asselin payette ecoscience 06

13 (2): 135-142 (2006)

The forest-tundra biome is a large ecotone between the boreal forest to the south and the treeless tundra to the north (Payette, 1983). The basic landscape unit of the for-est-tundra is a typical vegetation toposequence including a hilltop covered with shrubs and lichens, mesic lichen wood-lands midslope, and wet spruce–moss stands (sometimes peatlands) downslope (Payette, Fortin & Gamache, 2001). The contact between a forested valley and a tundra-cov-ered hilltop is called a subarctic tree line (Payette, Fortin & Gamache, 2001). In northern Québec, the landscape open-ing that led to the creation of the forest-tundra can be attrib-uted to post-fire black spruce (Picea mariana) regeneration

failure under the cool climate of the last 3000 y (Payette & Gagnon, 1985; Asselin & Payette, 2005a). The sum of growing degree-days > 5 °C in the northern forest-tundra is usually insufficient for black spruce to produce viable seeds (Sirois, 2000), and populations are thus maintained by layer-ing (Laberge, Payette & Pitre, 2001). Hence, black spruce is presently in disequilibrium with climate in northern Québec (Davis, 1986; Arseneault & Payette, 1997).

Temperatures in northern Québec have been rising since the end of the Little Ice Age (Payette et al., 1985) and are expected to continue to rise because of the increased greenhouse effect caused by anthropogenic overuse of fossil fuels (IPCC, 2001). Under climate warming, it is assumed that black spruce krummholz (more or less stunted individu-als) that are still present on slopes and in valley bottoms of the forest-tundra will develop tree growth forms (single, erect stems taller than 2.5 m). This would increase viable seed production as seeds are born on upper branches (Bégin & Filion, 1999). Subarctic tree lines would eventually

Origin and long-term dynamics of a subarctic tree line1

Hugo ASSELIN2 & Serge PAYETTE, NSERC Northern Research Chair, Centre d’études nordiques, Université Laval, Québec, Québec G1K 7P4, Canada.

Abstract: The basic unit of the forest-tundra landscape is a toposequence extending from a wet, forested valley to a xeric, deforested hilltop; the contact zone between these two environments being called a subarctic tree line. Dendrochronological analysis of living, dead, and subfossil black spruce, and radiocarbon dating of peat samples were used to reconstruct the dynamics of a subarctic tree line since its post-fire origin about 1000 y ago. Fire is not the sole disturbance to have influenced the dynamics of the toposequence. A regional-scale flooding event ca. 1120 AD killed many black spruce trees, growth of permafrost during the Little Ice Age, and its subsequent degradation in the 20th century, also had major consequences. The climate was favourable to black spruce growth between ca. 300 and 1100 AD, as evidenced by large growth rings and tree growth forms. Ring widths then decreased markedly between the 12th and 19th centuries and trees were replaced by stunted growth forms. Although climate warming during the 20th century resulted in increased ring widths, black spruces have still not produced tree growth forms, a necessary condition for viable seed production and eventual re-colonization of deforested hilltops.Keywords: black spruce, climate change, dendrochronology, fire, flooding, permafrost, Picea mariana, subarctic tree line, toposequence.

Résumé : L’unité de base du paysage de la toundra forestière est une toposéquence incluant un bas de pente humide et boisé et un sommet de colline sec et déboisé. La zone de contact entre ces deux milieux constitue une limite d’arbres subarctique. L’analyse dendrochronologique d’épinettes noires vivantes, mortes et subfossiles et la datation au radiocarbone d’échantillons de tourbe ont été utilisées afin de reconstituer la dynamique d’une limite d’arbres subarctique depuis son origine post-incendie il y a environ 1000 ans. Le feu n’est pas la seule perturbation à avoir affecté la dynamique de la toposéquence. Une inondation d’ampleur régionale survenue vers 1120 après J.-C. a causé la mort de plusieurs épinettes noires. La formation du pergélisol durant le Petit Âge Glaciaire et sa dégradation subséquente au XXe siècle ont également eu des conséquences majeures. Le climat a été favorable à la croissance de l’épinette noire entre environ 300 et 1100 après J.-C., tel que démontré par les cernes de croissance larges et les formes de croissance érigées. La largeur des cernes de croissance a ensuite diminué de façon appréciable entre les XIIe et XIXe siècles et les arbres ont été remplacés par des formes de croissance prostrées. Même si le réchauffement climatique du XXe siècle a résulté en une augmentation de la largeur des cernes de croissance, les épinettes noires n’ont toujours pas produit de forme de croissance érigée, une condition sine qua non à la production de graines viables et à la re-colonisation éventuelle des sommets de collines déboisés.Mots-clés : changement climatique, dendrochronologie, épinette noire, feu, inondation, limite d’arbres subarctique, pergélisol, Picea mariana, toposéquence.

Nomenclature: Crum & Anderson, 1981; Marie-Victorin, 1995; Brodo, Sharnoff & Sharnoff, 2001.

Introduction

1Rec. 2004-12-09; acc. 2005-10-04. Guest Editor: Yves Bergeron.2Author for correspondence. Present address: Chaire industrielle CRSNG-UQAT-resent address: Chaire industrielle CRSNG-UQAT-UQAM en aménagement forestier durable, Université du Québec en Abitibi-Témiscamingue, 445 boulevard de l’Université, Rouyn-Noranda, Québec J9X 5E4, Canada, e-mail: hugo.asselin�uqat.ca, e-mail: hugo.asselin�uqat.ca e-mail: hugo.asselin�uqat.caca

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Asselin & PAyette: Origin And dynAmics Of A subArctic tree line

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move upslope (Gamache & Payette, 2005) and re-colonize hilltops that shifted from forest to tundra over the last cen-turies (Arseneault & Payette, 1992). However, black spruce growth at the tree line and the ability of the species to colo-nize new habitats are not only influenced by the regional climate, but also by local factors like permafrost growth and decay (Camill & Clark, 1998; Osterkamp et al., 2000), snow depth (Vaganov et al., 1999), wind exposure (Daly & Shankman, 1985; Scott, Hansell & Erickson, 1993), cata-strophic floods (Payette & Delwaide, 2004), and seedbed suitability (Cowles, 1982; Houle & Filion, 2003). Although subarctic tree lines and their associated toposequences form the basic unit of the forest-tundra landscape (Payette, Fortin & Gamache, 2001), little is known about their dynamics from their post-fire origin to present times.

We used tree-ring analysis and radiocarbon dating of charcoal and peat samples to reconstruct the dynamics of a typical forest-tundra toposequence (peatland–mesic slope with black spruce–deforested hilltop) that developed follow-ing fire, ca. 1000 y ago. Tree-ring analysis of living, dead, and subfossil black spruce sampled in the peatland and on adjacent slopes was used to decipher the respective roles of and interactions between different climatic and environmen-tal variables in the secular dynamics of the subarctic tree line. Emphasis was also put on the links between the differ-ent constitutive zones of the toposequence.

Methodsstudy site

The study site is located in the Rivière Boniface area of northern Québec (57° 45' n, 76° 20' w), about 30 km east of Hudson Bay and 10 km south of the latitudinal tree line. The nearest weather station is located in Inukjuak (58° 28' n, 78° 04' w), about 125 km northwest of the study area. The mean annual temperature is -7.0 °C, and mean annual precipitation is ca. 460 mm, of which 40% falls as snow (Environment Canada, 2004).

The study site consists of three low, elongated hills sur-rounding a permafrost peatland (Figure 1). Two different zones can be distinguished in the peatland: an ombrotrophic zone in the centre, and a minerotrophic zone at the periphery (Figure 1). The ombrotrophic zone (a thawing palsa pla-teau) shows a chaotic pattern of hummocks and hollows with Dicranum and Sphagnum species, ericaceous shrubs, and stunted black spruce. The minerotrophic zone (a poor fen) is characterized by the predominance of Eriophorum and Carex species. Black spruce stands covering the hill-tops were eliminated by fire between 840 and 1080 AD (S. Payette, in prep.; Table I). Black spruce is absent from the hilltops, the species being relegated to the mesic slopes (high krummholz) and to the hummocks zone of the peat-land (low krummholz). Only two individuals could be quali-fied as trees (i.e., single, erect stem taller than 2.5 m) in the entire study site (ca. 24 ha), and they were growing close to each other in a well-protected microsite. In summary, the studied toposequence is composed of the following ele-ments (all mapped in the field using an infrared theodolite [Leica T1010, 0.0005-m precision, Leica Geosystem AG,

St. Gallen, Switzerland]): (1) a hummocks zone, (2) a poor fen, (3) mesic slopes with high spruce krummholz, and (4) hilltops covered with ericaceous shrubs, dwarf birch (Betula glandulosa), and lichens (mostly Cladina spp.) (Figures 1 and 2).

sAmPling

A 60-m trench was dug across the toposequence in order to describe soil profiles and to take peat samples for radiocarbon dating (Figures 1 and 2). Additional peat sam-ples for radiocarbon dating were taken along two other tran-sects (Figure 1). Living, dead, and subfossil black spruces were sampled from the different zones of the toposequence. Disks were sampled from six living and seven dead black spruces in the hummocks zone of the peatland and from 10 living and 23 dead individuals on mesic slopes. Many dead black spruces were found in the northwestern portion of the fen. A total of 40 disks were sampled from 20 individuals of the northwestern fen. Subfossil black spruces were sampled within a 700-m2 portion of the southeastern fen adjacent to the soil trench (47 samples) and from the bottom of a small pond north of hill 101 that was drained with a water pump prior to sampling (54 samples). Growth form (tree or krummholz) was described for each sampled black spruce.

figure 1. Schematic view of the study site showing the different zones of the toposequence. Living, dead, and subfossil black spruce were sampled in the southwestern pond (SW pond), in the southeastern and northwestern parts of the fen (SE fen and NW fen), and throughout the hummocks zone and the mesic slopes. Also shown are radiocarbon dates obtained from basal/upper sedge peat samples (Table I) and the position of the soil trench (A-B; Figure 2). The inset shows the location of the Rivière Boniface area in northern Québec.



rAdiOcArbOn dAting

Charcoal fragments were sampled below the lichen car-pet on the three hilltops and radiocarbon-dated in order to determine the time of fire occurrence (S. Payette, in prep.). Peat inception dates from different points in the peatland were inferred by radiocarbon-dating basal sedge peat, enabling us to reconstruct the lateral expansion of the peat-land. Dates of the permafrost aggradation that caused the rising of the palsa plateau were based on radiocarbon-dating of the uppermost hygrophilous sedge peat, at the contact with Sphagnum peat (Seppälä, 1988; Allard & Séguin, 1987a,b; Vasil’chuk & Vasil’chuk, 1998; Payette, 2001a). All radiocarbon dates were calibrated using version 4.4 of the CALIB program (Stuiver & Reimer, 1993) (Table I). To facilitate comparisons with tree-ring dates, radiocarbon dates are given in years BC/AD.

dendrOchrOnOlOgicAl AnAlysis

All black spruce wood samples were air-dried and sand-ed for growth ring dating and measurement. Cross-dating of the samples was done at 40X under a dissecting microscope, using diagnostic light rings, i.e., growth rings almost exclu-sively composed of earlywood cells, with very few layers of latewood cells (Filion et al., 1986). Light rings are caused by low summer temperatures induced by delayed springs, cool summers, or early-ending autumns (Yamaguchi, Filion

& Savage, 1993; Wang, Payette & Bégin, 2000). Two light-ring chronologies were used: a solid chronology covering 690 AD to present and a floating chronology, radiocarbon dated between 178 BC and 785 AD (Lavoie & Payette, 1997; Arseneault & Payette, 1997; 1998). Samples from the present study allowed us to fill the gap between the chronologies, yielding a 2184-y chronology of black spruce growth at tree line (Asselin et al., in prep.). Tree ring widths were measured at 40X under a dissecting microscope, using a Velmex micrometer (± 0.002 mm) linked to a computer for data recording.

It is commonplace in dendrochronological analysis to first remove age-related biological growth trends by detrending (or standardization) before pooling the series in a common chronology (Schweingruber, 1996). This ensures that the remaining variation can be mostly attrib-uted to climate. However, it has been shown that detrending techniques remove most of the low-frequency (long-term) variation when mean segment length is short relative to total chronology length (Cook et al., 1995). The regional curve standardization (RCS) technique has been suggested to preserve low-frequency variability (Briffa et al., 1992; Esper, Cook & Schweingruber, 2002). This method works by removing the same (regional) growth trend from all series, instead of computing a different growth trend for each series. However, the RCS technique requires a large

ÉcOscience, vOl. 13 (2), 2006

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tAble i. List of radiocarbon dates used in this study. Dates from soil charcoal fragments sampled below the lichen carpet on the three hill-tops (hill identification numbers as in Figure 1) give a maximum age of fire occurrence. Dates from basal and upper sedge peat samples (order of dates as in Figure 1 starting from upper left corner and going clockwise) give an estimation of the time of peat inception and palsa formation, respectively.

Site Lab. No. Dated material Measured Conventional date2 Median probability3 Date in radiocarbon age1 (y BP ± 1σ) (cal. y BP) years BC/ADhills101 Beta-35507 charcoal na 970 ± 60 870 1080 AD136 Beta-35515 charcoal na 1130 ± 50 1040 910 AD140 Beta-35519 charcoal na 1110 ± 50 1020 930 AD140 UL-2113 wood 1170 ± 60 1190 ± 70 1110 840 AD

PeAtlAnd UL-2338 basal peat 250 ± 60 220 ± 80 210 1740 AD UL-2337 basal peat 4490 ± 100 4460 ± 110 5100 3150 BC UL-2306 upper peat 360 ± 60 330 ± 80 380 1570 AD UL-2318 basal peat 2050 ± 70 2020 ± 90 1980 30 BC UL-2299 basal peat 2310 ± 60 2280 ± 80 2260 310 BC UL-2305 basal peat 900 ± 90 870 ± 100 800 1150 AD UL-2322 basal peat 1660 ± 70 1630 ± 90 1520 430 AD UL-2319 basal peat 2370 ± 70 2340 ± 90 2380 430 BC UL-2301 basal peat 2140 ± 90 2110 ± 100 2090 140 BC UL-2307 upper peat 640 ± 60 610 ± 80 600 1350 AD UL-2303 basal peat 3600 ± 70 3570 ± 90 3860 1910 BC UL-2298 basal peat 980 ± 60 950 ± 80 850 1100 AD UL-2281 basal peat 1220 ± 90 1190 ± 100 1110 840 AD UL-2292 basal peat 2030 ± 70 2000 ± 90 1950 0 AD UL-2293 upper peat 710 ± 60 680 ± 80 630 1320 AD UL-2285 basal peat 5510 ± 100 5480 ± 110 6250 4300 BC UL-2287 upper peat 650 ± 60 620 ± 80 600 1350 AD UL-2288 basal peat 5200 ± 100 5170 ± 110 5920 3970 BC UL-2282 basal peat 3550 ± 100 3520 ± 110 3800 1850 BC UL-2289 basal peat 4260 ± 100 4230 ± 110 4740 2790 BC1 Measured radiocarbon age is given only for Laboratoire de datation au 14C de l’Université Laval (UL) which does not normalize dates to account for isotopic

fractionation.2 Dates normalized for isotopic fractionation. Values of –27.0 ± 3 ‰ and –24.0 ± 2 ‰ were used to normalize UL dates obtained from peat and from charred

subfossil wood respectively (following Stuiver & Pollach, 1977). Dates from Beta Analytics (Beta) are conventional dates as provided by the laboratory.3 Calibrated median probability dates were obtained using CALIB 4.4 (Stuiver & Reimer, 1993) with the IntCal98 calibration dataset of Stuiver et al. (1998).




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sample set to be efficient (Briffa et al., 1992). Furthermore, krummholz do not show clear age-related growth trends and are sometimes standardized using a horizontal line (slope = 0). This is only recommendable when short time periods are studied or when all samples have grown concurrently. For these reasons, the chronologies discussed in the present study are presented as raw ring widths.

ResultsdynAmics Of the tOPOsequence

The radiocarbon dates obtained from soil charcoal frag-ments sampled below the lichen carpet on the three hilltops yielded ages of 910, 930, and 1080 AD for hilltops 136, 140, and 101, respectively (S. Payette, in prep.; Table I). A date of 840 AD was also obtained from a charred black spruce stem sampled in the southeastern portion of the fen, at the contact with the hilltop (S. Payette, in prep.; Table I). Characterization and measurement of hilltop soil horizons showed that the soils were poorly developed podzols, with 9 ± 5 cm of organic matter, a thin eluviated (Ae) horizon (3 ± 2 cm), and a lightly coloured illuviated (B) horizon (15 ± 12 cm) (soil nomenclature following Expert Committee on Soil Survey, 1987). The bedrock was some-times exposed or only covered with a thin layer of organic matter without mineral horizons. According to vegeta-tion surveys conducted on hilltop 140 (Asselin & Payette, 2005b), it is presently dominated by dwarf birch (Betula glandulosa), ericaceous shrubs (Vaccinium vitis-idaea and Ledum decumbens), lichens (mostly Cladina spp.), and sedges (mostly Carex bigelowii); these taxa accounted for more than 97% of the vegetation cover on the hilltop.

The mesic slopes marking the transition between the hilltops and the peatland were characterized by high black spruce krummholz and had more developed podzols than

those present on the hilltops. The organic, Ae, and B hori-zons were 7 ± 5, 4 ± 3, and 24 ± 11 cm thick, respectively. Furthermore, the eluviated horizon was paler, and the illuvi-ated horizon darker, than in hilltop soils.

According to radiocarbon dating of basal peat samples from the deepest parts of the peatland, peat started accumu-lating in 2790 BC in the southwestern pond (at that time a peatland) and in 4300 and 3150 BC in the southern and north-ern parts of the peatland, respectively (Figure 1; Table I). Other basal peat dates indicate that the peatland reached half its present size by ca. 30 BC and that maximum lateral expansion occurred between 430 and 1150 AD on the east-ern side and more recently (1740 AD) on the western side (Figure 1; Table I). Radiocarbon dating of the uppermost sedge peat beneath the Sphagnum carpet suggests that the formation of the palsa plateau (present hummock zone) occurred between 1320 and 1570 AD (Figure 1; Table I).

dendrOecOlOgicAl AnAlyses

The oldest black spruce individuals preserved in the peat established shortly before 300 AD in the southeastern fen and shortly after 400 AD in the southwestern pond area (Figure 3a,b). Ring width values for the period 278–1096 AD are almost always above the long-term average, and the growth curves obtained for the southeastern fen and the southwestern pond area show similar patterns for this period, except from 900 to 1096 AD (Figure 3a,b). The percentage of samples showing tree growth forms for the early portion of the record (278–1096) is always above 50%, except for five brief episodes: 548–573 (no sample), 641–693, 833–865, 961–993, and 1065–1096 (Figure 3g). No samples were found for the period 1097–1133 AD. Only krummholz lived in the southeastern fen and in the southwestern pond area between 1134 and 1535 (Figure 3g), and ring width values for this period are mostly below the long-term average (Figure 3a,b). From 1536 to present, no black spruce established in the minerotrophic part of the peatland or on the palsa that had formed in the southwestern pond area. However, black spruce krummholz colonized the newly formed palsa plateau starting around 1525 AD. Ring widths of these individuals reached the lowest values on the record in the first half of the 17th century (Figure 3c). Growth remained minimal and below the long-term aver-age until the 20th century, although an increasing trend is noticeable. Black spruce krummholz also established at the periphery of the fen (northwestern section) in the early 17th century. Although currently located in the fen, these spruces were rooted in mineral soil and produced adventitious roots. Ring width values were low during the 17th and 18th centuries, higher during the 19th century, and decreased again in the early 20th century (Figure 3d). These spruces were all dead by 1961. The oldest samples from the mesic slopes (all krummholz) date from 1580. Their growth was below the long-term average until ca. 1900 and has increased markedly since then (Figure 3e).

DiscussionhOlOcene dynAmics Of the tOPOsequence

According to the radiocarbon dates obtained from black spruce wood and soil charcoal fragments (S. Payette,

figure 2. Schematic view of the soil trench. Shown are radiocarbon dates obtained from basal and upper sedge peat samples (same dates as in Figure 1 and Table I). Vertical enhancement factor = ×10.




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in prep.; Table I), two fires probably occurred in the study area, one around 840–930 AD (hilltops 136 and 140) and the other around 1080 AD (hilltop 101). Taking into account the error associated with radiocarbon dating, it is impossible to determine if a single fire or two different events burned hilltops 136 and 140. The 840 AD date obtained from a charred black spruce stem is direct evidence that the fire that burned hilltop 140 reached the margin of the peatland in some areas. This is further evidenced by the presence of a charcoal layer between the organic and Ae soil horizons found beneath the high krummholz of the mesic slopes. Therefore, the fire margin was re-colonized to some extent from surviving individuals (located in portions of the mesic slopes where no charcoal was found between the organic and Ae soil horizons).

The Holocene dynamics of the studied peatland closely follow what is known from other northern Québec sites (Payette, 2001a,b). Peat started accumulating in the deepest parts of the peatland around 4300 BC, i.e., shortly after the retreat of the postglacial Tyrrell Sea (Lauriol & Gray, 1987). Similar dates of peat inception were obtained for other parts of northern Québec (Payette, 2001b). Accumulation continued for several hundred years, with peat expanding both vertically and laterally. By ca. 1150 AD, the peatland was almost at its present size (a small portion in the western area was absent). At that time the peatland was a treed fen, with no evidence of permafrost. Then, between 1320 and 1570 AD, permafrost developed in the peatland, forming a palsa plateau. The age range for permafrost aggradation in the studied peatland is comparable to those reported in other studies conducted in northern Québec (Payette & Séguin, 1979; Couillard & Payette, 1985; Allard & Séguin, 1987a,b; Gahé, Allard & Séguin, 1987; Payette & Delwaide, 2004) and elsewhere in the northern hemisphere (Van Vliet-Lanoë, Bourgeois & Dauteuil, 1998; Vasil’chuk & Vasil’chuk, 1998). Starting ca. 1740 AD, wetter conditions favoured peat expansion on the western side of the peatland. Greater humidity can be attributed to increased winter precipita-tion from 1750 AD to present (Payette & Delwaide, 2004). Climate warming starting in the mid-1800s induced thawing of the palsa plateau, the four ponds present in the peatland being direct evidence of recent permafrost degradation. This phenomenon is widespread in northern Québec (Allard & Séguin, 1987b; Allard, Séguin & Lévesque, 1987; Payette, 2001a; Payette et al., 2004) and elsewhere in the northern hemisphere (Osterkamp et al., 2000; Luoto & Seppälä, 2003; Agafonov, Strunk & Nuber, 2004).

dendrOecOlOgicAl recOnstructiOn Of the tOPOsequence dynAmics

Trees grew in the peatland (at that time a poor fen) between 278 and 1096 AD (Figure 3a,b). No sample (either tree or krummholz) was found for the period 548–573 AD. It is not possible to determine if this is due to a mass-mor-tality event or a sampling artifact, as samples are less abun-dant in the earliest portion of the chronology. Nevertheless, climatic conditions were favourable to tree growth between 278 and 1096 AD, as shown by high ring width values both in the southeastern fen and in the southwestern pond area.

Tree-ring curves for the southeastern fen and the south-western pond area suddenly diverged between 900 and 1096 AD (Figure 3a,b). This can be explained by the fact that hill 140 burned earlier than hill 101 (Table I) and is a further confirmation that these were separate fire events. Post-fire regeneration failure on hilltop 140 caused excess snow to accumulate in the peatland, as it was no longer trapped by black spruce on the hilltop. It is possible that this excess snow took a longer time to melt in the spring, keeping the soil frozen and thus causing a delayed growing season and reduced tree growth in the southeastern fen.

No sample was found in the peatland or in the pond for the period 1097–1133 AD. We hypothesize that this may be attributed to a mass-mortality event caused by a regional flooding episode that is known to have occurred between 1120 and 1155 AD in other sites of the Rivière Boniface area

figure 3. Black spruce growth curves (yearly values and 49-y low pass filter to show long-term trends) for the southwestern pond (a), the southeastern fen (b), the hummocks zone (c), the northwestern fen (d), and the mesic slopes (e). The long-term average of ring width values for all chronologies combined (0.237 mm) is indicated as a reference for each chronology (horizontal black line). Also shown is the number of samples included in each curve (f) and the percentage of samples showing a tree growth form (the remaining being krummholz) for each year of the record (g). The shaded areas represent the flooding event (1097–1133 AD) and the time of permafrost aggradation (1320–1570 AD). The arrows indicate the fires that burned hilltop 140 (930 AD) and 101 (1080 AD).




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(Payette & Delwaide, 2004). Black spruce re-colonized the southeastern fen and the southwestern pond area at the begin-ning of the 12th century (Figure 3a,b), probably meaning that the effects of the flood were no longer apparent. Krummholz grew in the southeastern fen and in the southwestern pond area between 1134 and 1535 AD. However, ring widths for this period are markedly thinner than during the preceding period (278–1096 AD). Another episode of massive death (after 1535 AD) can be attributed to the formation of the palsa plateau at the center of the peatland and a palsa in the southwestern pond area. According to the peat stratigraphy, permafrost growth started in the peatland between 1320 and 1570 AD (no date available for the pond area), favoured by the cold conditions prevailing during the first half of the Little Ice Age. The palsa plateau reached its full extension during the first half of the 16th century, drastically altering the drainage. Water accumulated in the surrounding (remaining) fen, killing black spruce krummholz growing there. Soil up-thrusting due to permafrost aggradation probably caused up-rooting of some individuals, a phenomenon likely to explain most of the mortality in the southwestern pond, where no spruce was found in growth position.

Soon after its formation, the palsa plateau was dry enough to be colonized by black spruce, which estab-lished in the 16th century and still grow in the present day (Figure 3c). Although ring widths have remained below the long-term average, a slight increasing trend is clearly visible and may be due in part to increased temperatures since the end of the Little Ice Age. Nevertheless, ring width values for the 20th century are far from being comparable to those of the period from 278 to 1096 AD (Figure 3). The palsa that once occupied the present location of the southwestern pond was not colonized by black spruce dur-ing recent centuries. Although it is difficult to pinpoint a specific explanation for the absence of colonization, it may be due to submersion of the palsa by rainwater. Before the formation of the palsa, rainwater coming from hilltop 101 fed the small peatland, and excess water drained to the river. It is likely that the palsa blocked the normal course of the water between hilltop 101 and the river, and water accumulated, rapidly covering the palsa. This hypothesis is supported by the absence of a Sphagnum peat cover on top of the sedge peat in the pond stratigraphy, as opposed to the palsa plateau stratigraphy.

The climate warming of the 20th century was accom-panied by wetter conditions. Increased wetness, which started ca. 1750 AD, is attributable to increased winter precipitation (Payette & Delwaide, 2004). Humid condi-tions favoured the lateral expansion of the western part of the peatland into the surrounding forest, gradually killing the spruces growing there and rooted in the mineral soil (Figure 3d). Interestingly, the long-term growth trends of spruce of the northwestern fen are the opposite of those of spruce growing in the hummocks zone (Figure 3c,d). This can be explained by the differential response of these indi-viduals to moisture fluctuations: increased moisture was detrimental to spruce growing in the peatland but beneficial to those growing at the periphery.

The black spruce growing on mesic slopes responded most strongly to the 20th century climate warming (Figure 3e),

although their ring width values do not yet exceed those characteristic of the period from 278 to 1096 AD. Moreover, only two trees (not sampled) are present in the study site today, while the majority of the samples of the period from 278 to 1096 AD were trees (Figure 3g). As black spruce krummholz can adopt a tree growth form in a few decades under warm climate (Payette et al., 1989), this confirms that conditions at the study site are not yet as favourable as they were between 278 and 1096 AD.

If climate warming continues, trees may eventually come back on the hilltops, a phenomenon already appar-ent in the southern part of the forest-tundra (Gamache & Payette, 2005). However, as thawing of permafrost will soon transform the peatland into a treeless poor fen, hilltop re-colonization will rely on tree-like spruce (high krumm-holz) presently growing on mesic slopes. These individuals must first adopt vertical (tree) growth forms and produce viable seeds (Sirois, 2000) for invasion of hilltops to pro-ceed. Seeds must then fall on suitable sites, i.e., mudboils or cracks in the lichen mat where mineral soil is surfacing (Cowles, 1982; Sirois, 1993; Houle & Filion, 2003).

AcknowledgementsMany thanks to M. Beauchemin, S. Champagne, and S. Vallée

for field assistance. Help from A. Delwaide and M.-C. Martel during lab work was greatly appreciated. This study was finan-cially supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Department of Indian Affairs and Northern Development, and the Ministère de la Recherche, de la Science et de la Technologie du Québec (FCAR program).

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