Fire–vegetation interactions during the Mesolithic ... - CiteSeerX

The Holocene 18,5 (2008) pp. 679–692

Fire–vegetation interactions during the Mesolithic–Neolithic transition at Lago dell’Accesa, Tuscany, Italy Daniele Colombaroli,1,2* Boris Vannière,3 Chapron Emmanuel,4 Michel Magny3 and Willy Tinner1,5 (1Paleoecology, Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland; 2Environmental Change Research, Department of Geography, University of Oregon, Eugene OR 97403-1251, USA; 3LCE – UMR 6565 CNRS – Univ. F-Comté, 16 route de Gray, F-25030 Besançon cedex, France; 4Geological Institute, Department of Earth Sciences, ETH Zürich, Universitätsstrasse 16, CH-8092 Zurich, Switzerland; 5 Institute of Terrestrial Ecosystems, Department of Environmental Sciences, ETH Zurich, Universitätsstrasse 16, CH-8092 Zurich, Switzerland) Received 9 May 2007; revised manuscript accepted 17 December 2007

Abstract: A new core from the centre of Lago dell’Accesa (Tuscany, Italy) was sampled for pollen and charcoal analyses to provide a high-resolution sequence from 8400 to 7000 cal. yr BP. We combined series of microscopic charcoal, macroscopic charcoal and pollen to address the response of vegetation to fire at different spatial scales. Before 7900 cal. yr BP, broadleaved evergreen forests of Quercus ilex were the most important vegetational type in the area of Lago dell’Accesa. The subsequent decline of Q. ilex occurred when humaninduced fires increased at the Mesolithic/Neolithic transition (c. 8000 cal. yr BP). Cross-correlation analyses show that fire was a key factor for vegetational change. Higher fire incidence affected the forest composition, converting evergreen forests to high-diversity open, partly deciduous forests and shrubby communities. The correlation is more pronounced at a local scale (macroscopic charcoal), whereas at a regional scale (microscopic charcoal) the vegetation followed the fire intervals with a more marked time lag (10–100 years). Climatic change, such as wetter periods inferred from lake levels, may have directly influenced the vegetational change, exacerbating the effect of human impact. Our study suggests that the disruption of evergreen broadleaved forests occurred when mean fire interval reached values as high as those of today’s highly disturbed Mediterranean ecosystems. Hence broadleaved evergreen forests may not be as fire-resilient as assumed according to modern ecological paradigms. In view of the projected increase in fire frequency as a consequence of global warming, the present relict forests of Quercus ilex will be strongly affected. Key words: Fire ecology, fire history, Mediterranean vegetation, Quercus ilex, climate change, microscopic charcoal, macroscopic charcoal, Mesolithic, Neolithic, Tuscany, Italy, Holocene.

Introduction Modern Mediterranean ecosystems are mostly the result of longterm human activity through the centuries (Solari and Vernet, 1992; Bacilieri et al., 1994; Butzer, 2005; Pausas, 2006), in which human-induced alterations of the fire regime play an important role (Moreno, 1998; Pausas, 2006; Viedma et al., 2006). Thus, it has been hypothesized that without such disturbances natural *Author for correspondence (e-mail: [email protected])

© 2008 SAGE Publications

Mediterranean forests, which are presently fragmented, would be more widespread (Barbero et al., 1990; Roberts, 1998). The effects of post-fire vegetation succession last for decades and centuries and thus cannot be exhaustively studied by conventional ecological approaches (Oldfield and Alverson, 2003; Bradley et al., 2003; Overpeck et al., 2003). Fire–vegetation history studies by means of charcoal and pollen data can overcome this problem and contribute to a better understanding of the long-term impact of fire on Mediterranean ecosystems, including both biotic and non-biotic processes (De Luis et al., 2003; Pardini et al., 2004; Bisson et al.,

10.1177/0959683608091779

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The Holocene 18,5 (2008)

Figure 1 (A) Location map of Lago dell’Accesa. (B) Bathymetry and sedimentary environments deduced from seismic reflection profiles (seismic grid is shown by white lines). The location of core AC05-B was selected within undisturbed sediments. (C) Core AC05-B lithology and gamma density. The core consisted of six sedimentary facies. F1, laminated calcareous and organic silty clays. F2, faintly laminated calcareous silts. F3, laminated organic and calcareous clayey silts. F4, faintly laminated calcareous clayey silts. F5, laminated fine calcareous silts. F6, laminated fine calcareous and organic silts

2005; Malak and Pausas, 2006; Shakesby and Doerr, 2006). Only a few studies have attempted to reconstruct palaeofire frequency and its effects on vegetation in the Mediterranean region and elsewhere (Carcaillet et al., 1997; Carrion and Van Geel, 1999). In the present study, we use high-resolution pollen and charcoal records to study the vegetation response to fire at Lago dell’Accesa, a lake located in Mediterranean Italy. We combine contiguous pollen, microscopic and macroscopic charcoal, time series analyses to reconstruct vegetation response to fire at different spatial scales during the Mesolithic/Neolithic transition. To test whether humans affected past fire regimes, we use pollen types indicative of anthropogenic activity (Behre, 1981) and we compare sedimentological evidence of climatic change (Magny et al., 2006) with pollen and charcoal records to address potential linkages between climate, vegetational composition and fire regimes (Keeley and Fotheringham, 2001).

Materials and methods The study area Lago dell’Accesa (latitude 42°59′N, longitude 10°53′E) is a karstic lake situated at 157 m a.s.l. in the southern part of Tuscany, 10 km from the Tyrrhenian Sea (Figure 1A). The area around the basin falls between geological formations of the ‘Serie Toscana’, including Permian and Eocene schists and Triassic limestone (Calcare Cavernoso e Verrucano; Merciai, 1933). It is surrounded by hills up to 300 m high (Colline Metallifere), covered by mixed evergreen/deciduous vegetation. Lago dell’Accesa has a subcircular shape (580 m × 400 m), a total surface of 16 ha and a depth of 37.5 m on a relatively flat floor surrounded by steep littoral slopes (Figure 1B). This karstic lake is fed by subaerial and subaquatic springs (‘L’Inferno’ and ‘Il Paradiso’, Figure 1B) situated on its western shore. The Bruna River in the east is the only outlet. It was dug in recent times to lower the lake level and thus extend arable land. The vegetation around the lake is ascribed to the Phragmitetum communis Koch 1926 and Holoschoenetum romani Br. Bl. 1931 by Rizzotto (1981). On the basis of a fuel-model map (Lopez et al., 2002), vegetation around Accesa is ascribed to perennial grass-

land, transitional woodland shrubs and broadleaved forests. The climate is Mediterranean, with precipitation around 740 mm/yr concentrated in October and November and scarce in summer. The mean annual temperature is 14°C, and the mean July and January temperatures are 22°C and 4°C, respectively. Close to the lake, an Etruscan village existed during the seventh century BC (Lobell, 2002) and the area was for centuries exploited for extensive mining (Serrabotini area).

Coring Short and long sediment cores were retrieved from the basin floor of Lago dell’Accesa with a gravity corer and a 3 m long UWITEC piston coring device operated from a barge. Coring sites were selected on the basis of a high-resolution seismic survey, which showed that the extension of littoral slopes resulted from slope failure events locally disturbing the basin floor (Figure 1B). For the present study, we refer to core AC05-B (Figure 1C), situated at a water depth of 35 m in the western border of the basin, where seismic profiles revealed undisturbed sediments (Matter, 2005). Sediment gamma density (Figure 1C) was measured every 5 mm with a GEOTEK multisensor for physical characterization of the sediment.

Radiocarbon dates and depth–age model For AMS (Accelerated Mass Spectrometry) 14C dating, a total of 70 sediment samples (volume c. 20 cm³; mean thickness of 5 cm) were sieved through a 100 µm mesh screen. Macrofossil analyses provided 12 terrestrial plant macrofossils for AMS dating (Table 1). The resulting 14C-dates were calibrated with the program Calib 5.0.2 (Reimer et al., 2004). Only one radiocarbon date was rejected (Poz-16244) on the basis of a reliabilityof-stratigraphy test, as implemented in the program Oxcal 3.10 (Ramsey, 2001). A model based on cubic splines (Telford et al., 2004) was used for the chronology. The near-optimal number of degrees-of-freedom was chosen to keep the final chronology (Figure 2) within the confidence interval of the generalized mixed-effect regression as proposed by Birks and Heegaard (2003) and Heegaard et al. (2005), as well as to force the age–depth model to pass through the error range of the original calibrated radiocarbon ages.

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Daniele Colombaroli et al.: Fire–vegetation interactions during the Mesolithic–Neolithic transition

Table 1

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AMS dating used for chronology

Core

Sample ID

Laboratory number

Master core depth (cm)

Materiala

AMS radiocarbon date BP

Cal. year BP 95% limitsb

Cal. yr BP in diagram

P7 AC05 AC05 AC05 AC05 AC05 AC05 AC05 AC05 AC05 AC05 AC05

P7-60 B1.2 99 C16–33 B1.4 35.5 B1.5 7.5 BB1.4 57.5 BB1.5 4.5 BB1.5 67.5 B1.7 22.5c B1.7 67.5 B1.8 32.5 B1.9 68

POZ-14759 POZ-12334 POZ14758 Poz-16242 Poz-16243 Poz-16248 Poz-16249 Poz-16250 Poz-16244 Poz-16245 Poz-16247 POZ-11448

66.5–67.5 191–192 237–238 267.5–277.5 328–333 392–397 425.5–426.5 486.5–491.5 532–537 577–582 625–630 768–769

W W W CH BS BS BS BS W W DL W

970 ± 30 2495 ± 30 3510 ± 35 3680 ± 50 4440 ± 50 4550 ± 110 5120 ± 70 5810 ± 50 3840 ± 100 7040 ± 80 8330 ± 50 9850 ± 50

795–933 2378–2732 3693–3876 3879–4151 4873–5285 4874–5572 5661–5997 6493–6734 3933–4520 7697–7998 9141–9472 11191–11389

851 2714 3597 4154 4941 5412 5758 6565 – 8047 9202 11268

a

W, wood; CH, charcoal; DL, dicotyledon leaves; BS, bud scale. Calibrated with CALIB Rev 5.0.2 (intcal04.14c; Stuiver and Reimer, 1993; Reimer et al., 2004). c Rejected date. b

(0.5 mm sieving and decanting) following standard procedures (Moore et al., 1991). Lycopodium markers (Stockmarr, 1971) were added to estimate pollen concentration (grains/cm³). For identification of pollen types we used keys (Moore et al., 1991; Beug, 2004) as well as the reference collection at the Institute of Plant Science in Bern. We analysed a total of 34 and 78 pollen samples for the framework and the high-resolution sequence, respectively, using a magnification of 400×. Water and wetland plants as well as spores were excluded from the pollen sum, which reached a minimum of 400 for AC05 and of 500 for AC05hr1. The pollen diagrams were plotted with Tilia 2 (Grimm, 1992). Local pollen assemblage zones (LPAZ) were defined for the whole sequence (AC05, 63 samples) and for the high-resolution record (Ac05hr1, 78 samples). According to the zonation method of optimal partitioning (Birks and Gordon, 1985), as implemented in the program Zone version 1.2 (Juggins, 1991). We then determined the number of statistically significant zones in both series (AC05 and AC05hr1) according to the broken-stick model, as implemented in the program BSTICK (Bennett, 1996).

Microscopic and macroscopic charcoal, MFI

Figure 2 Depth–age model for core AC05-B in Lago dell’Accesa, based on cubic splines (Telford et al., 2004). The curve is within the confidence interval (bracketing envelope) of the generalized mixedeffect regression model (Birks and Heegaard, 2003; Heegaard et al., 2005). Error bars indicate the error range of the original calibrated radiocarbon ages

Pollen analysis The cores were sampled for pollen and microscopic charcoal analyses at the Institute of Plant Science in Bern. The sequence was sampled continuously (samples in cubes of 1 cm³ and 1 cm thickness) between 595.5 and 517.5 cm depth for high-resolution analyses (Ac05-hr1) and every 8 or 16 cm between c. 440 and 720 cm depth (namely Ac05). The sediment samples were treated chemically (HCl, KOH, HF, acetolysis) and physically

Microscopic and macroscopic charcoal was estimated at the IPS of the University of Bern and at the LCE of the CNRS in Besançon, respectively. We used microscopic charcoal particles (>10 µm and 100 µm minimum diameter) to reconstruct local fire events (Whitlock and Larsen, 2001). Microscopic charcoal was estimated in the pollen slides with a light microscope at 250× magnification following Tinner and Hu (2003) and Finsinger and Tinner (2004); then charcoal concentration (particles/cm3) and influx (particles/cm2 per yr) were calculated with the same approach as for pollen. For macroscopic charcoal, contiguous 1 cm sediment samples of 2 cm3 were washed through a 150 µm mesh sieve and areas were measured under a stereomicroscope (×40) with a grid. Counts of charcoal particles give similar results as area estimation, and it is possible to convert number estimations to area estimations with a regression equation (Tinner and Hu, 2003). We then use microscopic and macroscopic charcoal influx (particles/cm2 per yr) as a proxy for past regional and local fire incidence (Whitlock and Larsen, 2001). To estimate fire occurrence from macroscopic charcoal (but not from microscopic, see MacDonald, 1991; Tinner et al., 1998), raw data for macroscopic charcoal

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The Holocene 18,5 (2008)

Figure 4 AC05-B

Figure 3 Overview pollen diagram between 11 000 and 6000 cal. yr BP in core AC05-B

High-resolution pollen diagram (AC05-hr1) from core

influx (particles/cm2 per yr) were transformed on a constant timescale (bin-width = 25 yr; see Long et al., 1998; Whitlock and Larsen, 2001; Brunelle et al., 2005). To eliminate the effect of background charcoal (Whitlock and Larsen, 2001), macroscopic charcoal influx was smoothed with a LOWESS function with a 200-yr time window, and the results were subtracted from the raw data to derive residual peaks (Long et al., 1998). The statistical distribution of residuals was made by the estimation of the proportion of peak accumulation values above a threshold P (Lynch et al.,

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Daniele Colombaroli et al.: Fire–vegetation interactions during the Mesolithic–Neolithic transition

2002). Sensitivity analyses allowed identifying how the proportion of peak accumulation rates changed with P (Clark et al., 1996; Lynch et al., 2002). The mean fire interval (MFI) referred to events identified as local fire events above the threshold value (in our case charcoal peaks> 0.6 particles/cm2 per yr). MFI was estimated as the number of years covered between the first and last fire events detected in the sediment section, divided by the number of intervals between all the fire events. To address potential linkages between vegetation and fire regime, we then calculated the mean fire interval before and after the limit representing the largest variance in the pollen data set (see numerical analyses). Explorative MFI analyses with an alternative method (ie, background by LOWESS smoothing of minimum, see Gavin et al., 2003) provided a different MFI value; nevertheless the selected method provided at our site the most conservative estimate of local fires.

Numerical analyses We used cross-correlations analyses of pollen and charcoal series (using the software Systat 11.0) to reconstruct the long-term dynamic linkages between vegetation, diversity and fire (Green, 1983; Tinner et al., 1999). Both charcoal influx and pollen percentages were linearly de-trended to extract the major trends from the time series, assuming that such trends are present in our pollen record (eg, decline of Q. ilex-type, expansion of Q. robur-type). To avoid spurious correlations resulting from potential common trends in sedimentation rates (eg, influx versus influx) or internally created changes (percentages versus percentages, Tinner et al., 1999), we compared microscopic and macroscopic charcoal influx with pollen percentages. However, pollen influx, concentration and percentages have similar trends, showing that pollen percentages are not significantly influenced by internally created changes. A total of 78 samples (517.5–594.5 cm depth) were used to calculate cross-correlation coefficients at ± 19 lags, corresponding to c. ± 360 yr. Cluster analyses were applied to identify groups of samples with similar variation (percentages, untransformed), by choosing the highest cophenetic correlation value among the Average Link Method, Nearest Neighbour, Furthest Neighbour, Ward’s minimum variance clustering and Weighted Average Link (Davis, 1986; Jongman et al., 1987). To estimate palynological richness, we applied rarefaction analyses with RAREPOLL (Birks and Line, 1992). CANOCO software was used for data ordination (PCA; using interspecies correlations divided by standard deviation and centred by species, see Smilauer, 1994; Kovach, 1995; ter Braak and Smilauer, 2002).

683

Finally, to test the statistical significance of the ordination axes, a broken-stick model was compared with the PCA axes.

Results Sedimentary data A continuous composite series 8 m long was established from two series of overlapping 3 m long sections, according to gamma density and marker horizons characterized by abrupt changes in colour. Up to six different sedimentary facies (F1 to F6, Figure 1C) were identified in these light brown to dark grey silts. According to our chronology, the high-resolution sequence between 595.5 and 517.5 cm depth (ac05hr1) covers 1450 years and has a mean (interval) sample age of 18.8 ± 3.2 years, which is also the mean number of years per centimetre of sediment. The high-resolution sequence includes a marked decrease in percentages of Quercus ilex-type pollen, which is the most abundant pollen tree taxon in the first part of the high-resolution sequence.

Pollen Since pollen influx and percentages have similar trends, only percentage values are shown and discussed. The middle local pollen assemblage zone of the overview sequence (AC05-2, AC05-3, AC05-4) covers the zone of high resolution, so the high-resolution sequence is subdivided into statistically significant subzones (AC05-3a, AC05-3b, AC05-4a, AC05-4b). A total of 119 pollen and 95 spore types were identified in the overview (34 samples) and high-resolution (78 samples) sequences, respectively. The overview sequence (Figure 3) shows phases dominated by pollen of deciduous oaks (AC05-2, Q. robur-t at 60%), evergreen oaks (AC05-3, Q. ilex-type pollen at 60%) and deciduous oaks (AC054, Q. robur-t at 40%). The high-resolution sequence (from 8400 to 7000 cal. yr BP, Figure 4) covers the transition from high to low pollen values of Q.ilex-t (from 60 to 30%, limit LPAZ AC05-3 and AC05-4, c. 7700 cal. yr BP), after a period of relatively stable values of tree pollen types around 80% (LPAZ AC05-2, from c. 10 800 to c. 8600 cal. yr BP). After a period of dominance by pollen of Q. ilex-t (60% in LPAZ AC05-3, c. 8600 to 7700 cal. yr BP), pollen percentages of Corylus, Poaceae and Q. robur-t increase markedly, whereas Q. ilex-t decreases from 60% to 40% (end of the subzone AC05-3b, c. 7700 cal. yr BP). This trend continues in zone 4, where pollen of Q. ilex-t decreases to 30%, while pollen of Q. robur-t, Poaceae and shrub taxa (Corylus, Ericaceae)

Figure 5 Macroscopic charcoal residuals and MFI. Continuous line represents macroscopic charcoal influx (particles/cm2 per yr) on a constant timescale of 25 yr (Long et al., 1998); dashed line is the resulting lowess smoothing (bandwidth = 25 yr) and the dotted line represents the residual peaks. Local fire events are identified by a threshold value of T > 0.6 (16 significative peaks, cross symbols) Downloaded from http://hol.sagepub.com at GEOBOTANISCHES INSTITUT on August 19, 2008 © 2008 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.

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The Holocene 18,5 (2008)

Figure 6 Cross-correlograms of microscopic and macroscopic charcoal (influx, detrended) versus pollen types (detrended percentages). One time lag corresponds to the average time difference between two adjacent sedimentary samples (containing both, charcoal and pollen) ie, 18.8 years. Lag 0 corresponds to the ordinary correlation coefficient between the two variables of interest (charcoal and pollen). Correlation coefficients for positive Downloaded from http://hol.sagepub.com at GEOBOTANISCHES INSTITUT on August 19, 2008 © 2008 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.

Daniele Colombaroli et al.: Fire–vegetation interactions during the Mesolithic–Neolithic transition

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Figure 7 Cross-correlograms summary (species and charcoal detrended). One time lag corresponds to the average time difference between two adjacent sedimentary samples (containing both, charcoal and pollen) ie, 18.8 years. Lag 0 corresponds to the ordinary correlation coefficient between the two variables of interest (charcoal and pollen or macroscopic and microscopic charcoal). Correlation coefficients for positive lags are a measure of the influence of the first variable on the second after a certain time lag, whereas correlation coefficients for negative lags are a measure for the influence of the second variable on the first with regard to the lag. Vertical axis show correlation coefficients; those outside the horizontal lines are significant at P = 0.05

increases. At the end of the subzone AC05-4a, pollen of all tree taxa recovers (up to 80%). In subzone AC05-4b (c. 7100–6900 cal. yr BP), pollen of forest tree taxa decreases again (60–70%), with the exception of Q. robur-t (40%). Other taxa, such as Corylus, Erica arborea and Poaceae, are present with changing values.

Microscopic and macroscopic charcoal Concentration and influx of microscopic and macroscopic charcoal (Figures 3 and 4) show moderate correlations. The microscopic charcoal influx reaches high values during the early Holocene (AC05-2), with peaks around 250 000 fragments/cm2 per yr. Subsequently, microscopic charcoal influx slightly decreases. Macroscopic charcoal influx shows a comparable decreasing trend, with the exception of one peak at 8600 cal. yr BP (up to 18 particles/cm2 per yr). Influx values remain lower between LPAZ 2 and 3a for both microscopic (between c.

100 000 and 200 000 fragments/cm2 per yr) and macroscopic charcoal (less than 4 particles/cm2 per yr). In subzone 3b, both macroscopic charcoal and microscopic charcoal increase, with microscopic charcoal lagged by c. 100 years (c. 200 000 particles/cm2 per yr). In subzone 4a, microscopic charcoal influx shows several peaks, the most important at the end of the subzone (around 7200 cal. yr BP, up to c. 300 000 particles/cm2 per yr), whereas macroscopic charcoal remains high (peaks up to 10 particles/cm2 per yr), with the exception of the period around 7160 cal. yr BP (less than 2 particles/cm2 per yr). Finally, in the subzone 4b (c. 7600 cal. yr BP), influx values of both microscopic charcoal and macroscopic charcoal decrease. In the case of macroscopic charcoal LOWESS smoothing (Figure 5) reduces the effects of variations in the background component. This charcoal component may have various origins, such as secondary charcoal deposition and regional fires (see discussion in Whitlock

Figure 6 (continued) lags are a measure of the influence of the first variable (eg, charcoal) on the second (eg, pollen) after a certain time lag, whereas correlation coefficients for negative lags are a measure for the influence of the second variable (eg, pollen) on the first (eg, charcoal) with regard to the lag. Vertical axis shows correlation coefficients; those outside the horizontal lines are significant at P = 0.05 Downloaded from http://hol.sagepub.com at GEOBOTANISCHES INSTITUT on August 19, 2008 © 2008 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.

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The Holocene 18,5 (2008)

Figure 8 Cross-correlograms of palynological richness and PCA (axis 1). One time lag corresponds to the average time difference between two adjacent sedimentary samples (containing both, charcoal and pollen) ie, 18.8 years. Lag 0 corresponds to the ordinary correlation coefficient between the two variables of interest (pollen and palynological richness or PCA axis 1 and pollen or palynological richness). Correlation coefficients for positive lags are a measure of the influence of the first variable on the second after a certain time lag, whereas correlation coefficients for negative lags are a measure for the influence of the second variable on the first with regard to the lag. Vertical axis shows correlation coefficients; those outside the horizontal lines are significant at P = 0.05

and Larsen, 2001). The residual macroscopic charcoal peaks increase in frequency after c. 8000 cal. yr BP, with high peak frequency, at c. 7800–7700 cal. yr BP and 7300–7100 cal. yr BP. With a total time span of c. 1250 years and 16 significative peaks, the MFI is around 83 ± 63 years. We assess the potential effects of vegetational composition on fire occurrence and vice versa by dividing the macroscopic charcoal record into two series, corresponding to the change in variance of the pollen data (limit LPAZ AC03 and AC04, see numerical analyses). The resulting MFI is 144 ± 94 yr for the period with dominance of evergreen broadleaved vegetation (Q. ilex-t) and 61 ± 30 yr for the period where mixed deciduous and evergreen vegetation prevailed. The MFI calculated according to the methods of Gavin et al. (2003) resulted in 27 significative peaks corresponding to a significantly shorter total MFI of 48 ± 38 years. However, we adopted the method proposed by Long et al. (1998) that in our case was the most conservative in estimating past fire events.

Correlation analyses Macroscopic charcoal is significantly negatively correlated with Q. ilex-t (peak from lags −1, c. −20 yr, Figure 6) and Q. robur-t (peak at lags −13 to +3, c. −240 to 56 years); each lag corresponding to the time difference between synchronous pollen and charcoal peaks. Significant positive correlations are present for Pinus (peak from lags −2, c. −38 years), Corylus (peak at lags −1 to 8, c. −18 to 150 years), E. arborea-t (peak from lags 5, c. from 38 years), Poaceae (peak at lags −3 to 5, c. −56 to 94 years), Plantago lanceTable 2

olata (peak at lags 0 to 1, c. 18 years), and Artemisia (peak at lags 2 to 3, c. 40 to 55 years). Generally, macroscopic charcoal is negatively correlated with trees (peak at lag −3, from c. −55) and positively with shrubs (peak at lags 3 to 7, c. 55 to 130 years) and herbs (peak at lags −3 to 5, c. −56 to 94 years, Figure 7). Just as with macroscopic charcoal, microscopic charcoal is significantly negatively correlated with Q. ilex-t (peak from lags 7 to 15, from c. 130 years, Figure 6) and Q. robur-t (peak at lags −6 to 4, c. −110 to 75 years). Positive correlations are present for Alnus (peak at lags −5 to 1, c. −95 to 18 years), Corylus (peak at lags − 5 to 0, from c. −95 to 0 years), Poaceae (peak from negative and positive lags −8 to 13, c. −150 to 250 years). As in the case of macroscopic charcoal, microscopic charcoal is negatively correlated with trees (peak at lags −5 to −3, c. −95 to −55 years, Figure 6) and positively with shrubs (peak from lags 10, c. from 180 years) and herbs (peak at positive and negative from lag −8, c. −150). Macroscopic charcoal is positively correlated with microscopic charcoal by a negative time lag (peak at lags −7 to 3, c. −131 to 55 years, Figure 7). Macroscopic charcoal is positively correlated with palynological diversity (peak at lags −6 to −4, c. −110 to −75 years), whereas microscopic charcoal does not show any significant correlation (Figure 8). In general, trees are significantly negatively correlated with palynological richness (peak from lags −4, +5, c. −75 to 95 years), whereas shrubs and herbs reach positive correlations (peak at lags −4 to 3, c. −75 to 55 years and peak at lags −6 to 5, c. −112 to 95 years, respectively). PCA-a1 (Figure 8)

DCA (standard deviation, SD) and PCA (percentage variance of species data for the high resolution (hr 1))

Total samples

Period (cal. yr BP)

Length of gradient (DCA )

PCA a1

PCA a2

Other PCA axis (sum)

78

8400–7000

1.1

89.9%

6.1%

4%

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Daniele Colombaroli et al.: Fire–vegetation interactions during the Mesolithic–Neolithic transition

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Figure 9 (a) PCA and palynological richness for the high resolution sequence (hr1). PCA axis 1 explains most of the data variance (89.9%). (b) Variance accounted by the nth PCA axis (dashed line) compared with values from a broken-stick model (continuous line), being only the first axis to be significant

was significantly positively correlated with trees (peak at lags − 5 to 5, c. −95 to 95 years) and negatively with shrubs (peak at lags −3 to 5, c. −55 to 95 years), herbs (peak at lags −6 to 4, c. −110 to 75 years) and pollen diversity (peak at lags −4 to −4, c. −75 to 75 years).

Numerical analyses Given the gradient length of DCA (percentage data, detrending by segment and with no rare taxa down-weighted) was below the value of 2 SD (standard deviation, Ammann et al., 2000), we used PCA ordination to summarize the variation in the pollen data from the period 8400 to 7000 cal. yr BP (see Table 2). We considered only the first PCA axis (89.9% of the data variance) after we tested the statistical significance with a broken-stick model (Figure 9a and b). PCA axis 1 was highly correlated with Q. ilex-t pollen percentages; cross-correlation analyses revealed also a significantly positive correlation of PCA axis 1 with pollen percentages of trees and a negative correlation with palynological diversity and percentages of shrubs and herbs (Figures 8 and 9). PCA scores indicate that two main groups of pollen samples can be distinguished, those below and those above the level of 562 cm depth (c. 7700 cal. yr BP), corresponding to the limit of optimal partitioning between the zones AC03 and AC04 as well as to the two first clusters of the Average Link method, which resulted in the least amount of distortion of all clustering methods (cophenetic correlation score = 0.84, Kaesler and Cairns, 1972).

Discussion Vegetation history The Holocene vegetation history of Lago dell’Accesa was described in a littoral core by Drescher-Schneider et al. (2007). Our new AC05 sequence is in agreement with these results but focuses on the interval from 11 000 cal. yr BP until 6500 cal. yr BP. At the beginning of the Holocene, forests became dense (10 600–10 300 cal. yr BP, AC05-2), and deciduous oak forests of

Quercus (probably Q. pubescens, see Drescher-Schneider et al., 2007) replaced open grasslands (Poaceae, R. acetosella) near the lake. Deciduous oak forests expanded with mesophilous Corylus avellana and Fraxinus excelsior and persisted until c. 8500 cal. yr BP, when evergreen forests of Q. ilex became the major component of forest ecosystems (AC05-3, 8500–7700 cal. yr BP). Pollen and spore data suggest that the decline of Quercus ilex around 7900–7600 cal. yr BP (from 60 to 40%) was associated with an increase of deciduous oak and plants indicating disturbance and/or human activities (Pteridium, Poaceae, Plantago lanceolata AC053b, c. 7900–7700 cal. yr BP). The slight expansion of Abies alba during this phase (5%) did not reach that observed in the littoral core (c. 20%, see Drescher-Schneider et al., 2007). The high amounts of Abies pollen in the littoral core might be related to local stands of Abies alba near the (humid) shore (see also Lowe, 1992; Andrieu-Ponel et al., 2000) or to over-representation owing to pollen buoyancy on the water surface (ie, higher concentration near the shore, see Ammann, 1994). A further persistent decline of Quercus ilex (40% to 20%) occurred 200 years later (562 cm depth, limit LPAZ AC05-3 and AC05-4, c. 7700 cal. yr BP), corresponding to the maximum data variance as indicated by PCA samples scores, cluster analyses and optimal partitioning of the general and high-resolution sequence. We thus assume that it reflects the most important vegetational change in the sequence. This drastic vegetational change is also coupled with an increase of taxa indicative of human impact, such as Plantago lanceolata, Rumex acetosella, Cichorioideae, Chenopodiaceae, Apium, Artemisia and Pteridium, which were probably growing in open habitats used for agriculture (eg, meadows, fields). This interpretation agrees with that based on the littoral core (Magny et al., 2006; Drescher-Schneider et al., 2007). Both cores suggest that land use in the area around Accesa played a key role in the drastic vegetational change at the Mesolithic–Neolithic transition.

Fire history Our sequence from the early Holocene (c. 10 500 cal. yr BP) to 7000 cal. yr BP covers the Mesolithic and early Neolithic period (c. 8000–4000 cal. yr BP). In the early Holocene (c. 10 500–9500 cal.

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yr BP), both microscopic and macroscopic charcoal show comparable trends, with high charcoal influx values at the beginning of the Holocene that subsequently decrease during the expansion of forested ecosystems (LPAZ AC05-2 c. 10 500–9000 cal. yr BP). High fire incidence inferred from charcoal peaks was coupled with a dominance of Pinus and taxa of open environments (eg, Rumex acetosella, Artemisa, Poaceae), thus not suggesting a fuel level control on fire (Marlon et al., 2006). This environmental setting was probably associated with drier than present climatic conditions in the Mediterranean area, as reported elsewhere (Rossignol-Strick, 1999; Carrion et al., 2001). Afterwards, deciduous oak expanded and fires decreased, perhaps as a result of increasing moisture availability. Fire incidence declined even more during the expansion of Quercus ilex (AC05-3a, 8400–7900 cal. yr BP). With the beginning of the Neolithic period (AC05-3, from c. 8000 cal. yr BP), fire incidence increased again (Figures 3, 4 and 5), but high-resolution analyses reveal that the increase in macroscopic charcoal and microscopic charcoal did not occur synchronously, possibly pointing to a different fire history at local to regional scales (eg, Tinner et al., 1998; Whitlock and Larsen, 2001). Around c. 8000 cal. yr BP, local fire incidence increased, preceding by c. 100 years the increase of regional fire incidence (microscopic charcoal peak at c. 7900 cal. yr BP, Figure 4). Cross-correlation analyses may help to assess possible linkages between regional and local fire regimes (eg, higher local fire frequencies ahead of increasing regional fire frequencies). Time-series analyses surprisingly show that microscopic charcoal is correlated with macroscopic charcoal not only at positive, but also at negative time lag (lags −7, +4 c. −130, +70, see Figure 7), making it difficult to assess whether local or regional fire activities increased first. On the basis of the macroscopic charcoal record, we estimate the local MFI to be 83 years for the whole sequence, whereas it is 144 years for undisturbed Q. ilex forests (see PCA axis 1) and only 61 for mixed evergreen-deciduous forests. Fire-history reconstructions by means of historical data and observations suggest mean fire return intervals of c. 53 years for Italy for the period 1980–1990 (Moreno, 1998). The range of modern fire frequencies observed (40 to even 2000 years, depending on the country) may result from the effect of forest fragmentation and land use north and south of the Mediterranean sea (Moreno, 1998). Our data (61–144 years) are in good agreement with the lower end of the modern range, ie, with those estimates for European Mediterranean countries (eg, Greece, Italy, Spain, see Moreno, 1998).

Impact of climatic change and fire on vegetation Dense Quercus ilex woodlands established abruptly at approximately 8500 cal. yr BP, although the species was (sparsely) already present before (continuous pollen curve 1–2%). This abrupt and massive expansion of Quercus ilex is likely to be a result of changing environmental conditions. In the littoral core, the expansion of Q. ilex forests (Drescher-Schneider et al., 2007) was associated with drier conditions as evidenced by lake-level changes, that were interrupted only by a short-term shift towards more humid conditions at around 8200 cal. yr BP (Magny et al., 2006). Modern studies show that Quercus ilex is drought-adapted (Pigott and Pigott, 1993). In the area of Lago dell’Accesa, dense and widespread forests of Q. ilex (pollen influx 13 000 grains/cm² per yr) declined gradually around 7900–7700 cal. yr BP (6000 grains/cm² per yr), when both microscopic and macroscopic charcoal increased. The evidence that charcoal influx increases with lake levels contrasts with the possible explanation that charcoal deposition was driven by lake level fluctuations (Lynch et al., 2004). However, the final demise at c. 7700–7000 cal. yr BP (3500 grains/cm² per yr) occurred only after 200 years of persistent burning (higher charcoal influx).

The dynamics of evergreen broadleaved forests around Lago dell’Accesa may be indirectly related to drought and frost (Pigott and Pigott, 1993; Conedera and Tinner, 2000; Grund et al., 2005). Increased winter frost or summer humidity would have favoured deciduous (taller-growing) oaks at the cost of evergreen oaks. Pollen-inferred vegetational changes at Lago dell’Accesa coincide with known episodes of climate change in the Mediterranean area, such as the 8200 cal. yr BP event, evidenced by lake-level change at the site (Magny et al., 2006). In upland Sicily, climate conditions were relatively humid until c. 7000 cal. yr BP (Sadori and Narcisi, 2001). Similarly, in the eastern Mediterranean, sapropel deposition and speleothems suggests humid conditions between 10 000 and 7000 cal. yr BP (Rossignol-Strick, 1999; BarMatthews et al., 2000), whereas in the western Mediterranean the climate was dry before to c. 7000 cal. yr BP (Reed et al., 2001). Q. ilex had already declined at our site by 7000 cal. yr BP. New studies from Lago dell’Accesa (littoral core) suggest that the lake level rose around 8200 and 7700 cal. yr BP (Magny et al., 2007), pointing to wetter and possibly cooler conditions at that time. The first rise of the lake is coeval with the 8200 cal. yr BP event (von Grafenstein et al., 1998; Tinner and Lotter, 2001; Heiri et al., 2004; Alley and Agustsdottir, 2005; Magny et al., 2006), and the second can be correlated with the beginning of drier conditions in the Alps (end of CE-3 at c. 7700 cal. yr BP; see Haas et al., 1998; Heiri et al., 2004), corresponding in the Mediterranean to more humid conditions (Magny et al., 2003). Considering the ecology of Q. ilex, it is conceivable that more humid conditions at 7700 cal. yr BP could have disadvantaged the species during its second decline, but not during the first decrease (7900 cal. yr BP, Figure 4). Increasing fire incidence may have contributed to the decline of Quercus ilex as also observed at Lago di Massaciuccoli, Tuscany (Colombaroli et al., 2007) and would explain the direction of the vegetational change towards more open environments instead of rather closed forests (Figure 4). In agreement, our time-series analyses suggest that the decline of Q. ilex was associated with an increase of local fires; this correlation is less pronounced at a regional scale, where the decline of Q. ilex occurs 130 years after the increase of fire (Figure 6). The correlations between macroscopic, microscopic and pollen sequences may be used to address spatial issues, such as whether local and regional fires had similar or dissimilar influences on vegetational composition. The assumption behind this procedure is again that microscopic charcoal is primarily a proxy for regional fires, whereas macroscopic charcoal is mainly a proxy for local fires (Whitlock and Larsen, 2001; Carcaillet et al., 2001; Tinner and Hu, 2003; Higuera et al., 2005; Tinner et al., 2006). Given the size of the basin (16 ha), we expect that the source area of pollen at our lake is intermediate (c. 20–30 km, see Prentice, 1985; Bradshaw and Webb, 1985; Sugita, 1993; Bunting et al., 2004; Sugita, 2007a, b), between that of macroscopic charcoal (c. 0–1 km, Clark et al., 1998; Ohlson and Tryterud, 2000; Lynch et al., 2004) and microscopic charcoal (0–100 km, MacDonald et al., 1991; Tinner et al., 1998; Whitlock and Larsen, 2001). Macroscopic CHAR suggests that local fire occurred synchronously with decline of forested ecosystems and expansion of shrublands and maquis or grasslands (Figure 7). Fire promoted the expansion of resprouters such as Corylus, Erica and Poaceae (Figures 6 and 7), which is in agreement with findings from other regions of Europe (Clark et al., 1989; Tinner et al., 2000). Modern ecological studies suggest that fire occurrence simplifies forest structure towards shrublands communities (Trabaud and Galtie, 1996; Pausas, 1999, 2006; Mouillot et al., 2003, 2005; Lloret et al., 2005). In particular, Mediterranean shrubland ecosystems are more resistant to fire (Calvo et al., 2002). Microscopic charcoal shows that regional fires probably did not influence the vegetation to the same extent as local fires.

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Daniele Colombaroli et al.: Fire–vegetation interactions during the Mesolithic–Neolithic transition

Resprouters such as Corylus and Alnus, as well as herbaceous taxa, expanded before the increase of regional fire activities (ie, negative lags in cross correlations, see Figures 6 and 7). Q. pubescens declined prior to the increase of regional fire incidence. Conversely Artemisia, Poaceae and Q. ilex declined about 130–220 years after the increase of regional fires (c. +7 +12 lags). In general trees, shrubs and herbs seem to respond about 100 years before and 100–200 years after increases in regional fire incidence (Figure 7). On the basis of this pattern, it appears likely that climate and human impact played different roles at different spatial scales. For instance, Tinner et al. (1999) suggested that human disturbance prior to the increase of regional fire frequencies may have led to the expansion of disturbance-adapted taxa, which is a pattern we observe also in our data (eg, significant positive correlations with negative lags for Corylus, Poaceae and Plantago lanceolata, see Figure 7). In any case, the similarities among the patterns of crosscorrelation analyses for microscopic charcoal and macroscopic charcoal (eg, negative significant correlations for Q. ilex) point to a significant disruption of Q. ilex stands or forests by fire. It is striking that Q. ilex, in contrast with the common beliefs of (palaeo)ecologists (see discussions in, eg, Reille, 1992; Carcaillet et al., 1997) is not favoured by fire in our time series, although responses of Quercus ilex may also vary according to local conditions (eg, hydrology and lithology ,see Bergkamp, 1998 and Lloret et al., 2004). However, massive declines of Q. ilex after fire have been documented at other sites in Tuscany (Lago di Massaciuccoli, Colombaroli et al., 2007). Palynological richness has been used in different biota to infer past species diversity (eg, Bennett et al., 1992; Seppa, 1998; Odgaard, 1999). Given the size of our basin (Sugita et al., 1993) the gradient of spatial scales covered by our pollen records ranges from community to plant ecosystem (10²–10³ km²), covering alpha and gamma diversity (Birks and Line, 1992). Given that PCA axis 1 reflects the trend of Q. ilex, we assume a relationship with the development of evergreen forest ecosystems. Cross-correlation analyses reveal that PCA axis 1 is negatively correlated with shrub and herb communities and with palynological richness (Figure 8), pointing to the presence of well-developed but rather species-poor forests of Quercus ilex at that time. On the other hand, palynological richness is positively correlated with local fire (macroscopic charcoal influx). Taken together this suggests that fire disturbance led to open environments, promoting colonization by new (non late-successional) species and not merely inducing an expansion of species-poor fire-adapted shrubby or tall-herb communities, whereas studies from the neighbouring southern Alps point to impoverishment of forested ecosystems after fire (Delarze et al., 1992). Mediterranean ecosystems are probably more fire-resilient than non-fire adapted communities in the nearby southern Alps (Tinner et al., 2000; Tinner and Ammann, 2005). Similarly, mixed evergreen-deciduous forests were converted to less diverse shrublands ecosystems at Lago di Massaciuccoli in northern Tuscany after 6000 cal. yr BP (Colombaroli et al., 2007). Such apparent contradictions are also found in ecological studies in the Mediterranean region, where fire is coupled with increased homogeneity or greater heterogeneity depending on its severity and intensity (Trabaud and Galtie, 1996); only in some cases floristic richness reached its maximum after fire occurrence (Trabaud and Lepart, 1980).

The causes of fire Our microscopic and microscopic charcoal data suggest that fire incidence increased markedly c. 8000–7900 cal. yr BP. Increasing fire frequencies have been related to climatic changes (Keeley and Fotheringham, 2001; Moritz et al., 2004; Salvador et al., 2005) and to increased agricultural activities during the Neolithic (eg,

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introduction of animal husbandry or the first wide-scale slash-andburn practices, Clark et al., 1989; Tinner et al., 1999). The fire increase at Lago dell’Accesa (between 573.5 and 577.5 cm; c. 8000–7900 cal. yr BP) may have occurred during a dry period that ended at c. 7700 cal. yr BP in Lago dell’Accesa (Magny et al., 2006, 2007). Interestingly, charcoal influx values (Figure 4) remained high during the high lake-level stand at Lago dell’Accesa (ie, after 7700 cal. yr BP, Magny et al., 2007), pointing to causes other than climate for increased fire frequencies. Although it is reasonable to think that dry conditions would result in higher fire frequencies, a number of palaeoecological studies from boreal biomes show that fire may increase also under wetter conditions (eg, Lynch et al., 2004). For instance, more summer drought and wetter winters could also result in an increase in fire and lake levels (Vannière et al., 2008) at our site, but the shortterm record presented here is not able to rigorously test this hypothesis. Several studies show that during the Neolithic fire represented an important tool to increase openness in central and southern Europe (Carcaillet, 1998; Fyfe et al., 2003; Tinner et al., 2005). In the Mediterranean area, human impacts on natural landscapes increased significantly at the transition from (Mesolithic) hunters and gatherers to (Neolithic) agricultural societies (Solari and Vernet, 1992; Malone, 2003; Kirch, 2005; Ruiz, 2005). In fact, our pollen data suggest a strong increase in human influence at the Mesolithic/Neolithic transition, when open lands expanded markedly. Similar patterns suggesting land-use changes at the beginning of the Neolithic have been observed in other cases where high-resolution time-series analyses were applied (Tinner et al., 1999).

Conclusions High-resolution series of pollen, microscopic and macroscopic charcoal from Lago dell’Accesa during the period 8400 to 7000 cal. yr BP help improve our understanding of the fire history in Mediterranean environments. Contiguous pollen and charcoal data show the interactions between fire and vegetation at both local and regional scales. Between 8500 and 7900 cal. yr BP, extensive monospecific and thus rather species-poor evergreen forests of Quercus ilex were present around the area of Lago dell’Accesa. After 7900 cal. yr BP, increased fire occurrence, probably resulting from land-use activities at the beginning of the Neolithic, affected the forest ecosystems near the lake. The loss of evergreen forests created open areas and disturbance-adapted species gradually replaced evergreen broadleaved forests. The charcoal data at Lago dell’Accesa suggest a MFI of about 150 years under natural conditions. Transformation of evergreen/broadleaved ecosystems occurred at 7900 cal. yr BP, when MFI decreased to c. 50 years, an interval comparable with that of the modern highly disturbed Mediterranean ecosystems (Moreno, 1998). Our study suggests that increased fire activity led to an expansion of maquis and deciduous trees at the cost of evergreen oaks, a finding that is surprising given the hypothesis that Q. ilex expanded as a response to more frequent fires (see eg, Reille, 1992; Carcaillet et al., 1997). Pollen data show that the area around the lake probably was occupied by Neolithic settlements at c. 8000 cal. yr BP, and fire was a primary tool to create open environments for agricultural and grazing purposes. Considering the importance of Mediterranean environments for biodiversity (Allen, 2003; Vogiatzakis et al., 2006), it is of great interest to evaluate ecosystem resilience to past disturbances under quasi-natural or natural conditions. One concern for the future is that fire may significantly exacerbate the local effects of global change (Overpeck et al., 1990; Torn and Fried, 1992; Reisigl et al., 1992; Kasischke et al., 1995; Roberts, 1998; Frey

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and Lösch, 1998; Rupp et al., 2000; Intergovernmental Panel on Climate Change (IPCC), 2001; Allen, 2003). Our results suggest that the remnant Q. ilex forests are not fire-resilient and that they may decline rapidly if fire frequency was to increase. Given that only a few Quercus ilex forests are preserved in the entire Mediterranean area and that fire frequencies will probably increase as a consequence of global change, according to the IPCC scenarios (2001), these relicts of the former natural Mediterranean vegetation seem particularly endangered.

Acknowledgements We are deeply indebted to Jacques-Louis de Beaulieu, Robert Hoffmann, Cecile Matter, Willi Tanner, Elisa Vescovi for their help during the coring in 2005. We would like to thank Florencia Oberli for laboratory help, Jacqueline van Leeuwen for valuable assistance during pollen analyses and Petra Kaltenrieder for help in macrofossil identification. We also thank Herbert Wright, Jr, Cathy Whitlock and an anonymous reviewer for improvements of the manuscript. Brigitta Ammann and all the participants to the 30th Moor-excursion are acknowledged for fruitful discussions. This study was partly financed by the Swiss National Science Foundation (Project number 3100A0-102272).

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