The long-term stability of the boreal forest limit in subarctic Quebec.
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.
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.
Allard, M., M. K. Seguin, and R. Levesque. 1987. Palsas and mineral permafrost mounds in northern Quebec. Pages 285-309 in V. Gardiner, editor. International geomorphology 1986, part II. John Wiley, New York, New York, USA.
Bennett, K. D. 1985. The spread of Fagus grandifolia across eastern North America during the last 18 000 years. Journal of Biogeography 12:147-164.
-----. 1988. Holocene geographic spread and population expansion of Fagus grandifolia in Ontario, Canada. Journal of Ecology 76:547-557.
Brubaker, L. B. 1986. Responses of tree populations to climatic change. Vegetatio 67:119-130.
Couillard, L., and S. Payette. 1985. Evolution holocene d'une tourbiere a pergelisol (Quebec nordique). Canadian Journal of Botany 63:1104-1121.
Cwynar, L. C., and R. W. Spear. 1991. Reversion of forest to tundra in the central Yukon. Ecology 72:202-212.
Desponts, M., and S. Payette. 1993. The Holocene dynamics of jack pine at its northern range limit in Quebec. Journal of Ecology 81:719-727.
Diaz, H. F., J. T. Andrews, and S. K. Short. 1989. Climate variations in northern North America (6000 BP to present) reconstructed from pollen and tree-ring data. Arctic and Alpine Research 21:45-59.
Environment Canada. 1993. Canadian climate normals, Quebec, 1961-90. Atmospheric Environment Service, Environment Canada, Ottawa, Ontario, Canada.
Filion, L. 1984. A relationship between dunes, fire and climate recorded in the Holocene deposits of Quebec. Nature 309:543-546.
Filion, L., S. Payette, L. Gauthier, and Y. Boutin. 1986. Light rings in subarctic conifers as a dendrochronological tool. Quaternary Research 26:272-279.
Filion, L., D. Saint-Laurent, M. Desponts, and S. Payette. 1991. The late Holocene record of aeolian and fire activity in northern Quebec, Canada. Holocene 1:201-208.
Gagnon, R., and S. Payette. 1981. Fluctuations holocenes de la limite des forets de melezes, riviere aux Feuilles, Nouveau-Quebec: une analyse macrofossile en milieu tourbeux. Geographie physique et Quaternaire 35:57-72.
Gajewski, K., and S. Garralla. 1992. Holocene vegetation histories from three sites in the tundra of northwestern Quebec, Canada. Arctic and Alpine Research 24:329-336.
Gajewski, K., S. Payette, and J. C. Ritchie. 1993. Holocene vegetation history at the boreal-forest-shrub-tundra transition in north-western Quebec. Journal of Ecology 81:433-443.
Glaser, P. H. 1981. Transport and deposition of leaves and seeds on tundra: a late-glacial analog. Arctic and Alpine Research 13:173-182.
Hengeveld, R. 1989. Dynamics of biological invasions. Chapman and Hall, London, UK.
Houghton, J. T., G. J. Jenkins, and J. J. Ephraums, editors. 1990. Climate change: the IPCC scientific assessment. Cambridge University Press, Cambridge, UK.
Huntley, B., and H. J. B. Birks. 1983. An atlas of past and present pollen maps for Europe: 0-13,000 years ago. Cambridge University Press, Cambridge, UK.
Hyvarinen, H., and J. C. Ritchie. 1975. Pollen stratigraphy of Mackenzie pingo sediments, N.W.T., Canada. Arctic and Alpine Research 3:261-272.
Johnson, W. C., and C. S. Adkisson. 1985. Dispersal of beech nuts by blue jays in fragmented landscapes. American Midland Naturalist 113:319-324.
Johnson, W. C., and T. Webb, III. 1989. The role of blue jays (Cyanocitta cristata L.) in the postglacial dispersal of fagaceous trees in eastern North America. Journal of Biogeography 16:561-571.
Kay, P. A. 1979. Multivariate statistical estimates of Holocene vegetation and climate change, forest-tundra transition zone, N.W.T., Canada. Quaternary Research 11:125-140.
Khotinskiy, N. A. 1984. Holocene vegetation history. Pages 179-200 in H. E. Wright and C. W. Barnosky, editors. Late Quaternary environments of the Soviet Union. University of Minnesota Press, Minneapolis, Minnesota, USA.
LaMarche, V. C., Jr. 1973. Holocene climatic variations inferred from treeline fluctuations in the White Mountains, California. Quaternary Research 3:632-660.
Larsen, J. A. 1980. The boreal ecosystem. Academic Press, New York, New York, USA.
-----. 1989. The northern forest border in Canada and Alaska. Springer-Verlag, New York, New York, USA.
Lauriol, B. 1982. Geomorphologie quaternaire du sud de l'Ungava. Paleo-Quebec 15, Universite du Quebec a Montreal, Montreal, Quebec, Canada.
Lavoie, C. 1994. Dynamique holocene de la limite des forets au Quebec subarctique. Dissertation. Universite Laval, Sainte-Foy, Quebec, Canada.
Lavoie, C., and S. Payette. 1994. Recent fluctuations of the lichen-spruce forest limit in subarctic Quebec. Journal of Ecology 82:725-734.
Lonsdale, W. M. 1993. Rates of spread of an invading species - Mimosa pigra in northern Australia. Journal of Ecology 81:513-521.
MacDonald, G. M., and L. C. Cwynar. 1991. Post-glacial population growth rates of Pinus contorta ssp. latifolia in western Canada. Journal of Ecology 79:417-429.
MacDonald, G. M., T. W. D. Edwards, K. A. Moser, R. Pienitz, and J. P. Smol. 1993. Rapid response of treeline vegetation and lakes to past climate warming. Nature 361:243-246.
Mack, R. N. 1985. Invading plants: their potential contribution to population biology. Pages 127-142 in J. White, editor. Studies on plant demography. Academic Press, London, UK.
Martel, Y. A., and E. A. Paul. 1974. The use of radiocarbon dating of organic matter in the study of soil genesis. Soil Science Society of America Proceedings 38:501-506.
Monserud, R. A., N. M. Tchebakova, and R. Leemans. 1993. Global vegetation change predicted by the modified Budyko model. Climatic Change 25:59-83.
Neilson, R. P., and L. H. Wullstein. 1983. Biogeography of two southwest American oaks in relation to atmospheric dynamics. Journal of Biogeography 10:275-297.
Nichols, H. 1975. Palynological and paleoclimatic study of the Late Quaternary displacements of the boreal forest-tundra ecotone in Keewatin and Mackenzie, N.W.T., Canada. Institute of Arctic and Alpine Research, Occasional Paper 15, Boulder, Colorado, USA.
-----. 1976. Historical aspects of the northern Canadian treeline. Arctic 29:38-47.
Occhietti, S., and C. Hillaire-Marcel. 1977. Chronologie 14C des evenements paleogeographiques du Quebec depuis 14000 ans. Geographie physique et Quaternaire 31:123-133.
Payette, S. 1983. The forest tundra and present tree-lines of the northern Quebec-Labrador peninsula. Pages 3-23 in P. Morisset and S. Payette, editors. Tree-line ecology. Proceedings of the Northern Quebec Tree-Line Conference, 22 June-1 July 1981, Kuujjuarapik, Quebec, Canada. Nordicana 47.
-----. 1993. The range limit of boreal tree species in Quebec - Labrador: an ecological and palaeoecological interpretation. Review of Palaeobotany and Palynology 79:7-30.
Payette, S., L. Filion, A. Delwaide, and C. Begin. 1989a. Reconstruction of tree-line vegetation response to long-term climate change. Nature 341:429-432.
Payette, S., and R. Gagnon. 1985. Late Holocene deforestation and tree regeneration in the forest-tundra of Quebec. Nature 313:570-572.
Payette, S., and C. Lavoie. 1994. The arctic tree line as a record of past and recent climatic changes. Environmental Reviews 2:78-90.
Payette, S., and C. Morneau. 1993. Holocene relict woodlands at the eastern Canadian treeline. Quaternary Research 39:84-89.
Payette, S., C. Morneau, L. Sirois, and M. Desponts. 1989b. Recent fire history of the northern Quebec biomes. Ecology 70:656-673.
Pigott, C. D., and J. P. Huntley. 1978. Factors controlling the distribution of Tilia cordata at the northern limits of its geographical range. I. Distribution in north-west England. New Phytologist 81:429-441.
Richard, P. J. H. 1981. Paleophytogeographie postglaciaire en Ungava par l'analyse pollinique. Paleo-Quebec 13, Universite du Quebec a Montreal, Montreal, Quebec, Canada.
Ritchie, J. C. 1984. Past and present vegetation of the far northwest of Canada. University of Toronto Press, Toronto, Ontario, Canada.
-----. 1987. Postglacial vegetation of Canada. Cambridge University Press, Cambridge, UK.
Ritchie, J. C., L. C. Cwynar, and R. W. Spear. 1983. Evidence from north-west Canada for an early Holocene Milanko-vitch thermal maximum. Nature 305:126-128.
Ritchie, J. C., and F. K. Hare. 1971. Late-Quaternary vegetation and climate near the arctic tree line of northwestern North America. Quaternary Research 1:331-342.
Ritchie, J. C., and G. M. MacDonald. 1986. The patterns of post-glacial spread of white spruce. Journal of Biogeography 13:527-540.
Sauer, J. D. 1988. Plant migration. University of California Press, Berkeley, California, USA.
Sirois, L., and S. Payette. 1991. Reduced postfire tree regeneration along a boreal forest-forest-tundra transect in northern Quebec. Ecology 72:619-627.
Smith, T. M., H. H. Shugart, G. B. Bonan, and J. B. Smith. 1992. Modeling the potential response of vegetation to global climate change. Advances in Ecological Research 22:93-116.
Sorenson, C. J. 1977. Reconstructed Holocene bioclimates. Annals of the Association of American Geographers 67:214-222.
Sorenson, C. J., J. C. Knox, J. A. Larsen, and R. A. Bryson. 1971. Paleosols and the forest border in Keewatin, N.W.T Quaternary Research 1:468-473.
Spear, R. W. 1983. Paleoecological approaches to a study of tree-line fluctuation in the Mackenzie delta region, Northwest Territories: preliminary results. Pages 61-72 in P. Morisset and S. Payette, editors. Tree-line ecology. Proceedings of the Northern Quebec Tree-Line Conference, 22 June-1 July 1981, Kuujjuarapik, Quebec, Canada. Nordicana 47.
-----. 1993. The palynological record of Late-Quaternary arctic tree-line in northwest Canada. Review of Palaeobotany and Palynology 79:99-111.
Thebaud, C., and M. Debussche. 1991. Rapid invasion of Fraxinus ornus L. along the Herault River system in southern France: the importance of seed dispersal by water. Journal of Biogeography 18:7-12.
Webb, S. L. 1986. Potential role of the passenger pigeons and other vertebrates in the rapid Holocene migrations of the nut trees. Quaternary Research 26:367-375.
Willson, M. F. 1993. Dispersal mode, seed shadows, and colonization patterns. Vegetatio 107-108:261-280.
Wilson, J. B., and W. G. Lee. 1989. Infiltration invasion. Functional Ecology 3:379-380.
Woods, K. D., and M. B. Davis. 1989. Paleoecology of range limits: beech in the upper peninsula of Michigan. Ecology 70:681-696.
|Printer friendly Cite/link Email Feedback|
|Author:||Lavoie, Claude; Payette, Serge|
|Date:||Jun 1, 1996|
|Previous Article:||Using landscape indices to predict habitat connectivity.|
|Next Article:||Replacement patterns of beech and sugar maple in Warren Woods, Michigan.|