It is not uncommon for plant macrofossils to be found far beyond the range of the particular species today. Where climatic controls on present-day plant distributions are known, their former distribution may be interpreted paleoclimatically, from dated macrofossils. Fluctuations of three major biogeographical boundaries have been studied in considerable detail using macrofossils: the arctic treeline; the alpine treeline; and the lower or "dryness" treeline of semiarid and arid regions. In each case, the precise definition of treeline poses considerable problems as there is rarely a clearly demarcated boundary. Commonly, there is a gradual transition from mature dense forest through more open, discontinuous woodland to isolated trees or groups of trees, which may include dwarf or krummholz (deformed) forms, particularly in the alpine case (LaMarche and Mooney, 1972). Topoclimatic factors are particularly important in determining the precise limit of trees. It is not necessary to dwell on this at length here, but sufficient to note that the location of the modern treeline is itself often problematic, and may make the interpretation of macrofossils somewhat difficult. For further discussion of the problem, see Larsen (1974) and Wardie (1974).
Macrofossil evidence of formerly more extensive boreal forests has been found throughout the Northern Hemisphere, in tundra regions of Alaska, northern Canada, and the former USSR (Miroshnikov, 1958; Tikhomirov, 1961; McCulloch and Hopkins, 1966; Ritchie, 1987). In addition, paleopodsols (relict forest soils) and charcoal layers (relating to forest fire episodes) have been found in many tundra areas of Keewatin, north-central Canada (Bryson et al., 1965; Sorenson and Knox, 1974). In most areas the macrofossil evidence is episodic in nature, made up of radiocarbon dates on isolated tree stumps located north of the modern treeline. In Keewatin a number of dates (also on organic material in paleopodsols, and on charcoal layers) have enabled a time series of the forest/tundra boundary during the latter part of the Holocene to be constructed (Fig. 8.1; Sorenson, 1977). According to these data, the northern treeline was 250 km or more north of the modern tree-line between 6000 and 3500 yr B.R (Moser and MacDonald, 1990; Gajewski and Garralla, 1992). Less extensive northward migrations occurred around 2700-2200 yr B.P. and 1600-1000 yr B.P. By contrast, the presence of arctic brown paleosols (relict tundra soils) buried beneath more recent podsols south of the modern tree-line, suggests that the treeline was at least 80 km farther south around 2900, 1800, and 800 yr B.P. (see Fig. 8.1).
What paleoclimatic significance can be ascribed to such fluctuations? Several authors have noted the correspondence of northern treelines with isotherms of summer or July mean temperatures (Larsen, 1974), so a northward migration of the ecotone may indicate warmer summer conditions. A reconstruction of treeline migration in terms of July temperatures was made by Nichols (1967). Nichols assumed that, when the forest limit moved northward 250 km, July temperatures at the modern treeline were similar to locations 250 km south of the treeline today. In this way, July paleotemperatures were reconstructed for Keewatin, using paleosol, macrofossil, and palynological evidence. Modern treeline is also closely related spatially to the mean or modal position of the arctic front in summer over North America and the median front position over northern Eurasia (Fig. 8.2; Bryson, 1966; Krebs and Barry, 1970). Whether this is a causative factor in the location of the northern forest border or whether the vegetation boundary itself largely determines the climatic differences noted across the vegetation boundary is difficult to assess, although general circulation model experiments show that the treeline has strong feedback effects on climate (Foley et al., 1994). Mid-Holocene warming of the 60-90° N zone due to orbital forcing alone was about 2°C, but experiments with a
Southward migration ^00-
Was this article helpful?