French Alps - lia montabor

16 (1): 13-22 (2009)

Fires control spatial variability of subalpine vegetation dynamics during the Holocene in the Maurienne valley (French Alps)1 Aurélie GENRIES2, Centre de Bio-Archéologie et d'Écologie (UMR 5059 CNRS), Université Montpellier 2, and Paléoenvironnements et Chronoécologie (PALECO EPHE), Institut de Botanique, 163 rue Broussonet, 34090 Montpellier, France; and CNRS, Institut des Sciences de l'Évolution, CC061, Place Eugène Bataillon, 34095 Montpellier, cedex 05 France, e-mail: [email protected] Serge D. MULLER, Université Montpellier 2 and CNRS, Institut des Sciences de l'Évolution, CC061, Place Eugène Bataillon, 34095 Montpellier, cedex 05 France. Lény MERCIER, Loïc BIRCKER & Christopher CARCAILLET, Centre de Bio-Archéologie et d'Écologie (UMR5059 CNRS), Université Montpellier 2, and Paléoenvironnements et Chronoécologie (PALECO EPHE), Institut de Botanique, 163 rue Broussonet, 34090 Montpellier, France. Abstract: Due to stresses resulting from their high altitudes, subalpine forests are sensitive to disturbances, including fire. This study analyzes the long-term relationships between fire and subalpine vegetation in the western Alps. High-resolution analyses of charcoal, pollen, macroremains, and other palynomorphs were performed on sedimentary cores from 2 small peaty ponds located above 2000 m asl. in the Maurienne valley, France. Results reveal similar long-term vegetation dynamics, with differences concerning the structure and composition of local and surrounding plant communities. The vegetation pattern appears partially related to local fire occurrence, which was most frequent between 8900 and 6500 cal. BP at one lake and between 4100 and 1800 cal. BP at the second. Fires notably triggered the development and occurrence of populations of Acer and Alnus incana-type during a 2000-y period and the asynchronous alteration of Pinus cembra forests at both sites. Results show that the low-competitive species, i.e., Larix decidua or Pinus uncinata, were never stimulated by increasing fire frequency. This highlights the past importance of local-scale processes such as fire, which favoured pioneer broad-leaved species but did not threaten the resilience of the subalpine forests dominated by the cembra pine. Keywords: charcoal, disturbance, macroremains, mountain, plant dynamics, pollen. Résumé : En raison du stress lié à l'altitude, les forêts subalpines sont sensibles aux perturbations, y compris le feu. La présente étude vise à analyser les relations à long terme entre le feu et la végétation subalpine dans l'ouest des Alpes. Des analyses à haute résolution du charbon de bois, du pollen, des macrorestes et d'autres palynomorphes ont été effectuées sur des carottes de sédiments provenant de 2 petits étangs tourbeux situés au-dessus de 2000 m dans la vallée de la Maurienne, France. Les résultats révèlent une dynamique végétale à long terme similaire, mais des différences dans la structure et la composition des communautés végétales locale et environnante. Le patron de végétation semble relié en partie à la fréquence locale de feu, plus élevée entre 8900 et 6500 cal. BP dans un des étangs et entre 4100 et 1800 cal. BP dans l'autre. À noter que les feux ont provoqué le développement et la présence de populations d'Acer et d'Alnus de type incana durant 2000 ans et une modification asynchrone des forêts de Pinus cembra aux deux sites. Les résultats montrent que les espèces peu compétitives, c'est-à-dire Larix decidua ou Pinus uncinata, n'ont jamais été stimulées par l'augmentation de la fréquence des feux. Cela met en évidence l'importance passée des processus à l'échelle locale comme le feu, favorisant les essences feuillues pionnières, mais sans menacer la résilience des forêts subalpines dominées par le pin cembro. Mots-clés : charbon de bois, dynamique végétale, macrorestes, montagne, perturbation, pollen. Nomenclature: Tutin et al., 1968–1993.

Introduction

Mountain forests are sensitive to climate variability, changes in human practices, and disturbances (e.g., Grace, Berninger & Nagy, 2002). Disturbances are defined as “relatively discrete events in time that disrupt ecosystem, community, or population structure and change resources, substrate availability, or the physical environment” (White & Pickett, 1985). In the European Alps, mountain landscapes 1Rec.

2008-03-04; acc. 2008-08-26. Associate Editor: Renzo Motta. 2Author for correspondence. DOI 10.2980/16-1-3180

are strongly shaped by natural, e.g., windstorms, wildfires, or avalanches, and anthropogenic disturbances, e.g., cuttings, grazing and browsing, or litter collecting (Schumacher & Bugmann, 2006; Gimmi, Bürgi & Stuber, 2008). Numerous paleoecological studies have been performed in the Alps to evaluate the influence of external factors on vegetation changes (e.g., Burga, 1988; David, 1995b; Nakagawa, de Beaulieu & Kitagawa, 2000; Ali et al., 2005; Carcaillet & Muller, 2005; Muller et al., 2007). Nevertheless, our knowledge of the modality of disturbance influences remains weak with respect to changes in fire regimes and the duration or magnitude of fire related processes on plant

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Genries et al.: Fires control subalpine vegetation dynamics

communities. Changes in the frequency and intensity of natural and anthropogenic disturbances are expected in the coming decades as a consequence of global climate change (Schär et al., 2004; Schumacher & Bugmann, 2006; Fischlin et al., 2007). The changing relationships between vegetation and disturbances in extreme ecosystems such as subalpine forests raise questions about their stability and their future response to such events, particularly fire, which is recognized as a major control of terrestrial ecosystems (Wright, 1974; Bond, Woodward & Midgley, 2005). Studies conducted in the Alps suggest that fires occurred during the Holocene (Vorren, Mørkved & Bortenschlager, 1993; Wick, 1994; Tinner, Ammann & Germann, 1996; Carcaillet, 1998), but very few of these studies have undertaken paleo-fire frequency reconstruction (Carcaillet et al., 2009). Unlike boreal ecosystems (e.g., Lynch, Hollis & Hu, 2004; Carcaillet et al., 2007), subalpine forests of the European Alps appear to have experienced complex fire regimes in space and time (Carcaillet, 1998; Tinner et al., 2005; Stähli et al., 2006); a similar pattern has been observed in the subalpine Canadian Cordillera (Gavin et al., 2006). Fires could thus have locally impacted vegetation trajectories during the Holocene, altering or stimulating vegetation changes triggered by large-scale processes such as climate (Overpeck, Rind & Goldberg, 1990; Willis et al., 1997). The present study aims at distinguishing the influence of regional and local-scale processes on subalpine vegetation changes in a French alpine valley, with a particular focus on fire regimes. Charcoal, pollen, and macroremains analyses were carried out on sediment profiles from 2 small subalpine ponds; these analyses were then used to reconstruct the long-term changes in fire and vegetation at these sites. Comparison of the 2 sites using the multi-proxy approach was employed to explore the past function of regional and local-scale processes on vegetation trajectories, enabling us to examine the role of fire.

Methods Study sites Lac du Thyl (45° 14' 26" n, 06° 29' 59" e; 2038 m asl) and Lac du Lait (45° 18' 52" n, 06° 48' 56" e; 2180 m asl) are 2 small, peaty ponds (1300 and 2000 m 2, respectively), 26 km apart, situated on south-facing slopes in the Maurienne valley (northern French Alps). Both sites were previously investigated: Lac du Thyl by David & Barbero (2001) under the name Pré Bérard, and lac du Lait by David (1993; 1995a,b) and David & Barbero (1995). The Maurienne valley, located at the northern limit of the Mediterranean climatic influence, is one of the driest areas of the Alps. Its intra-annual variability and mean annual amplitude of precipitation and temperature are within the range of the European continental climate (Ozenda, 1985). Land-use abandonment, which began during the second half of the 19th century, has resulted in an intense woody biomass build-up. Today, the lower subalpine belt (ca. 1700– 1900 m asl) is covered by coniferous woodlands dominated by Pinus sylvestris, Larix decidua, and Picea abies, with understoreys characterized by Juniperus communis, Vaccinium vitis-idaea, Arctostaphylos uva-ursi, and Ononis 14

rotundifolia. Above ca. 1900 m asl, the upper subalpine belt is dominated by sparse stands of Pinus cembra, Pinus uncinata, Larix decidua, and Picea abies, alternating with meadows and scattered Juniperus sibirica, Vaccinium spp., and Rhododendron ferrugineum. Locally, the upper treeline reaches 2350-2400 m. Traditional land-use comprises cattle and sheep husbandry and meadows for haymaking, resulting in large areas of grassland. The alpine-tundra belt (> 2300 m asl) is covered with boulders and short-grasses dominated by Carex curvula and Nardus stricta. Today, the surroundings of the studied lakes are grazed by sheep, cattle, or horses from June to October, and the vegetation cover is characterized by highly diversified meadows dominated by Carex sempervirens and Festuca rubra (Ozenda, 1985). Sampling design and chronological control Cores were sampled with a Russian corer (1000 × 75 mm) within 1 m2 in each of the 2 ponds. Charcoal and pollen were analyzed on the longest cores (495 and 345 cm length at Lac du Thyl and Lac du Lait, respectively), and plant macroremains were analyzed on parallel shorter cores (395 and 300 cm, respectively). Chronologies were obtained from the longest cores, based on 12 AMS 14C datings and a series of 17 210Pb measurements at Lac du Thyl and on 7 AMS 14C datings at Lac du Lait (Carcaillet et al., 2009). 210Pb measurements were obtained by alpha-counting (Genries et al., 2009). AMS measurements were carried out on terrestrial plant macroremains (13 samples) or on bulk sediment when macroremains were not available (6 samples). Calibrated ages (cal. BP) were computed with the Calib 5.0 program (Stuiver & Reimer, 1993), using the calibration data set INTCAL04 (Reimer et al., 2004). Charcoal analyses Contiguous sediment samples of 1 cm3 were collected along the cores for charcoal quantification. They were soaked in a 3% NaP2O 4 solution and sieved through a 160-µm mesh. Fragment surfaces were classified under a dissecting microscope (40×) using an ocular-grid with 100 squares, each of 0.0625 mm2, in height size-classes increasing exponentially. The total surface area of charcoal was calculated for each sample by determining the mean surface area by size-class and multiplying it by the number of particles. Charcoal measurements were reported as charcoal concentrations (mm2·cm–3) and charcoal accumulation rates or charcoal influxes (CHAR; mm2·cm–3·y–1). The fire reconstructions derive from Carcaillet et al. (2009). Pollen and macrofossil analyses For pollen analysis, sediment samples of 1 cm3 were taken along the core to reach a constant temporal resolution of ca. 100 y. Samples were soaked using 10% hot KOH, and carbonates and silicates were eliminated using 20% HCl and 70% HF, respectively. Part of the organic matter was removed by acetolysis, and then the samples were mounted in glycerin on glass slides. Pollen identifications were based on pollen atlases (Reille, 1995–1999) and on comparisons with the reference collection of the University of Montpellier 2. Pollen percentages were calculated based on

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ÉCOSCIENCE, vol. 16 (1), 2009

pollen sums exceeding 500 terrestrial pollen grains, excluding Cyperaceae, aquatic species, and Pteridophyta spores. For macrofossil analysis, the cores were sliced into 1-cm-thick samples, representing volumes of about 22 cm3. Each sample was soaked for 45 min at 90 °C in a 10% KOH solution. The bulk solution was sieved at 160 µm, and plant remains were identified under a dissecting microscope (60×). Identifications were achieved by comparisons with the reference collection and atlases of plant remains (e.g., Berggren, 1969; Schoch, Pawlik & Schweingruber, 1988). Numerical analyses and temporal zonation Correspondence analyses (CA) were computed on charcoal influxes and pollen percentages to estimate the relationship between fires and plant composition and to analyze the vegetation trajectories per site. Macroremains were not considered because of their low diversity of taxa and their restricted source area (a few tens of metres around the peat). The assemblages identified by the CA enabled us to delimit pollen zones. Correlation analyses were performed with Statistica version 6 (StatSoft France, 2001). The correlations were calculated between the number of fire events per 1000 y and pollen percentages on the whole sequences. Correlations were considered significant for P-values ≤ 0.05 and ≤ 0.01.

Results Chronology Four radiocarbon datings were excluded from the age/ depth model of Lac du Thyl due to methodological or taphonomical problems. Two of the measurements were inferred from macroremains that were too old (reworking), and 2 were derived from bulk sediments containing old carbon from the coal-rich bedrock (Permo-Carboniferous schists and sandstones; Service géologique national, 1988). The measurements on bulk sediment should have been done on total organic carbon. The chronology of Lac du Thyl shows a break in the sedimentation rate, revealing a likely sedimentary hiatus from 3900 to 1600 cal. BP, although no change is evident by visual observation of the sediments. This hiatus could be confirmed with palynomorphs analysis. To compare the 2 ponds, we kept only the Holocene sequence for Lac du Lait, and present the results for the period 10 200– 0 cal. BP (Figure 1b). Pollen and macrofossil data Lac du Thyl Seven pollen zones were recognized (see Figure 1a). The pollen assemblages from 8900 to 8600 cal. BP (zone 1) are characterized by Pinus, Betula, and herbs (Apiaceae, Poaceae, Asteraceae, Rumex, Filipendula), and the macrofossil record attests seeds and catkin scales of Betula. Around 8600 cal. BP (zone 2), the pollen percentages of Abies, Acer, and Alnus incana-type increase. In these first 2 zones 8 fire events are recorded. Around 7200 cal. BP (zone 3), Pinus cembra expands, according to the increase in pollen and macrofossil abundances. This date also corresponds to a rise in both Cyperaceae pollen percentages and macroremains influxes of Carex sp., sug-

gesting the multi-millennial progressive development of a marginal peat-forming wet meadow. This process, which lasts 700 y until 6500 cal. BP, precedes the disappearance of pollen and macroremains of Acer and Alnus incana-type (zone 4). The end of zone 4 (5500 cal. BP) is marked by replacement of microscopic algae by rhizopods and Carex radicels, indicating a transition from open waters to peat. This sedimentological change, which induces a drop in the input of detritical material from run-off, happens after one fire. This may explain the contemporaneous decrease in both pollen percentages and macroremains influx of Pinus cembra. The beginning of zone 5, around 5500 cal. BP, is marked by a new increase in pollen percentages of Pinus and a drop of Pinus cembra and other tree macroremains, associated with a decrease in pollen of Abies, Poaceae, and other herbs. Zone 6 (4900–3900 cal. BP) records, both in pollen and macroremains assemblages, the local disappearance of Betula and the extension of Carex meadows over the entire site. The replacement of rhizopods by Pediastrum boryanum between zones 6 and 7 indicates a return to semi-open waters, supporting the hypothesis of a sedimentary hiatus between 3900 and 1600 cal. BP. The late Holocene (zone 7) is characterized by fluctuating and declining pollen records of Pinus, Abies, Betula, and Apiaceae and a simultaneous increase in Picea, Alnus viridis-type, Fagus, anthropization indicators (cultural proxies: Juglans, Castanea, Plantago, Cannabinaceae, and cereals), and other herbs. This leads to a vegetation similar to that of the present for at least 1600 y. Lac du Lait Generally speaking, Lac du Lait recorded fewer macroremains than Lac du Thyl, probably resulting from its larger size. Four pollen zones were recognized (Figure 1b), with a homogeneous record in the first 3 zones. Between 10 200 and 8700 cal. BP (zone 1), Pinus, Abies, Betula, other trees, and Poaceae dominate the pollen percentages and very few macroremains are recorded. The beginning of zone 2, around 8700 y ago, shows a weak decrease in pollen percentages of Abies, Betula, and other trees, an increase in pollen percentages Pinus cembra and of other herbs, and expansion of Pinus cembra, Betula, and Carex macroremains. A moderate fire frequency, i.e., 9 fires within 4600 y, is recorded in this zone. Around 4100 cal. BP (zone 3), Abies and Alnus incana-type pollen percentages increase, as does fire frequency (13 fire events recorded within 2300 y). Finally, zone 4 (1800 cal. BP) is marked by a decrease in fire frequency (only 1 fire recorded), a decrease in Pinus, Abies, and Betula pollen percentages, and an increase in pollen percentages of Picea, Alnus-viridistype, other trees, anthropization indicators (cultural proxies: Juglans, Castanea, Plantago, Cannabinaceae, and cereals), Cyperaceae, and other herbs. The tree macroremains disappear at 1500 cal. BP, replaced by Cyperaceae. The last millennium is characterized by an increase in Cyperaceae macroremains and the replacement of Tetraedron and Pediastrum boryanum by Carex radicels, rhizopods, and Botryococcus, which reveal the expansion of peat-forming marginal communities. Cyperaceae dominance both in

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Figure 1. Simplified diagrams of pollen percentages, macrofossil influxes, palynomorph occurrences, and charcoal influxes at Lac du Thyl (a) and Lac du Lait (b). The taxa include only those reaching mean pollen percentages ≥ 1% on at least 1 of the 2 sites. Anthropization indicators include plants related to agro-pastoral practices at the regional scale: Juglans, Castanea, Plantago, Cannabinaceae, and cereals (Behre, 1981). Dashed lines separate the pollen zones. The fire reconstructions derive from Carcaillet et al. (2009). Charred particles > 0.0625 mm2 were tallied after sieving and bleaching at high resolution all along the cores according to Carcaillet et al. (2001). The peaks of charcoal accumulation rates (mm2·cm–3·y–1) are used as fire proxies after statistical analyses of frequency distribution of detrended charcoal series (Gavin et al., 2006; Carcaillet et al., 2007). These analyses enable the date of occurrence of each fire, and thus the number of fires for each pollen zone of the diagram, to be determined. The identified fires are indicated by arrows. 16

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terms of pollen and macroremains shows the development of subalpine tree-less meadows. Correspondence analyses Lac du Thyl The correspondence analysis (CA) performed on pollen assemblages (Figure 2a,b) distinguishes well the 7 zones described above. The F1/F2 factorial map depicts about 2/3 of the total variance of the assemblages (axis 1 [F1] ca. 32% and axis 2 [F2] ca. 30%), which is a consequential result. F1 shows on the positive side assemblages of zones 1 to 6, and on the negative side only zone 7 (Figure 2a), whereas the F2 axis distinguishes pollen assemblages rich in Pinus cembra and Cyperaceae (positive side) from the others on the negative side (Figure 2b). This indicates that the last few centuries of the chronology, characterized by pollen of Fagus, Picea, Alnus viridis-type, and anthropization indicators, are very different from the rest (Figure 2b). The position of zone 2 on the positive side of F1 and the negative side of F2, outside the general trajectory, is due to pollen assemblages particularly rich in Acer and Alnus

incana-type between 8600 and 7200 cal. BP. The CA clearly demonstrates the close link between these taxa and charcoal abundances (Figure 2b). Lac du Lait The CA (Figure 2c) distinguishes well only pollen zone 4, which features greater heterogeneity than the other zones. Axis 1 (F1) incorporated ca. 64% of the variance and axis 2 (F2) ca. 12%. F1 opposes zones 1, 2, and 3 to zone 4, indicating that the main change in vegetation recorded at this site during the Holocene period is the one that occurred 1800 y ago. The vegetation of zone 4 is characterized by Fagus, Picea, Alnus viridis-type, anthropization indicators, and Cyperaceae (Figure 2c-d). The position on the positive side of F1 of both Alnus incana-type and charcoal abundances highlights their close connection (Figure 2d). Statistical comparison of site trajectories The CA based on a single matrix of all pollen assemblages of Lac du Lait and Lac du Thyl (Figure 3) utilized

Figure 2. Correspondence analyses of Lac du Thyl a) assemblages; b) taxa and Lac du Lait c) assemblages; d) taxa. The zone numbers refer to the pollen zones described in Figure 1. Taxa are labelled as follows: Psyl, Pinus sylvestris-type; Pcem, Pinus cembra; Puni, unidentified Pinus; Abie, Abies; Pice, Picea; Betu, Betula; Qpub, Quercus pubescens-type; Acer, Acer; Ainc, Alnus incana-type; Avir, Alnus viridis-type; Auni, unidentified Alnus; Fagu, Fagus; Otre, Other trees; Anti, Anthropization indicators; Apia, Apiaceae; Aste, Asteraceae; Poac, Poaceae; Cype, Cyperaceae; Oher, Other herbs; Spor, Spores; Char, Charcoal.

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a large number of assemblages (90 for Lac du Thyl, 100 for Lac du Lait) and taxa (20). The first 2 axes of the CA correspond to ca. 58% of total variance, which is a solid statistical result. Figure 3a enables to distinguish the similarity and differences between the trajectories of the 2 sites. Before ca. 6500 cal. BP, Lac du Lait appears very stable and no significant trend is perceptible, whereas the assemblages of Lac du Thyl show a large variability. Lac du Thyl is characterized by assemblages rich in Acer and Alnus incanatype, whereas Lac du Lait is characterized by high Pinus percentages, including P. cembra (Figure 3b). About 6500 y ago, the pollen assemblages of Lac du Thyl change, indicating a transformation of the surrounding vegetation, and become more similar to those of Lac du Lait (Figure 3a). The sites present convergent trajectories only for the last 1600 y, indicating a similar controlling process during this period. The pollen assemblages characterizing the lateHolocene (the last 2000 y) on the right side of axis 1 show the arrival of Picea and Fagus in the valley, as indicated by pollen, suppression of trees, and the development of meadows in the subalpine belt (anthropization indicators, increase in Poaceae and Cyperaceae pollen, and macroremains) and the spread of a disturbance-resistant taxon, i.e., Alnus viridis-type (Figure 3b) along with fire suppression (Figure 1a,b; Thyl: 2 events; Lait: 1 event).

1800 cal. BP around Lac du Lait. We detected only 1 fire, dated at almost 8900 cal. BP, that occurred at the Lac du Lait site during the first millennia of the Holocene. This result suggests that fires did not occur around Lac du Lait, although evidence of fires was found at Lac du Thyl and in southward valleys based on charcoal from travertine (Ali et al., 2005; 2006). The low influx of wood charcoal recorded at Lac du Lait during the Early Holocene (Figure 1b) is therefore surprising, but it may be linked to the low abundance of Pinus cembra and Betula macroremains (Figure 1b). The charcoal we found may have resulted from extremely low-severity local fires in a low-tree-biomass ecosystem, or from the transportation of charred particles from a regional source area. The very small size of the particles found also suggests that they may have been the result of regional transportation (Tinner et al., 2006). The variability of fire regime at the scale of a single valley confirms the absence of a clear temporal pattern of fire dynamics in the western Alps (Carcaillet, 1998; Tinner et al., 1999; 2000; Stähli et al., 2006). It moreover implies that fires never spread over large areas within the studied zone, and that they depend on local processes, overlying the regional climate. Biomass composition and structure, topography, heterogeneity of substrates, and elevation all affect fire ignition and spread.

Discussion

Long-term vegetation dynamics Comparison of the 2 diagrams (Figure 1) and the CA (Figure 2) reveals extremely different pollen records, much more heterogeneous at Lac du Thyl than at Lac du Lait. This suggests a more local record at Lac du Thyl, which is the smallest site (Jacobson & Bradshaw, 1981); this is also suggested by the close similarity between pollen and macroremains assemblages (Figure 1a). The homogeneous pollen record at Lac du Lait (Figures 1b and 3a), on the other hand, may represent an averaged long-distance input, although it converges with the macroremains pattern. Comparison of the 2 diagrams and the CA also reveals differences in the composition and structure of the surrounding

Holocene fire dynamics Whereas fires are currently extremely rare in the subalpine belt of the western Alps due to forest suppression for agro-pastoral activities in recent centuries, the charcoal records (Figure 1) show that fires were periodically frequent during the Holocene. The records at the 2 sites appear very different, however. First, the charcoal influx is more regular at Lac du Lait, which suggests a more extended source area for pollen. Second, the timing of fire frequency changes is asynchronous between the 2 sites: the highest frequencies occurred between 8900 and 6500 cal. BP around Lac du Thyl and from 7000 to 5000 cal. BP and 4100 to

Figure 3. Synthetic correspondence analyses of Lac du Thyl and Lac du Lait a) assemblages; b) taxa. Taxa are identified as follows: Psyl, Pinus sylvestris-type; Pcem, Pinus cembra; Puni, unidentified Pinus; Abie, Abies; Pice, Picea; Betu, Betula; Qpub, Quercus pubescens-type; Acer, Acer; Ainc, Alnus incana-type; Avir, Alnus viridis-type; Auni, unidentified Alnus; Fagu, Fagus; Otre, Other trees; Anti, Anthropization indicators; Apia, Apiaceae; Aste, Asteraceae; Poac, Poaceae; Cype, Cyperaceae; Oher, Other herbs; Spor, Spores. 18

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plant communities. Between 8700 and 4100 cal. BP, the simultaneous decrease in Abies pollen percentages and appearance of Pinus cembra macroremains around Lac du Lait (Figures 1b; 2c,d; 3a,b) could be due to a densification of the pine canopy (Muller et al., 2006). The subsequent 1800-y period (zone 3) of slow increase observed in pollen percentages of Abies (but no macroremains) and Alnus incana-type (Figure 1b) in relation to fires (Table I) would then mark a canopy-opening phase. The positive relation of Abies to fires, which challenges the conclusions of Tinner et al. (2000), is probably an artefact due to canopy opening by fires, leading to a better Abies pollen record. The best record of Alnus incana-type is in agreement with its present-day pioneer ecology (Rameau et al., 1993). The dominant regional pollen input, which dilutes the local record, probably explains the constant elevated Pinus pollen percentage and the inconsistent positive correlation between fire events and Pinus cembra pollen percentages (Table I). The macroremains record, which reflects the local vegetation, shows that infrequent fires did not alter the local Pinus cembra population. Around 1000 cal. BP, the record of Carex radicels and rhizopods associated to the increasing Cyperaceae pollen percentages and macroremains depicts the progressive development of Cyperaceae peaty margins related to the terrestrialization of the pond (Korhola, 1995). Unlike Lac du Lait, Lac du Thyl records, between 8500 and 6500 cal. BP, an original community composed of Acer and Alnus incana-type and little abundance of cembra pine (Figures 1a; 2a,b; 3a,b). This community is evidenced to be anew in relation with the occurrence of frequent fires. (Figure 1a; Table I). The pollen record of Acer at Lac du Thyl, which closely matches its present-day upper limit (Rameau et al., 1993), is consistent with the charcoal from this taxon found in soil profiles at similar altitudes (Talon, Carcaillet & Thinon, 1998; Carcaillet & Brun, 2000). The significant correlation between Acer and charcoal abundances (Table I) shows that the local persistence of a maple stand over 2000 y results from frequent fire disturbances, as suggested by David & Barbero (2001). The CA notably reveals a clear opposition between Pinus cembra and charcoal, Acer, and Alnus incana-type on axis 2 (Figure 2b). The significant negative correlation of Pinus cembra with fire at Lac du Thyl (Table I) suggests that P. cembra was disadvantaged by fire (Figure 1a). The results could indicate that fires constrained pine populations around the site, by controlling their spatial structure. At 5500 cal. BP, the record of Carex radicels and rhizopods indicates the final closure of the Cyperaceae peat that had begun at least 1500 y earlier, as indicated by the increase in Cyperaceae pollen percentages and macroremains (Figure 1a). The fossil record, which earlier had resulted from both aerial and run-off inputs into open waters, shifts at this point to mainly aerial input because the peaty marginal belt filters

the run-off waters. This development may be the reason for the decrease in the macroremains record at that time. Macroremains were nevertheless recorded throughout the whole sequence (Figure 1a), so this filtering effect must have been limited, probably because of the small size of the pond (1300 m2). Charcoal and pollen grains, on the other hand, are transported by both run-off and wind, and the filtering effect is thought to have less influence on these particles. Consequently, the records should accurately reflect the general variations in fire frequency and vegetation during the second part of the Holocene. From 5500 cal. BP, the vegetation trajectories at the 2 sites, hitherto different, become similar (Figure 3a), with a closing of the pine canopy. It must be noted that the canopy closing at Lac du Thyl is dated 1000 y earlier in the study of David and Barbero (2001) than in our analysis. Local and regional-scale controlling processes Climate and human activities are regional-scale processes; the former transformed the Holocene vegetation dynamics and species migrations (de Beaulieu, Kostenzer & Reich, 1993), while the latter has profoundly modified the landscape (e.g., T. Nakagawa, unpubl. thesis). In our study, long-term analysis of the vegetation dynamics around the sites indicates similar histories, with a dominance of pine from the early mid-Holocene until almost 1800 cal. BP (Figure 1). The last 1800 y have seen a profound change of the vegetation structure (Figure 1) and composition (Figures 2 and 3), with a significant change in the plant cover (Figure 1). This period is characterized by a great heterogeneity (Figure 2a,c) and a similar trajectory for the 2 sites (Figure 3a), associated with Alnus viridis-type, Fagus, Picea, and cultural proxies (Figures 1; 2b,d; 3b). Alnus viridis-type is recorded almost exclusively during the last 2 millennia, which underlines its close association with human history (de Beaulieu, 1977; Muller, David & Wicha, 2000; Nakagawa, de Beaulieu & Kitagawa, 2000), especially phases of land-use abandonment since grazing and trampling may prevent regeneration and growth of green alder (Richard, 1990). Anthropogenic fragmentation of the landscape into small units and the clearing of woody communities for pastures caused the local treed ecosystems, dominated by Pinus cembra and Betula, to be replaced by grass-dominated meadows (Figure 1) throughout the Alps (e.g., de Beaulieu, 1977; Muller, David & Wicha, 2000; Muller et al., 2006; Nakagawa, de Beaulieu & Kitagawa, 2000; ). The establishment of pastures led to suppression of fires because of a lack of fuel and a decrease in woody population connectivity. Only 2 fires were recorded around Lac du Thyl and 1 around Lac du Lait during the last 2 millennia, whereas these sites experienced higher frequencies earlier during the Holocene (Figure 1). While fire is a long-term climate-sensitive disturbance (Power et al., 2008), the periods of high fire occurrence

Table I. Correlation analyses (r2 values) for Lac du Thyl and Lac du Lait. Significant correlations are indicated by * when P-values ≤ 0.05 and by ** when P-values ≤ 0.01. Thyl Lait

Pinus cembra – 0.15 ** 0.31 **

Total Pinus – 0.21 ** 0.29 **

Alnus viridis-type – 0.01 – 0.10 **

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Alnus incana-type 0.32 ** 0.08 **


Abies 0.06* 0.17 **

Acer 0.24 ** – 0.01

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were not synchronous (Figure 1). Nevertheless, the study highlights the close link between some taxa and fire occurrence, and it demonstrates the existence of common trajectories between the sites when vegetation is submitted to high fire frequency. Indeed, when fires became frequent (8900–6500 cal. BP at Lac du Thyl, 4000–1800 cal. BP at Lac du Lait), local pine abundance decreased, benefitting Alnus incana-type and Acer at Lac du Thyl and Alnus incana-type at Lac du Lait (Figure 1, David, 1995b; David & Barbero, 2001). The CA notably shows that the persistence of open environments is related to the regular occurrence of fires (Figure 2b,d), which is confirmed by the significant positive correlations between fire occurrence and the pollen percentages of Acer and Alnus incana-type (Table I). In spite of phases with frequent fires, Pinus cembra was the main dominant tree during the first part of the Holocene, as it was in the western Alps (Ali et al., 2005). The presence of cembra pine during the periods of high fire frequency, attested by both pollen and macrofossil records (Figure 1a,b), clearly shows its role as a major fuel for fire spread within the upper subalpine belt and as a key functional species explaining fire-regime variability. Moreover, our results indicate that the fires between 7200 and 4700 cal. BP at Lac du Lait did not suppress the pine forest. Finally, they suggest complex interactions between vegetation and fire, since vegetation trajectories partly depend on fire frequencies, while in turn fire patterns are highly dependent on vegetation structures (Turner et al., 1994). This shows the importance of studying past fire and vegetation histories and interactions in order to better understand the consequences of current environmental changes with the potential to act on fire risk and spread (Schumacher & Bugmann, 2006), notably fuel build-up linked to land-use abandonment (Motta & Lingua, 2005; Chauchard, Carcaillet & Guibal, 2007) and the increase in drought occurrence in southern Europe (Pal, Giorgi & Bi, 2004; Sheffield & Wood, 2008).

Conclusion Despite the fact that both subalpine sites have hosted conifer ecosystems since at least 8000 cal. BP, the 2 sites have had heterogeneous fire histories. Fires were probably not climatically controlled and were not controlled by large-scale vegetation dynamics. Nevertheless, fires have locally influenced the vegetation in terms of both composition, favouring Acer and Alnus incana-type, and structure, through the opening of Pinus cembra subalpine forests (Muller et al., 2006), thus promoting grass-dominated communities. The vegetation patterns of the 2 sites were somewhat similar, but their trajectories were not synchronous. Our study reveals that fires and human practices locally have impacted the vegetation trajectories, overshadowing the direct impact of climate. The predicted increase in fire risk for southern Europe in response to climate warming thus should not constitute a major threat for high altitude Pinus cembra forest ecosystems. Nevertheless, given the rate of current climate change, the temporal resolution of our study may be too weak to adequately represent the vegetation response to fires. We stress the need for further studies and improvements in the analysis of short-term vegetation 20

responses to fires to increase our knowledge of the link between fire and subalpine ecosystems. Acknowledgements Financial support was provided by the Institut National des Sciences de l'Univers (INSU-CNRS, France), the national program ECCO (to C. Carcaillet), and a research allocation from the French Ministère de l'Enseignement Supérieur et de la Recherche (to A. Genries). We offer our grateful thanks to A. Ali, S. Ivorra, F. Roiron, and B. Vannière for field assistance and to J. Ferrier and P. Schevin for their help during the laboratory work. This publication is contribution ISE-M n° 2008-053.

Literature cited Ali, A. A., C. Carcaillet, B. Talon, P. Roiron & J.-F. Terral, 2005. Pinus cembra L. (arolla pine), a common tree in the inner French Alps since the early Holocene and above the present tree line: A synthesis based on charcoal data from soils and travertines. Journal of Biogeography, 32: 1659–1669. Ali, A. A., M. Martinez, N. Fauvart, P. Roiron, G. Fioraso, J.-L. Guendon, J.-F. Terral & C. Carcaillet, 2006. Incendies et peuplements à Pinus mugo Turra dans les Alpes occidentales (Val de Suse, Italie) durant la transition Tardiglaciaire–Holocène: une zone refuge évidente. Comptes Rendus Biologies, 329: 494–501. Behre, K. E., 1981. The interpretation of anthropogenic indicators in pollen diagrams. Pollen et Spores, 23: 225–245. Berggren, G., 1969. Atlas of Seeds and Small Fruits of NorthwestEuropean Plant Species, Part 2, Cyperaceae. Swedish Museum Natural History, Stockholm. Bond, W. J., F. I. Woodward & G. F. Midgley, 2005. The global distribution of ecosystems in a world without f ire. New Phytologist, 165: 525–538. Burga, C. A., 1988. Swiss vegetation history during the last 18,000 years. New Phytologist, 110: 581–602. Carcaillet, C., 1998. A spatially precise study of Holocene fire history, climate and human impact within the Maurienne valley, North French Alps. Journal of Ecology, 86: 384–396. Carcaillet, C. & J.-J. Brun, 2000. Changes in landscape structure in the northwestern Alps over the last 7000 years: Lessons from soil charcoal. Journal of Vegetation Science, 11: 705–714. Carcaillet, C. & S. D. Muller, 2005. Holocene tree-limit and distribution of Abies alba in the inner French Alps: Anthropological or climatic changes? Boreas, 34: 1–9. Carcaillet, C., A. A. Ali, O. Blarquez, A. Genries, B. Mourier & L. Bremond, 2009. Spatial variability of fire history in subalpine forests: From natural to cultural regimes. Écoscience, 16: 1–12. Carcaillet, C., Y. Bergeron, P. J. H. Richard, B. Fréchette, S. Gauthier & Y. T. Prairie, 2001. Change of fire frequency in the eastern Canadian boreal forests during the Holocene: Does vegetation composition or climate trigger the fire regime? Journal of Ecology, 89: 930–946. Carcaillet, C., I. Bergman, S. Delorme, G. Hornberg & O. Zackrisson, 2007. Long-term fire frequency not linked to prehistoric occupations in northern Swedish boreal forest. Ecology, 88: 465–477. Chauchard, S., C. Carcaillet & F. Guibal, 2007. Patterns of landuse abandonment control tree-recruitment and forest dynamics in Mediterranean mountains. Ecosystems, 10: 936–948. David, F., 1993. Altitudinal variation in the response of the vegetation to Late-glacial climatic events in the northern French Alps. New Phytologist, 125: 203–220.

©Écoscience



David, F., 1995a. Mise en place des forêts d'altitude en Vanoise et périphérie. Travaux Scientifiques du Parc National de la Vanoise, 19: 91–106. David, F., 1995b. Vegetation dynamics in the northern French Alps. Historical Biology, 9: 269–295. David, F. & M. Barbero, 1995. De l'histoire du genre Betula dans les Alpes françaises du nord. Review of Palaeobotany and Palynology, 89: 455–467. David, F. & M. Barbero, 2001. Les érables dans l'étage subalpin: une longue histoire. Comptes Rendus de l'Académie des Sciences de Paris, Série Sciences de la Vie, 324: 159–164. de Beaulieu, J.-L., 1977. Contribution pollenanalytique à l'histoire tardiglaciaire et holocène des Alpes méridionales françaises. Thèse d'état, Université Aix-Marseille 3, Marseille. de Beaulieu, J.-L., J. Kostenzer & K. Reich, 1993. Dynamique forestière holocène dans la haute vallée de l'Arve (Haute Savoie) et migrations de Abies et Picea dans les Alpes occidentales. Dissertationes Botanicae, 196: 387–398. Fischlin, A., G. F. Midgley, J. T. Price, R. Leemans, B. Gopal, C. Turley, M. D. A. Rounsevell, O. P. Dube, J. Tarazona & A. A. Velichko, 2007. Ecosystems, their properties, goods, and services. Pages 211–272 in M. L. Parry, O. F. Canziani, J. P. Palutikof, P. J. van der Linden & C. E. Hanson (eds.). Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. Gavin, D. G., F. S. Hu, K. Lertzman & P. Corbett, 2006. Weak climatic control of stand-scale fire history during the late Holocene. Ecology, 87: 1722–1732. Genries, A., L. Mercier, M. Lavoie, S. D. Muller, O. Radakovitch & C. Carcaillet, 2009. Role of fire frequency in local alteration of cembra pine populations. Ecology, 90: 476–486. Gimmi, U., M. Bürgi & M. Stuber, 2008. Reconstructing anthropogenic disturbance regimes in forest ecosystems: A case study from the Swiss Rhone Valley. Ecosystems, 11: 113–124. Grace, J., F. Berninger & L. Nagy, 2002. Impacts of climate change on the tree line. Annals of Botany, 90: 537–544. Jacobson, G. L., Jr. & R. H. W. Bradshaw, 1981. The selection of sites for paleovegetational studies. Quaternary Research, 16: 80–96. Korhola, A., 1995. Lake terrestrialization as a mode of mire formation: A regional review. Publications of the National Board of Waters and the Environment, A 207: 11–21. Lynch, J. A., J. L. Hollis & F. S. Hu, 2004. Climatic and landscape controls of the boreal forests fire regime: Holocene records from Alaska. Journal of Ecology, 92: 477–489. Motta, R. & E. Lingua, 2005. Human impact on size, age, and spatial structure in a mixed European larch and Swiss stone pine forest in the Western Italian Alps. Canadian Journal of Forest Research, 35: 1809–1820. Muller, S. D., F. David & S. Wicha, 2000. Impact de l'exposition des versants et de l'anthropisation sur la dynamique forestière dans les Alpes du Sud (France). Géographie physique et Quaternaire, 54: 231–243. Muller, S. D., T. Nakagawa, J.-L. de Beaulieu, M. Court-Picon, S. Fauquette & A. Genries, 2006. Paléostructures de végétation à la limite supérieure des forêts dans les Alpes françaises internes. Comptes Rendus Biologies, 329: 502–511. Muller, S. D., T. Nakagawa, J.-L. de Beaulieu, M. Court-Picon, C. Carcaillet, C. Miramont, P. Roiron, C. Boutterin, A. A. Ali & H. Bruneton, 2007. Post-glacial migration of silver fir (Abies alba Mill.) in the south-western Alps. Journal of Biogeography, 34: 876–899.

Nakagawa, T., J.-L. de Beaulieu & H. Kitagawa, 2000. Pollenderived history of timber exploitation from the Roman period onwards in the Romanche valley, central French Alps. Vegetation History and Archaeobotany, 9: 85–89. Overpeck, J. T., D. Rind & R. Goldberg, 1990. Climate-induced changes in forest disturbance and vegetation. Nature, 343: 51–53. Ozenda, P., 1985. La végétation de la chaîne alpine dans l'espace montagnard européen. Masson, Paris. Pal, J. S., F. Giorgi & X. Bi, 2004. Consistency of recent European summer precipitation trends and extremes with future regional climate projections. Geophysical Research Letters, 31: L13202.1-L13202.4. Power, M. J., J. Marlon, N. Ortiz, P. J. Bartlein, S. P. Harrison, F. E. Mayle, A. Ballouche, R. H. W. Bradshaw, C. Carcaillet, C. Cordova, S. Mooney, P. I. Moreno, I. C. Prentice, K. Thonicke, W. Tinner, C. Whitlock, Y. Zhang, Y. Zhao, A. A. Ali, R. S. Anderson, R. Beer, H. Behling, C. Briles, K. J. Brown, A. Brunelle, M. Bush, P. Camill, G. Q. Chu, J. Clark, D. Colombaroli, S. Connor, A.-L. Daniau, M. Daniels, J. Dodson, E. Doughty, M. E. Edwards, W. Finsinger, D. Foster, J. Frechette, M.-J. Gaillard, D. G. Gavin, E. Gobet, S. Haberle, D. J. Hallett, P. Higuera, G. Hope, S. Horn, J. Inoue, P. Kaltenreider, L. Kennedy, Z. C. Kong, C. Larsen, C. J. Long, J. Lynch, E. A. Lynch, M. McGlone, S. Meeks, S. Mensing, G. Meyer, T. Minckley, J. Mohr, D. M. Nelson, J. New, R. Newnham, R. Noti, W. Oswald, J. Pierce, P. J. H. Richard, C. Rowe, M. F. Sanchez Go–i, B. J. Shuman, H. Takahara, J. Toney, C. Turney, D. H. Urrego-Sanchez, C. Umbanhowar, M. Vandergoes, B. Vannière, E. Vescovi, M. Walsh, X. Wang, N. Williams, J. Wilmshurst & J. H. Zhang, 2008. Changes in fire regimes since the Last Glacial Maximum: An assessment based on a global synthesis and analysis of charcoal data. Climate Dynamics, 30: 887–907. Rameau, J.-C., D. Mansion, G. Dumé, A. Lecointe, J. Timbal, P. Dupont & R. Keller, 1993. Flore forestière française, Guide écologique illustré. 2, Montagnes. Institut pour le Développement forestier, Ministère de l'Agriculture et de la Pêche, Direction de l'Espace rural et de la Forêt, École nationale du Génie rural, des Eaux et des Forêts, Paris. Reille, M., 1995–1999. Pollen et Spores d'Europe et d'Afrique du Nord. Laboratoire de Botanique Historique et Palynologie, Marseille. Reimer, P. J., M. G. L. Baillie, E. Bard, A. Bayliss, J. W. Beck, C. J. H. Bertrand, P. G. Blackwell, C. E. Buck, G. S. Burr, K. B. Cutler, P. E. Damon, R. L. Edwards, R. G. Fairbanks, M. Friedrich, T. P. Guilderson, A. G. Hogg, K. A. Hughen, B. Kromer, F. G. McCormac, S. W. Manning, C. B. Ramsey, R. W. Reimer, S. Remmele, J. R. Southon, M. Stuiver, S. Talamo, F. W. Taylor, J. van der Plicht & C. E. Weyhenmeyer, 2004. IntCal04 Terrestrial radiocarbon age calibration, 26–0 ka BP. Radiocarbon, 46: 1029–1058. Richard, L., 1990. Écologie des mégaphorbiaies subalpines à aune vert de la Vanoise et des régions environnantes (première partie) - compréhension de la répartition actuelle des aulnaies. Travaux Scientifiques du Parc National de la Vanoise, 17: 127–158. Schär, C., P. L. Vidale, D. Lüthi, C. Frei, C. Häberli, M. A. Liniger & C. Appenzeller, 2004. The role of increasing temperature variability in European summer heatwaves. Nature, 427: 332–336. Schoch, W. H., B. Pawlik & F. H. Schweingruber, 1988. Botanishe Makroreste. Paul Haupt, Berne. Schumacher, S. & H. Bugmann, 2006. The relative importance of climatic effects, wildfires and management for future forest landscape dynamics in the Swiss Alps. Global Change Biology, 12: 1435–1450.

©Écoscience


21


Service géologique national, 1988. Carte géologique de la France à 1/50 000, Modane 775, Feuille 3534. Ministère de l'Industrie, des Postes et Télécommunications et du Tourisme, BRGM, Orléans. Sheffield, J. & E. F. Wood, 2008. Projected changes in drought occurrence under future global warming from multi-model, multiscenario, IPCC AR4 simulations. Climate Dynamics, 31: 79–105. Stähli, M., W. Finsinger, W. Tinner & B. Allgöwer, 2006. Wildfire history and fire ecology of the Swiss National Park (Central Alps): New evidence from charcoal, pollen and plant macrofossils. The Holocene, 16: 805–817. StatSoft France, 2001. STATISTICA version 6. StatSoft France, Maisons-Alfort. Stuiver, M. & P. J. Reimer, 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon, 35: 215–230. Talon, B., C. Carcaillet & M. Thinon, 1998. Études pédoanthracologiques des variations de la limite supérieure des arbres au cours de l'Holocène dans les Alpes françaises. Géographie physique et Quaternaire, 52: 195–208. Tinner, W., B. Ammann & P. Germann, 1996. Treeline fluctuations recorded for 12,500 years by soil profiles, pollen, and plant macrofossils in the central Swiss. Arctic and Alpine Research, 28: 131–147. Tinner, W., P. Hubschmid, M. Wehrli, B. Ammann & M. Conedera, 1999. Long-term forest fire ecology and dynamics in southern Switzerland. Journal of Ecology, 87: 273–289. Tinner, W., M. Conedera, E. Gobet, P. Hubschmid, M. Wehrli & B. Ammann, 2000. A palaeoecological attempt to classify fire sensivity of trees in the southern Alps. Holocene, 10: 565–574.

22

Tinner, W., M. Conedera, B. Ammann & A. F. Lotter, 2005. Fire ecology north and south of the Alps since the last ice age. The Holocene, 15: 1214–1226. Tinner, W., S. Hofstetter, F. Zeugin, M. Conedera, T. Wohlgemuth, L. Zimmermann & R. Zweifel, 2006. Long-distance transport of macroscopic charcoal by an intensive crown fire in the Swiss Alps: Implications for fire history reconstruction. Holocene, 16: 287–292. Turner, M. G., W. W. Hargrove, R. H. Gardner & W. H. Romme, 1994. Effects of fire on landscape heterogeneity in Yellowstone National Park, Wyoming. Journal of Vegetation Science, 5: 731–742. Tutin, T. G., N. A. Burges, A. O. Chater, J. R. Edmonson, V. H. Heywood, D. H. Moore, D. H. Valentine, S. M. Walters & D. A. Webb, 1968–1993. Flora Europaea. Vols. 2–5, Vol. 1. 2nd Edition. Cambridge University Press, Cambridge. Vorren, K.-D., B. Mørkved & S. Bortenschlager, 1993. Human impact on the Holocene forest line in the Central Alps. Vegetation History and Archaeobotany, 2: 145–156. White, P. S. & S. T. A. Pickett, 1985. Natural disturbance and patch dynamics: An introduction. Pages 3–13 in S. T. A. Pickett & P. S. White (eds.). The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, New York, New York. Wick, L., 1994. Early Holocene reforestation and vegetation change at a lake near the alpine forest limit: Lago Basso (2250 m asl), northwestern Italy. Dissertationes Botanicae, 234: 555–563. Willis, K. J., M. Braun, P. Sümegi & A. Toth, 1997. Does soil change cause vegetation change or vice versa? Ecology, 73: 740–750. Wright, H. E., 1974. Landscape development, forest fires, and wilderness management. Science, 186: 487–495.

©Écoscience
