History of fires and vegetation since the Neolithic in the Cantabrian Mountains (Spain)

Fire has been one of the main causes of disturbance of vegetation over time, and since the Neolithic has become an irreplaceable tool for the opening of forest spaces and maintenance of pastures. Previous studies showed that the intensity and effects of wildfires are related to the biomass and controlled by climate factors. However, in regions such as Cantabria, where agriculture and livestock have spread throughout the territory since prehistory, fires should also be closely related to human land uses. The aim of this paper was to investigate the history of fires and vegetation since the Neolithic in the Cantabrian Mountains, using sedimentary charcoal and pollen data to study the role of human activities in the processes that have shaped ecosystems throughout the Holocene. The asynchrony and quantitative differences in the results obtained at different sites indicate significant variations in fire patterns at regional scale since the Neolithic, although the type and size of each basin also had a strong influence on charcoal accumulation. Maximum values for charcoal accumulation rate at La Molina were observed between the Neolithic and the Bronze Age but occurred after about 3500 cal years BP at El Cueto de la Avellanosa. At El Sertal, low charcoal accumulation rate values were observed, probably because the sequence begins in a space that already had been cleared; the maximum values occurred during the most recent millennium. These data provide evidence that fire has been a key factor in forest retreat and in maintaining open landscapes since the Neolithic.


| Fire as an ecological factor
In recent decades, research in ecosystems throughout the world has shown sedimentary charcoals to be a useful proxy helping to explain the role of fire as a key ecological factor (Beffa et al., 2015;Feurdean et al., 2012;Power et al., 2008;Whitlock & Larsen, 2001). The impact of fire on the landscape during the Last Glacial and since the beginning of the Holocene has been analyzed in relation to climatic episodes and to the availability of biomass in various parts of the world, including central and southern Europe (Carcaillet, Hörnberg, & Zackrisson, 2012;Gil-Romera et al., 2014;López Sáez et al., 2017;Rius, Galop, Doyen, Millet, & Vannière, 2014).
Fire signal and pollen data facilitate an understanding of historical disturbances in the landscape (Burjachs & Expósito, 2015; González-Analysis of the deposition of sedimentary charcoal particles provides information on fuel availability, the extension of a historical fire, and the estimated distance between the fire and the coring site (Ali et al., 2012;Higuera, Gavin, Bartlein, & Hallett, 2011). The fire regime can be inferred from the total number of charcoal particles per unit of sediment, which is proportional to the total biomass burned in a given site's surroundings (Marlon, Bartlein, & Whitlock, 2006;Thevenon, Bard, Williamson, & Beaufort, 2004). Thus, sedimentary charcoals (≥150 μm) have been used in the study of long-burning fire events (local scale fire, 500 m-1 km) in mountain zones (Clark, 1988). Fires in European mountain zones have been studied in the Alps (Carcaillet et al., 2007), Pyrenees Rius, Vannière, Galop, & Richard, 2011), and the Iberian Central System (García Álvarez, Bal, Allée, García-Amorena, & Rubiales, 2017;López Sáez et al., 2017). Considering the available evidence since the Neolithic period, fire appears to be a great ally for the opening of forest spaces and the maintenance of open landscapes (Cunill, Soriano, Bal, Pèlachs, & Pérez-Obiol, 2012;Feurdean et al., 2012;Gil-Romera et al., 2014;; García-Amorena, Morla, , a practice that survives in Cantabria to the present day (Carracedo, 2015).
In Cantabria, only one study of historical fires has been based on sedimentary charcoals (Pérez-Obiol, García-Codron, Pèlachs, Pérez-Haase, & Soriano, 2016); it points out the importance of fire since Neolithic times in one of the Iberian regions most affected by fires in forests and pasture lands. The region also has some of the richest and best known prehistoric archaeology in Europe (González Morales & Estévez, 2004;Moure Romanillo, 1995).
At present in Cantabria, fires are generally intentional and mainly burn scrub (Carracedo, 2015); previously, tree biomass was involved but controlled by climate factors. For example, Bond cycles (Bond et al., 2001) associated with quasi-periodic climate fluctuations have sometimes coincided with cooling periods or dry periods (Burjachs & Expósito, 2015); in Cantabria, dry climatic phases coincided with Bond Cycles three and four (Pérez-Obiol et al., 2016), which could favor greater recurrence of fires.
Although it has been assumed that fire synchronicity is linked to climate, there may be a correlation with a human factor linked to land use. The type of landscape at each site and the role of biomass in fire history can be assessed by pollen analysis. The main objective of this study was to analyze the history of fires and vegetation since the Neolithic in the Cantabrian Mountains using sedimentary charcoals and pollen data in core samples obtained from three peat bogs (La Molina, El Sertal, and el Cueto de la Avellanosa).
In order to make reliable comparisons, the synchronicity of fire episodes and the observed differences associated with altitude, as well as topographic factors and human activities, were included in the analysis.

| Study area
The peat bogs analyzed are located in the central-western sector of Cantabria, in northern Spain ( Figure 1). The Cantabrian Mountains are an obstacle to the prevailing winds of maritime origin and give rise to a strong climatic dissymmetry. The northern slope has a temperate and rainy oceanic climate with seasonally distributed annual precipitation totals ranging from 1,000 to 1,600 mm (Cfb, in the Köppen-Geiger classification), whereas the southern slope shows continental Mediterranean traits (Csb) with cold winters and a water deficit in the summer. The highest sectors have a narrow, intercalated strip of mountainous climate (Dfc/Dsb/Dsc), with annual precipitation -occurring in the form of snow for much of the year-that may exceed 2,000 mm (AeMet, 2011 is located on a small mountain range, at 940 m a.s.l., near Peña Sagra. The peat bog is on the top of a hill, in an elevated, well-ventilated area without lateral protections (Mariscal Alvarez, 1986). Communities of Nardus stricta dominate locally, and nearby are moist Atlantic heaths with E. ciliaris and E. tetralix and some remains of Q. pyrenaica and Q. robur. El Cueto de la Avellanosa (43°06′50″ N-4°21′52″ W) also lies in a mountain pass, located at 1,320 m a.s.l. on the northern slope of the Cordel range. It rests on a substrate of impermeable sandstone, forming weak aqueous layers in which Sphagnum grows (Mariscal, 1983). At present, the communities of Nardus stricta and oro-Mediterranean and dry European heaths dominate, whereas on the slopes, there are forests of F. sylvatica and B. pubescens.

| Archaeological context
The archaeological heritage of Cantabria is exceptionally rich and provides us with abundant information on the ways of life and the use of the territory in the region throughout the period between the Late Palaeolithic (Late Pleistocene s.l.) and the end of Prehistory (Figure 1).
Most of the remains dating to the Palaeolithic have been found in low valleys and coastal areas, but this situation changes in the Neolithic, throughout the sixth millennium BP, with the appearance of the first groups of herders and farmers who quickly colonized most of the regional territory (see Table 1). Subsequently, an economy based on cattle ranching and the exploitation of mountain areas was consolidated, leading to an effective system of spatial and productive organization that is considered the direct antecedent of the one that has survived until the contemporary period (Ortega, 1987). Knowledge about this historical process of cultural evolution and colonization of the territory provides a large amount of proxy data of great value for understanding the results obtained in relation to the use of fire and the evolution of the vegetation in the region.
Copper and tin deposits are very scarce in Cantabria; the region was relatively isolated from the new cultural currents, and no important economic or social changes occurred during the Bronze Age (de Blas Cortina & Fernández, 1992). However, we know that during this period (ca. 3800-2700 cal years BP), peasant societies continued their consolidation and agricultural and livestock farming activities experienced strong growth, which encouraged the occupation of new areas (Arias, 1999).
Archaeology of the Iron Age is also very limited in this region (Arias, 1999;de Blas Cortina & Fernández, 1992). There is evidence of large deforested areas at mid-mountain summits. The population was distributed throughout the region in small fortified nuclei situated in easy-to-defend positions, such as hydrographic boundaries or high points that allowed a good control of the territory. This settlement system was likely the most common until Roman times (Cisneros Cunchillos, Marco Simón, Pina Polo, & Ramírez Sádaba, 2008;Peralta, 2000).  respectively. At El Cueto de la Avellanosa, it was possible to take advantage of the vertical cut that remained accessible after exploitation of the peat bog was abandoned to a depth of 406 cm. Cores from La Molina and El Sertal were cut into 302 and 57 samples, respectively, below the centimeter. At El Cueto de la Avellanosa, the core was sampled every 7 cm for 58 samples.
The chronology of this study was based on 11 AMS 14 C dates (Beta Analytic Inc.) (Table 2A-C), calibrated at 2σ from the INTCAL13 database (Reimer et al., 2013) using Calib Rev. 7.0.4. At La Molina, the dating did not respond to parametric adjustment. Therefore, an agedepth model was developed using Clam modeling software (Blaauw, 2010) with the R statistical software platform (R Core Team, 2015).
Models for El Sertal and El Cueto de la Avellanosa underwent polynomial adjustment to obtain age-depth models ( Figure 3).

| Macroscopic charcoal analysis and organic matter content
All samples from each site were used for the analysis of sedimentary charcoal and organic matter, for 302 from La Molina, 57 from El Sertal, and 58 from El Cueto de la Avellanosa. To identify fire events, the number of charcoals ≥150 μm was estimated (Carcaillet, Bouvier, Fréchette, Larouche, & Richard, 2001) in the Physical Geography laboratory of the Geography Department at the Universitat Autònoma de Barcelona.
This charcoal particles approach does not allow taxonomic identification but does establish the relative magnitude of fire events. The standard protocol (Carcaillet et al., 2001(Carcaillet et al., , 2007, as adjusted by Pérez-Obiol et al. (2016), was followed. KOH was used as a deflocculant solution and 15% NaClH as a bleaching solution (Finsinger, Kelly, Fevre, & Magyari, 2014). Samples were heated to 70°C for 90 min and sieved with a 150-μm mesh. The number of macroscopic carbon particles was estimated under a stereomicroscope at 40×. The surface area of charcoal fragments was estimated using an ocular grid with 100 squares of 0.0625 mm 2 each. Charcoal fragments were classified into exponential size-classes according to their area (Carcaillet et al., 2007). The sedimentary charcoals concentration (CHAC, mm 2 g −1 ) was used to obtain Ortega (1987), Ceballos (2001) the charcoals accumulation rate (CHAR, mm 2 g −1 year −1 ), based on the sedimentation rate estimated by age-depth models for each site Loss on ignition analysis was carried out in the Physical Geography Laboratory of the Geography Department at the Universitat Autònoma de Barcelona, modifying the standard procedures to achieve combustion at 550°C for 4 hr (Dean, 1974;Heiri, Lotter, & Lemcke, 2001).

| Palynology
Information from La Molina (Pérez-Obiol et al., 2016) and El Cueto de la Avellanosa (Mariscal, 1983) allowed us to obtain a complete and comparative view of vegetation changes. The pollen diagram was FIGURE 3 Age-depth models with location of the radiocarbon samples, values for organic matter, and description of sediment. The main sedimentology of La Molina is peat. At El Sertal, there are three main sediment units (peat, organic silt, and silt). At Cueto de la Avellanosa, the peat lies above the lithological substrate (comprising Permian lutites and sandstones with large blocks of siliceous conglomerates of the same age).
Note: 14 C dates for these sequences are shown in Table 2  under an optical microscope using reference collections and codes (Reille, 1992(Reille, , 1998. The results were expressed in relative percentages, excluding aquatic plants and spores from the total sum. The pollen diagram was constructed using TILIA and TILIAGRAPH software (Grimm, 1991). The most intense fires coincide with the highest arboreal pollen values. Significant fire events were identified from the very base to the end of the sequence; the highest frequency and intensity occurred between 5800 and 3500 cal years BP, when there was also a dense forest cover The pollen values at El Cueto de la Avellanosa have been redrawn based on Mariscal (1983). The fire events detected at El Sertal and El Cueto de la Avellanosa appear to follow a different pattern. At El Cueto de la Avellanosa, CHAR increased ca. 3600 cal years BP, and the maximum is seen at ca. 2650 years cal BP, coinciding with Bond Cycle 2. At El Sertal, the highest CHAR values were found during the most recent millennium, when there has been a greater local and regional anthropic pressure A comparison of all the sequences ( Figure 6) showed a fire pattern that was differentiated but coincided with the main points when changes in trends were observed. This framework of trends was established between 3000 and 4200 cal years BP.

| Pollen analysis
The pollen diagram for El Sertal (Figure 7) indicates the main vegetation changes since Neolithic times. The arboreal pollen curve showed a decreasing trend throughout the entire sequence.
From 6700 to 4600 cal years BP, despite the highest values for arboreal pollen (mean value of 74%) and a large Corylus presence, high percentages of Poaceae (up to 32%) and Pteridium, as well as the presence of cereals, provided evidence of Neolithic human impact. Pine The quantitative differences between the three sites may be due to several factors: basin type and size, patterns and rate of deposition, number of woody plants burned, and anthropic factors (Rius et al., 2011). In this study, basin type seems to have a lot of weight. Macroscopic charcoal is usually not carried more than a few hundred meters and, therefore, is mainly a local fire proxy (Clark, 1988;Higuera, Peters, Brubaker, & Gavin, 2007;Ohlson & Tryterud, 2000;Whitlock & Larsen, 2001). The concave La Molina topography could have contributed to the quantitative results of charcoal particle deposits, both locally and on the slopes that form the basin. However, El Sertal and El Cueto de la Avellanosa occupy two hills and have almost no basin to receive particles from nearby slopes. This difference shows the sensitivity of the sedimentary records to the morphology of each site, along with the importance of considering likely potential biases (Higuera, Sprugel, & Brubaker, 2005).   (Mercuri, 2014;Rius et al., 2009;Vannière, Power, Roberts, Tinner, & Carrión, 2011).

| Fire regime
Nevertheless, other studies found human impact on fire regime and frequencies since the Neolithic (8000 cal BP; Vannière et al., 2008).
Thus, the existence of dissimilar chronological patterns between the three peat bogs makes it difficult to interpret the data based solely on climate factors. However, the Cantabrian Region has been characterized by the predominance of broad-leaved forests since the beginning of the Holocene and by a large and sustained human presence since the Late Glacial period, which suggests that human control has been a major factor in the region's fires since the Neolithic.  during the most recent millennium, when there has been a greater local and regional anthropic pressure.
From 3000 to 2800 cal years BP, the percentage of Ericaceae pollen increased and arboreal pollen decreased simultaneously, a pattern very similar to El Cueto de la Avellanosa (Mariscal, 1983). At this site, the increase in sedimentary charcoals was more coincident with indicators of the opening of the landscape, which suggests either that the fires were closer to the site or that its basin better reflects the events. Mariscal (1983) Galop, Carozza, Marembert, & Bal, 2007;Gassiot et al., 2014;Pèlachs et al., 2011). 4.3 | Fire and landscape dynamics It is important to keep in mind that fire linked to agricultural activities has had great influence on the configuration of the environment intensify continuously between the Bronze Age and the Roman period (Arias, 1999

| CONCLUSIONS
The comparisons between the three sequences provide evidence that fire has been a key factor in the retreat of the forest and subsequently in the maintenance of open landscapes since the Neolithic. Over the past 7000 years, the intensity and chronology of fire events was unequal at the three sites studied, which can be interpreted as the result of asynchronous human activity in the high and low mountain areas and, at the same time, of the type of fuel involved in each zone.
The patterns of fires have varied significantly on a regional scale since the Neolithic period. In La Molina, the maximum values occurred between 5800 and 3500 cal years BP, whereas in El Cueto de la Avellanosa, they were observed between 3600 and 1700 cal years BP. At El Sertal, the fire events occurred mainly during the past millennium, although open spaces are evident in the record from the very base of the sequence.
The phases of decline in organic matter, which could reflect more erosive periods, do not seem to be directly related to the rates of sedimentary charcoal accumulation. The catchment area at La Molina quantitatively favored the deposition of local charcoal as well as that from surrounding hillsides. At El Sertal and El Cueto de la Avellanosa, the peat bogs are located on hills and provide a much more local reflection of fire events.
At both El Sertal and El Cueto de la Avellanosa, the increase in Ericaceae around 3000 cal years BP is synchronous with the reduction in arboreal cover. At El Cueto de la Avellanosa, this landscape transformation coincides with an increase in sedimentary charcoals, defining a model of anthropic pressure intensification related to the use of fire.
Even though much of the Neolithic control of Cantabrian fires was basically human, we cannot rule out synergies with the climate and its influence in the processes of human adaptation to mountain areas.