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The long-term stability of the boreal forest limit in subarctic Quebec.

INTRODUCTION

The most dramatic changes in climate projected by models of greenhouse warming are at high-latitude sites (Houghton et al. 1990). In particular, the arctic tree line and the nearby forest stands are typically cold-stressed ecosystems that are likely to experience pole-ward displacements in response to warming (Smith et al. 1992, Monserud et al. 1993). The positions of the arctic tree line and of the attendant marginal forests are considered reliable climatic markers at different time scales (Ritchie 1984, 1987, Payette and Lavoie 1994).

Several studies have attempted to reconstruct past forest fluctuations in the Yukon, Northwest Territories (N.W.T), and northern Quebec, Canada. There are pollen and macrofossil evidence of a spruce forest limit retreat ([approximately equal to]20-70 km) in central Yukon and the Tuktoyaktuk Peninsula (N.W.T) between 10 000 and 3500 yr BP (Ritchie and Hare 1971, Hyvarinen and Ritchie 1975, Ritchie et al. 1983, Spear 1983, 1993, Ritchie 1984, Cwynar and Spear 1991). Additionally, pollen, diatom, and geochemical data suggest a 25-km shift of the spruce forest limit north of its modern position between 5000 and 4000 yr BP (MacDonald et al. 1993) near Great Slave Lake (N.W.T). Other studies in Keewatin (N.W.T) have reported major movements (50-350 km) of the forest limit since 5000 yr BP (Sorenson et al. 1971, Nichols 1975, Kay 1979), but these reconstructions were based on few macrofossils (charcoal) or on pollen data from peat profiles. In northern Quebec, as a result of late deglaciation, the forests reached their northernmost position at [approximately equal to]5000-4500 yr BP, later than in the Yukon and N.W.T. (Payette 1993). Most pollen studies suggest that there has been no major movement ([greater than]20 km) of the forest limit since the postglacial forest maximum (Richard 1981, Gajewski and Garralla 1992, Gajewski et al. 1993). Larch (Larix laricina [DuRoi] K. Koch) logs found 5 km north of the modern forest limit in the Riviere aux Feuilles area indicate a minor forest advance between 3500 and 2700 yr BP (Gagnon and Payette 1981).

Forest limit advances or retreats, such as those recorded in Canada, result from the migration of tree populations. The term "migration" was defined by Sauer (1988:2) as ". . . any change in the distribution of successfully established mature plants as the years and generations pass. The term includes both advances and retreats and involves both continuous and disjunct ranges." A plant species can use two migration strategies to colonize a new area. The first strategy, "neighbourhood diffusion" (Hengeveld 1989), involves short-distance dispersal of diaspores around the parent plant. The second strategy, called "stratified diffusion" (Hengeveld 1989) or "infiltration invasion" (Wilson and Lee 1989), involves long-distance dispersal of diaspores. Wind (Glaser 1981, Ritchie and MacDonald 1986, Willson 1993), animals (Johnson and Adkisson 1985, Webb 1986, Johnson and Webb 1989), or waterways (Huntley and Birks 1983, Thebaud and Debussche 1991, Lonsdale 1993) are the main agents allowing seed dispersal of several kilometres. The long-distance strategy is more effective for the rapid spread of a plant species (Mack 1985, Hengeveld 1989, Woods and Davis 1989), and it appears that many North American trees, such as beech (Fagus grandifolia Ehrh.) and pine (Pinus contorta Loud. var. latifolia Engelm., Pinus banksiana Lamb.), used this strategy during their postglacial spread (Bennett 1985, 1988, MacDonald and Cwynar 1991, Desponts and Payette 1993).

For the last 18 000 yr, climate change has been the major factor controlling plant migrations. However, several factors, unrelated to climate change, can restrict plant migration and lead to a disequilibrium between climate and the spatial distribution of a taxon. Moreover, clonal growth can maintain some individuals for a long time at a particular place in the absence of site disturbance, even when climatic conditions are unfavorable for sexual reproduction (LaMarche 1973, Pigott and Huntley 1978, Neilson and Wullstein 1983, Brubaker 1986, Payette et al. 1989a). Therefore, the dynamics of tree line and forest fluctuations during the Holocene period may be used to test current hypotheses on the responsiveness of tree species to climate change. According to the ecotone hypothesis (Nichols 1976, Sorenson 1977, Larsen 1980, 1989), one would expect the forest limit to shift rapidly in response to the ever-changing climate. The deforestation hypothesis (Payette and Gagnon 1985, Payette and Lavoie 1994), on the other hand, states that biological mechanisms underlying forest limit advance or retreat are not easily triggered, and that there is no close relationship between forest limit position and climatic conditions at any particular time. To test both hypotheses, we analyzed macrofossil remains across a long transect from forest-tundra to tundra, to determine whether or not the eastern Canadian forest limit shifted in unison with climate change during the late Holocene.

STUDY AREA AND METHODS

The study area is located between the Riviere Boniface watershed (57 [degrees] 45 [minutes] N, 76 [degrees] 15 [minutes] W; 130 km southeast of Inukjuak) and the Lac Ujarasutjulik (58 [degrees] 10 [minutes] N, 76 [degrees] 30 [minutes] W), 60 km northward [ILLUSTRATION FOR FIGURE 1 OMITTED], in northern Quebec. The mean annual temperature at the Inukjuak weather station is -7 [degrees] C, the mean temperature for the coldest month (February) is -26 [degrees], and that of the warmest month (July) is 9 [degrees]. The mean frost-free period is [approximately equal to]60 d. Mean annual precipitation is 418 mm, 42% of which falls as snow (Environment Canada 1993).

Vegetation was established [approximately equal to]6000 yr BP after deglaciation and marine regression (Lauriol 1982, Gajewski et al. 1993). Pollen and macrofossil analyses indicate that spruce was not common in the area until 4600 yr BP (Payette 1993). The modern forest limit (defined by a cover of [greater than or equal to]10% conifer trees) is located 10 km north of the Riviere Boniface [ILLUSTRATION FOR FIGURE 1 OMITTED]. South of the forest limit, northern forest-tundra consists of moss-shrub black spruce (Picea mariano [Mill.] B.S.P.) forests in lowlands and lichen-heath communities on most well-drained sites (Payette 1983). Only two tree species, black spruce and larch, occur in the area, but larch is rare. North of the forest limit, spruce is scattered and grows as a depressed shrub. The northernmost black spruce is located in the Lac Ujarasutjulik area.

Forests and krummholz (tree and stunted spruce stands, respectively) on dry-mesic sites and peatlands were mapped using aerial photographs in a 10 km wide transect from the Riviere Boniface area to the black spruce limit [ILLUSTRATION FOR FIGURE 1 OMITTED]. The transect was wider (25 km) in the Riviere Boniface watershed where other paleoecological data are available (Payette and Morneau 1993, Lavoie 1994). To investigate past displacements of the forest limit in response to climate change, we sampled in the tundra for dry peat produced by stunted clones of black spruce, and north and south of the modern forest limit for logs in peatlands and charcoal in organic topsoil.

All peatlands south of the forest limit and up to 20 km north of the forest limit were investigated to recover buried spruce logs. Particular attention was paid to palsas (peat mounds with a frozen core), where the collapse of peat layers following the melting of permafrost may bring logs to the surface. Three large tundra peatlands ([greater than]5 [km.sup.2]) were also surveyed for logs at 20, 30, and 40 km north of the forest limit. The growth form (tree or stunted) of each spruce log recovered was described, and a cross section was taken at the base of the stem for 14C dating and cross dating.

Charcoal was sampled in the eastern half of the transect, from 4 km south to 16 km north of the forest limit. The charcoal transect was subdivided into 20 1 x 5 km quadrats, and the three lowest well-drained hills (where the probability of finding charcoal is highest; Payette and Morneau 1993) in each quadrat were systematically sampled. In sites where charcoal was absent, the thickness of the dry peat (organic layer with predominant moss, lichen, and spruce remains) laid down by each clonal spruce was measured. A 1-cm slice of the thickest peat was sampled at the contact with the mineral horizon. The production of dry peat by spruce is common in undisturbed, fire-free sites north and south of the tree line. Only well-decomposed dry peat produced by spruce was collected.

Spruce logs, charcoal, and peat were cleaned to remove surficial organic material and processed for 14C dating. The 14C dates of logs and charcoal give a minimum 14C age for the presence of a spruce tree in wet and dry-mesic sites, respectively. The 14C dates of basal dry peat beneath spruce clumps correspond to the residence time of carbon in the organic topsoil at the site, depending on local turnover conditions associated with drainage, nutrients, soil temperature, and spruce productivity (Martel and Paul 1974). Thus, each 14C date gives the minimum 14C age for the presence of stunted clonal spruce at the site, black spruce being the sole species able to produce this type of thick organic layer in the area (Lavoie 1994).

RESULTS AND DISCUSSION

The palsa sampled contained 10-25 buried logs/[less than]100 [m.sup.2] per site [ILLUSTRATION FOR FIGURE 1 OMITTED], with most (90%) showing a normal tree growth form. Thirty-four of 146 spruce logs recovered were 14C dated between 4580 and 1540 yr BP (Table 1), and an additional 34 logs were cross-dated with 14C dated spruces (3010-1540 yr BP), using diagnostic tree rings (i.e., light rings sensu Filion et al. 1986). No spruce fossils were found in peatlands north of the forest limit. Palsas cover only 1.6% of the tundra, in contrast to a cover of [greater than]11% in the forest-tundra. Other tundra peatlands have shallow peat ([less than]20 cm) and did not contain spruce logs.

South of the forest limit, charcoal with abundant spruce wood fragments, cones, and leaves was widespread under the organic topsoil in lichen-spruce woodlands or krummholz, and dated between 1920 and 1140 yr BP ([ILLUSTRATION FOR FIGURE 1 OMITTED]; Table 2). Charcoal (dated at 550 yr BP) was found only at one tundra site, located 12 km north of the forest limit. At this site, charcoal was restricted to a small area ([less than]1 ha) and was composed of birch (Betula glandulosa Michx) remains with one spruce twig and needle. Across the south-north transect, the transition between lichen-spruce sites with charcoal and lichen-heath sites without charcoal was abrupt ([less than] 10 m) and coincided exactly with the modern forest limit.

Relatively thick (12.2 [+ or -] 6.4 cm, mean [+ or -]1 SD) humified peat occurred under tundra spruce clones, in contrast to the thin 1.1 [+ or -] 0.7 cm organic layer in nearby lichen-heath (Lavoie 1994). In the tundra, thick organic layers are patchily distributed, confined to areas occupied by clonal spruce and generally restricted to a zone 8 km north of the forest limit. The residence time of the 13 basal peat layers sampled in the tundra ranged in age from 3040 to 630 yr BP (Table 2).
TABLE 1. Radiocarbon dates of buried spruce logs in the Riviere
Boniface area, subarctic Quebec, Canada.


Position
south of
the forest      Altitude            Age              Laboratory
limit (km)        (m)        (yr BP [+ or -] 1 SD)     number


2.8               135           1730 [+ or -] 60       GSC-5532
3.0               125           2040 [+ or -] 70       GSC-5533
3.0               110           1540 [+ or -] 80       UL-995
4.5               110           1640 [+ or -] 60       GSC-5591
6.1               125           1910 [+ or -] 70       UL-942
7.0               110           2230 [+ or -] 60       GSC-5574
7.0               110           2610 [+ or -] 80       UL-933
8.3               110           2520 [+ or -] 60       GSC-5530
8.3               110           2390 [+ or -] 60       GSC-5568
8.5               135           2410 [+ or -] 80       GSC-5314
9.0               125           2660 [+ or -] 70       UL-917
9.0               125           2600 [+ or -] 70       UL-922
9.0               125           2330 [+ or -] 80       UL-923
9.0               125           1890 [+ or -] 80       UL-930
9.0               125           2540 [+ or -] 80       UL-931
10.0              135           2880 [+ or -] 60       GSC-5346
10.0              135           2860 [+ or -] 60       GSC-5579
10.0              135           2980 [+ or -] 60       GSC-5598
10.0              135           3020 [+ or -] 80       UL-780
10.0              135           2440 [+ or -] 90       UL-784
10.0              135           2470 [+ or -] 90       UL-785
10.0              135           4580 [+ or -] 140      UL-802
10.5              140           1760 [+ or -] 50       Beta-40498
10.5              140           4560 [+ or -] 70       Beta-40502
10.5              140           1710 [+ or -] 80       UL-783
10.5              140           1800 [+ or -] 70       UL-786
11.0              140           2870 [+ or -] 60       GSC-5355
11.5              120           1910 [+ or -] 70       GSC-5421
11.5              120           2170 [+ or -] 60       GSC-5565
12.2              115           2770 [+ or -] 60       Beta-40500
12.2              115           3130 [+ or -] 70       Beta-40501
12.2              115           2690 [+ or -] 90       UL-774
12.2              115           3010 [+ or -] 70       UL-775
12.2              115           3180 [+ or -] 90       UL-787


Buried spruce in peatlands and charcoal in dry-mesic lichen-spruce sites south of the forest limit confirm the presence of spruce since at least 4600 yr BP and forest since at least 3000 yr BE All buried stems in palsa mounds were older than 1540 yr BP and contemporaneous with the period of fen development prior to palsa formation, which occurred after 1500 yr BP in most subarctic Quebec peatlands (Couillard and Payette 1985, Allard et al. 1987). Recent dates from buried spruce in fens in the Riviere Boniface area confirm that spruce has occurred continuously in peatland for the last 3000 yr (D. Arseneault, personal communication).

The 14C dates of charcoal in spruce sites south of the forest limit correspond to a period of good postfire regeneration (Payette and Morneau 1993). Dating of the oldest trees ([approximately equal to]AD 1500; Lavoie 1994) at these sites does not give the approximate age of these forests (or krummholz) because the longevity of spruce at these latitudes is much shorter than the fire-free interval (Payette et al. 1989b). However, 14C dates of charcoal can be considered as the maximum age of the postfire forests (krummholz) because regeneration must take place a few years after the fire (Sirois and Payette 1991). Because the young-aged charcoal 12 km north of the forest limit was rare and contained only two small pieces of spruce, in contrast with the forest-tundra sites where spruce remains (wood, leaves, and cones) in the organic topsoil were very abundant and ubiquitous, the prefire vegetation was probably lichen-heath with one scattered, stunted spruce clone, as found nearby today. The absence of logs in peatlands and the absence of charcoal beneath topsoil in the tundra zone, together with the geographic coincidence of the charcoal limit and forest limit [ILLUSTRATION FOR FIGURE 2 OMITTED], strongly suggest that the forest limit has remained stable during the last 2000-3000 yr BP. Although negative evidence for the presence of spruce remains in arctic peatlands is no direct proof that trees never colonized the sites, the probability of finding such subfossil trees appears low for two reasons. Firstly, the mean residence time of 500-600 yr of spruce subfossils lying at or slightly above the soil surface in this area (Payette et al. 1989a, Payette and Morneau 1993) and, secondly, the thin peat deposits of the studied peatlands, allowing easy recording of logs, suggest that trees were absent during the past millenia. Old peat produced by tundra spruce is additional evidence for the stability of the forest limit. Because only spruce is able to accumulate such a thick organic layer [ILLUSTRATION FOR FIGURE 2 OMITTED], the old basal peat dates indicate that spruce clones have developed continuously in the absence of fire disturbance over the last 3000 yr BP.

The long-term stability of the forest limit in the Riviere Boniface area suggests either the absence of major and/or prolonged climatic change during the last millenia or unresponsiveness of the forest limit following climate change. Climatic reconstruction in the Hudson Bay area using eolian, gelifluction, and fire records indicates peak dune activity between 3650 and 2700 yr BP, 1650 and 950 yr BP, and 700 and 100 yr BP ([ILLUSTRATION FOR FIGURE 3 OMITTED]; Filion et al. 1991). These paleoecological records were interpreted as a response to increased fire occurrences under dry, often cool conditions inimical to tree regeneration and conducive to sustained postfire erosion of sand dunes (Filion 1984, Filion et al. 1991). Postfire gelifluction events in snowpatch environments reached maxima around 1500-1100 yr BP and 750 yr BP, indicating increased periglacial activity. Also, large-scale postfire deforestation of the forest-tundra occurred after 2000 yr BP, suggesting a long-term trend of temperature lowering by [approximately equal to]1 [degree]-1.5 [degrees], preventing spruce regeneration (Payette and Gagnon 1985). This trend was also recorded in palynological data (Richard 1981, Diaz et al. 1989, Gajewski et al. 1993).

Our study shows that the forest limit has been unresponsive [TABULAR DATA FOR TABLE 2 OMITTED] to climate change over the last 2000-3000 yr BP, and that mechanisms underlying forest limit advance or retreat are not easily triggered by climatic change. However, even though the range of spruce has not measurably changed, dendroecological data from the same area suggest that the forest limit was located [approximately equal to]4 km south of the modern limit during the Little Ice Age, [approximately equal to]AD 1800-1850 (Lavoie and Payette 1994). In this particular example, there was no population shift induced by seed dispersal and establishment. This rapid response of northernmost spruce populations to the last major climatic change, from the Little Ice Age to present warming, consisted of the shifting dominance of arboreal and shrubby individuals. Thus, change in the forest limit resulted from a changing growth form dominance of pre-established spruce (from forest to krummholz) during a cold period, and stand reversion since the late 1800s, without any movements of the species' range.

Most paleoecological studies using macrofossils reported a major retreat of the forest limit in northwestern Canada and Russia around 5000-3500 yr BP (Ritchie et al, 1983, Khotinskiy 1984, Cwynar and Spear 1991, Spear 1993). For the last 4000-3500 yr BP, however, forest displacements inferred from microfossil data (Nichols 1975, Kay 1979, MacDonald et al. 1993) remain to be substantiated with macrofossil evidence. The stability of the forest limit during the last 2000-3000 yr suggests that only major temperature changes of the order of several degrees Celsius can induce back-and-forth movements of the boreal forest.

When analyzing the response of tree species to climate change, it is necessary to consider the various responses of tree populations through time. Results from the current study should provide insights for inferences about vegetation development from pollen diagrams in subarctic and arctic sites, particularly those suggesting a rapid response of the spruce forest limit to past climate changes. These records may correspond to changing local and regional abundances rather than to tree species displacements. Furthermore, in the absence of macrofossil evidence, the pollen data suggesting forest expansion could be explained by the "switching on" of pollen production by previously "silent" krummholz and/or transport of pollen produced regionally in a large-scale response south of the forest limit. This does not, of course, negate the pollen evidence of climate change, but it cautions against interpreting the data in terms of a model of tree line response that may turn out to be inappropriate for the latter part of the Holocene.

ACKNOWLEDGMENTS

This research has been financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds pour la formation des chercheurs et l'aide a la recherche du Quebec (FCAR) (grants to S. Payette), The Department of Indian and Northern Affairs (Canada), the Geological Survey of Canada, NSERC, FCAR, and Eco-Research provided grants or doctoral fellowships to C. Lavoie. We thank F. Cadrin, I. Leduc, G. and M. Levesque, e. Marcoux, e. Poirier, R. Saint-Arneault, and N. Samson for field and laboratory assistance. Comments of D. Arseneault, M. Edwards, D. R. Foster, G. Houle, P. Morisset, P. J. H. Richard, and an anonymous reviewer on an earlier draft were greatly appreciated.

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Author:Lavoie, Claude; Payette, Serge
Publication:Ecology
Date:Jun 1, 1996
Words:4848
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