, given its low light requirements and its opportunistic nature (Rühland et al., 2008; Willén, 1991). Redon Lake also evidenced long-lasting ice cover during this period (Catalan et al., 2009). With the thawing of the ice cover, littoral habitats become first available to benthic and periphytic diatoms such as small Fragilarioid species, adapted to cold waters, short growing seasons and prolonged ice cover (Schmidt et al., 2004). This period represents the end of the MCA–LIA transition (AD 1300–1600) characterized by fluctuating, moist conditions and relatively cold temperatures in the IP and Southern Pyrenees (Morellón et al., 2011a). Some lakes, such as Basa de la Mora Lake, increased their water levels (Pérez-Sanz et al., 2013), whereas Redon Lake did not show remarkable shifts in its planktonic percentages, suggesting small changes in water depth (Catalan et al., 2009) (Figure 6). However, Burg Lake increased its Cyperaceae frequencies, while Sparganium decreased, implying shallower waters (Bal et al., 2011; Gacia et al., 2008). Estanya Lake also registered aridity and fluctuating water levels (Riera et al., 2004). Bassa Nera does not show evidence of moist conditions; this could be related to the possibility that local factors obscured any plausible regional relationship between climate and peat bog development (Mäkilä, 1997).

Phase IV. 177.5–90 cm, AD 1618–1823

During this phase, the vegetation was dominated by coniferous forest with some deciduous Quercus and Corylus. The low montane ratio indicates that the montane boundary remained below the altitude of the lake and suggests low temperatures during this period, matching with the second phase of the LIA (AD 1600–1850) (Morellón et al., 2011a). A notable increase in Potentilla, Urtica and Chenopodiaceae/Amaranthaceae suggests an intensification of human disturbance through grazing (Ejarque et al., 2010). The maximum values of Olea and evergreen Quercus imply intensified agricultural practices in the lowlands and an expansion of meadows (Cañellas-Boltà et al., 2009). Nearby lakes (Bal et al., 2011; Pérez-Sanz et al., 2013) and other parts of the Pyrenees also recorded high proportions of Olea at approximately the same time (Reille and Lowe, 1993) (Figure 6). The low macrophyte diversity, combined with higher frequencies of damaged pollen, might indicate periods of aerial exposure.

Low diatom concentrations and less pelagic assemblages seem to indicate shallower waters because of the infilling process and the development of the peat bog in the pond edges. In this regard, it is important to note that the study of a single record adds some uncertainty to the evaluation of a general circumstance of the pond in front of a particular transition on the coring site because the rates of peat growth vary in different parts of the bog according to hydrological, topographical and edaphic factors (Mäkilä, 1997). The high values of Cr/Di suggest poor nutrient conditions (Smol, 1985). Some authors found similarly high proportions of cysts in littoral semi-aquatic mosses, where epiphytic diatoms are restricted (e.g. Duff et al., 1995). However, the stable frequencies of monoletes and triletes do not suggest a significant increase in mosses and ferns. The disappearance of the sponge Ephydatia could be due to a decrease in water temperature below 15°C (Økland and Økland, 1996) or limitations in food availability. The absence of wetland plants and the influx of humic acids from degrading peat-banks enclosing the lake (Pérez-Haase and Ninot Sugrañes, 2006) would have changed the biochemical conditions of the pond, colouring the clay sediment to brown-red and perhaps favouring a decrease in Botryococcus (Demetrescu, 1998). This would have prompted the replacement of planktonic diatoms by S. exiguiformis, A. alpigena and T. flocculosa, which are periphytic forms that can attach to mosses (Krammer and Lange-Bertalot, 2004b).

The second major phase of the LIA was remarkably cold in Europe (Mann et al., 2009), while in the Southern Pyrenees it was characterized by colder temperatures, higher humidity and maximum glacier advances (González Trueba et al., 2008; Morellón et al., 2011a). The Bassa Nera record does not evidence increasing moisture, as also occurred in nearby basins as Basa de la Mora Lake and Perafita Valley (Bal et al., 2011; Miras et al., 2010). However, other lakes in the pre-Pyrenees such as Estanya show periods of large hydrological fluctuations (Morellón et al., 2011b; Riera et al., 2004) (Figure 6). On the other hand, the peat accumulation in Bassa Nera could have been favoured by the cold conditions (Martinez-Cortizas et al., 1999).

Phase V. 90–0 cm, 1823 to present

At the onset of this phase, the montane ratio remains low, suggesting the downward continuity of the montane vegetation belt boundary and low temperatures until the past century, when it increases again. A slight decrease in Abies at approximately AD 1921 and the increase in deciduous elements suggest a recent dominance of mixed forest. The higher proportion of herbs during the past century indicates an open landscape around the catchment. These non-forested areas are prone to be eroded and may produce higher sediment input to the pond, explaining the switch from clay to sandy silt during this period. The charcoal abundance suggests two periods of frequent regional and local fires. The increase in Poaceae, Pinus and Corylus at approximately AD 1915 could be due to their ability to reappear after fires.

The type and acidophilus character of diatoms as well as the significant presence of chrysophytes indicate the continuity of shallow waters and the expansion of periphytic habitats (Douglas and Smol, 1995). A minor shift in diatom assemblage during the past century, marked by an initial decline in S. venter and P. alvareziae with a strong peak in the periphytic A. minutissimum and S. pinnata, reflects an increase in subaquatic vegetation and greater nutrient-rich conditions (Van Dam et al., 1994). This phase includes the end of the LIA in the IP (Morellón et al., 2011a) and the onset of a warmer and more arid period, coinciding with the IR and the CGW (Seager et al., 2007).

Comparison with other peat bogs

The Bassa Nera record shows some peculiarities in aquatic and vegetal trends compared with those found by other studies in the nearby region (Bal et al., 2011; Cunill et al., 2013; Pérez-Obiol et al., 2012). Some of these unique features might be due to differences in ecosystem sensitivity to climate between peat bogs and lakes. Our results agree with those from other peat bogs that show analogous conditions and are more geographically distant. In the IP, the distribution of peatlands is mainly in the northern areas, within the Eurosiberian bioclimatic region (Hernández-Beloqui et al., 2015; López-Moreno et al., 2010; Martınez-Cortizas et al., 2001; Pérez-Díaz et al., 2016; Pérez-Díaz and López-Sáez, 2014). Some Northwestern peat bogs also recorded wet periods between AD 1110–1210 and AD 1345–1475 (Mighall et al., 2006) with several rapid and brief dry episodes between AD 1200–1300, AD 1400–1450 and AD 1600–1700 (Castro et al., 2015) and higher intra-annual fluctuations after the mid-16th century (Silva-Sánchez et al., 2016) that could match with the hydrological fluctuations of Bassa Nera. In the Alps, the Mauntschas mire showed a change in hydrological conditions from AD 1572, favouring peat bog development and decreasing the water level (Van der Knaap et al., 2011). Similarly, in northern Poland, the Kusowskie Bagno bog presented water table fluctuations from AD 1150, increasing abruptly at approximately AD 1240 and then decreasing by AD 1500 (Lamentowicz et al., 2015). Carpathian peatlands also showed a substantial increase in the water table after AD 1400 and a marked change to drier conditions after AD 1580 (Schnitchen et al., 2006). Most of these peat bogs responded to the MCA with water level fluctuations, and all of them recorded an abrupt change to drier conditions with the onset of the LIA. The ecological response and sensitivity revealed by these palaeoenvironmental records provide insights into the nature and timing of the response that we may expect the CGW to trigger. Note that peat bogs are known to impact the global water cycle because of their water-retention properties (Moore, 2002). This capacity might be important in buffering the effects of precipitation decrease due to global warming expected for the Mediterranean region (Giorgi and Lionello, 2008). Therefore, a surveillance network along latitudinal gradients of peatlands would act as a sentinel and help to apply the appropriate measures of conservation and management.

Lag between pollen and diatom responses

Figure 7 displays the trends of montane and P/F ratios representing 35 samples taken at the same core depth. For this purpose, we considered only local vegetation, since the presence of the genera used in montane ratio imply the nearby occurrence of the pollen source. Our results show a possible faster response of diatoms than the local vegetation to the global climatic signal in all the climatic periods studied in this work, with a lag of several years to decades, coinciding with Scussolini et al. (2011). These trends, however, must be interpreted with caution because of the relative large fluctuations of both ratios and the likely influence of anthropogenic disturbances. The most coincident trends occur during MCA and IR, suggesting more similar response times between the two proxies during warm conditions. However, during the IR, the vegetation seemed to respond more intensely, while diatoms had a weaker response, most likely explained by the impoverished tychoplanktonic assemblage resulting from peat bog infilling. During the LIA, the response of these proxies clearly differs. At the species level, the peak in P. alvareziae (Figure 4) is interpreted as a manifestation of LIA cooling. Since then, the aquatic organisms show a decreasing trend during the rest of the sequence. In contrast, Abies peaks shortly before the Fragilariaceae, while Pinus and Prunus decline (Figure 3), and the vegetation strongly fluctuates thereafter, showing range expansions and contractions.

Figure 7.

Lag between pollen and diatom responses to the same climatic pressure defined in the MCA, LIA and IR periods.

Human impact

The presence of human-related species along the whole diagram indicates that the Aiguamòg Valley has been subjected to low-intensity anthropogenic pressure throughout the studied period. The increase in Artemisia (AD 990) and Cerealia-t (AD 1144), with the high frequency of local and regional charcoal, suggests the use of fire to maintain open spaces for cultivation during the MCA (Bal et al., 2011; Pérez-Obiol et al., 2012) (Figure 6). Later, the increase in S. cereale and ruderals (Plantago and Potentilla) reflect an increase in farming activity and local grazing in the first stages of the MCA–LIA transition (AD 1300–1600), which might have increased lake turbidity. Other studies performed in adjacent regions have also recorded extensive S. cereale cultivation during this period (Cunill et al., 2013; Miras et al., 2010; Pérez-Obiol et al., 2012). However, these results contrast with nearby pre-Pyrenees regions, where wars and the devastating ‘black death’ epidemic (AD 1347-1353) prompted the abandonment of lands and crops (Rull et al., 2011). In contrast, Bassa Nera increased and shows diversified crops between AD 1300 and 1500. This finding could be due to the significant migration from towns to farmland after the large epidemic, together with the Querimonia (Gómez, 2007), a privilege document signed by King Jaume II in AD 1313 that granted the ownership of mountain lands to Aranese institutions and allowed their free use for grazing and farming. From a geographical point of view, the harsh winters and poor communication across the mountain passes of Viella and Bonaigua that surround the region favoured the geographic isolation of the Aran Valley (Boya-Busquet and Cerarols-Ramirez, 2015), affording protection against disease outbreaks and favouring regional activities. By the end of the MCA–LIA transition, the disappearance of S. cereale at AD 1500 suggests that crops suddenly ceased to occur, with only some grazing evidence remaining (Potentilla and Urtica), indicating that people abandoned the farming of the high ranges to develop farming in the lowlands; only the livestock persisted because of the increasingly colder conditions. The low charcoal values in our record between AD 1583 and 1736 indicate a low frequency of fires in the area. However, the short-term decreases in Pinus and the increase in Artemisia and Potentilla could indicate periods of forest clearance and increases in grazing lands at higher altitudes, as also observed in nearby Basa de la Mora Lake sequence (Pérez-Sanz et al., 2013). This pattern contrasts with nearby high-mountain valleys that experienced higher pastoral pressure and the use of fires for forest clearance or metallurgical activities (Catalan et al., 2013; Ejarque, 2009; Pèlachs et al., 2009), highlighting the differences in land use management between regions. By the mid-19th century, the frequent fires, forest clearance and the Poaceae peak might be the result of an increased need for supplies and raw materials during the IR (Ferrer i Alòs, 2012). The social and economic changes during the mid-20th century forced migration from the Pyrenees to cities. Therefore, the abandonment of rural lands and the establishment of Aiguestortes i Estany de Sant Maurici National Park in AD 1955 and the protection of its surroundings in AD 1990 favoured the expansion of arboreal taxa, such as Pinus (Améztegui et al., 2010; Bal et al., 2011). As in Bassa Nera, the abandonment of traditional land uses in the Pyrenees and many Mediterranean mountains has led to recolonization of deforested areas, shrub encroachment and densification of treelines (MacDonald et al., 2000). This afforestation reduced the grassland extensions and landscape diversity, increased water consumption and evapotranspiration and produced marked alterations in hydrological responses (Barrio et al., 2013; López-Moreno et al., 2010). These large amounts of accumulated deciduous biomass and the expected concomitant increase in temperatures and drought events (IPCC, 2007) might cause a shift to flammable material and trigger fire frequency and large-scale fire hazards, as occurred in the mid-Holocene (Gil-Romera et al., 2014; Lasheras-Alvarez et al., 2013). Applying strategies that minimize the impact of CGW to biodiversity may become essential, such as traditional grazing activities and associated management practices with ecological forest management (Ninot et al., 2008). Our palaeoecological results are in agreement with Pérez-Sanz et al. (2011), indicating that climate changes have not only influenced environmental evolution during recent times but might also have modulated the degree of human pressure in the high ranges.

Expected future mountain scenarios under CGW

In this study, the observed increase in montane vegetation by the mid-20th century points to an ongoing upward range extension in species distribution in the Aiguamòg region because of the CGW, which would possibly match with the rapid response of alpine Pyrenean treelines to climate registered by Camarero et al. (2015). Other studies have also demonstrated recent vegetation shifts in high-mountain ecosystems (Gottfried et al., 2012; Thuiller et al., 2005). From approximately 1940 to 1968, the montane ratio increased from 0.57 to 1.6. In this period, the mean temperature in the Pyrenees increased at an average rate of +0.3°C per decade (López-Moreno et al., 2010) and raised tree establishment and density within the treeline ecotone (Camarero and Gutiérrez, 2004). Considering that the predicted temperature increase for mountain regions by 2030 is 0.5–1.5°C (IPCC, 2013) and 1.5–2°C by 2021–2050 in the Pyrenees (IPCC, 2007), we could expect an acceleration of the upward shift of the montane belt in the Pyrenean region in a short period of time. This, together with a likely decrease in snow accumulation and reinforced Mediterranean summer droughts (IPCC, 2007; López-Moreno et al., 2009), could significantly reduce the available area for subalpine and alpine ecosystems in the Mediterranean mountains.

Conclusion

The multi-proxy approach used in this work has helped to produce a detailed and comprehensive picture of the main events that occurred during the last millennium in the surroundings of the Bassa Nera as an example of a temperate high-mountain environment. This study shows that the vegetation of the Bassa Nera catchment responded strongly to climate with altitudinal shifts and is most likely currently responding to the CGW. From the MCA to the MCA–LIA transition, the montane–subalpine ecotone reached the Bassa Nera catchment. We might expect that with the current temperature projections for 2100, this ecotone will eventually reach the Bassa Nera again. This scenario could be extrapolated to other high-mountain environments of the Mediterranean region.

To the best of our knowledge, this study is the first attempt to link a pollen ratio to past altitudinal shifts in the montane–subalpine ecotone. The montane ratio has proven the usefulness of good pollen indicators for revealing vegetation trends, providing a suitable tool for palaeoecological studies and for monitoring regional changes in natural communities in response to CGW. This pollen analysis is site-specific, and the application of this ratio for interpreting different biogeographic locations should be adapted by including local species with similar ecological characteristics. The use of the montane ratio in highland peat bog ecosystems would help with early detection of the replacement of vegetation predicted by IPCC (2007).

Additionally, human management of natural resources has changed over the past millennium. Through the MCA and MCA–LIA transition, the people of the region used fires to open the forests for cultivating and grazing. With the LIA cooling, grazing was the main form of resource exploitation. During the IR, some farming activities were still conducted until the authorities restricted resource exploitation by creating the National Park.

Aquatic taxa, diatom communities and sedimentary units allowed to describe the peat bog development at the coring site and its infilling at approximately AD 1565. During the past millennium, the small Fragilarioid species dominated the community. These opportunistic species had a particularly higher incidence during peat bog development in the LIA period, seizing an advantage when the diatom diversity diminished due to unfavourable and cold conditions.

Consistent shifts in vegetation, fire activity and aquatic communities throughout the sequence are clearly related to climatic signals such as the MCA and LIA phases. Although the studied lakes nearby such as Basa de la Mora, Burg and others located in the pre-Pyrenees (Pérez-Sanz et al., 2013; Riera et al., 2004; Scussolini et al., 2011) had shallow waters during the arid conditions of the MCA, the Bassa Nera might have maintained or increased its water levels. This finding underlines the contrasting responses of lakes and peat bogs to similar climatic pressures.

In this study, we have observed the past biotic responses to climate changes and considered possible future responses under the scenarios of the CGW. Because high spatial resolution has shown that the forecasted climatic changes will not be uniform throughout regional areas (Barrera-Escoda and Cunillera, 2011), the information provided by this study will help to better understand spatial variability in the impacts of climate change on high-mountain ranges.

This study has been based on a single record recovered from the shore of Bassa Nera and mainly reflects the evolution of the sedimentological, ontogenic and palaeoecological processes occurred in the littoral zone in order to improve the obtained palaeoecological and palaeoenvironmental reconstructions and inferences, and it would be interesting to investigate other depositional environments like the pelagic zone of the pond and other parts of the peat bog and catchment.

Footnotes

Acknowledgements

Otto Huber, Arantzazu Lara, Encarni Montoya and Sandra Nogué carried out the fieldwork sampling and coring. The authors acknowledge Iosune Uriz for the Ephydatia taxonomic determination. Special thanks to Owen S. Wangensteen for reviewing the English language and helping with the montane ratio. Santiago Giralt lent support with the age–depth model construction and Joan Gomà with diatom identification. Two anonymous reviewers improved a former version of this manuscript with their constructive comments.

Funding

This research has been funded by the project PIRIMOD, granted to Valentí Rull by the ‘Institute for Catalan Studies’ (IEC).

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