Fig. 5.22 A proposed division of the Holocene into three stages based on climatic data in relation to the dynamics of the polar treeline on the Yamal Peninsula in Siberia. The graph illustrates the relative distance of sampled tree remains from the present position of the most northerly open stands of larch in river valleys and the number of tree samples for each radiocarbon date. (Diagram based on data from Hantemirov & Shiatov, 1999, and reproduced with permission from Juday, 2005.)
A common presumption in assessing the direction of future change in high latitude biomes as a result of climatic warming is that forest is the natural climax vegetation and this will move north. However, in areas subject to maritime climatic influences, and these can extend inland considerable distances from the sea, there is always the possibility that the natural climax vegetation may be bog rather than forest (Klinger, 1996). In oceanic environments mineral soil impoverishment is frequently aggravated, as soil leaching accelerates nutrient removal and results in iron pan formation. Waterlogging then follows and impedes mineralization and nitrogen fixation, leading to the process of palu-dification and establishment of bogs.
Paludification, however, is not just limited to the coastal fringe areas of western Europe. Extensive regions of Newfoundland, north-west Europe and northern Siberia have become covered in extensive bogs during the past 6000 years (Crawford, 2000). Whether the tundra-taiga interface is moving north or south or remains stationary at present is uncertain and requires further investigation, particularly in areas where cool moist summers may accelerate the rate of peat formation as the growing season extends its length and facilitates bog growth.
Oceanicity is a complicating factor in many studies on climatic warming. In the past, some of the major influences of warm temperatures observed on plant distribution have been due to oceanic influences as manifested through increased precipitation and milder winters. Already, examples are available where milder conditions at high latitudes are producing more variable winters. One such consequence of climatic warming has been ice encasement. Instead of a continuous covering of snow, thaw periods with rain falling on frozen ground have resulted in vegetation ice encasement which can completely destroy large areas of improved pasture with disastrous results for agriculture in the north (Gudleifsson, 1997) as well as causing high mortality rates from winter starvation in reindeer. The natural grasslands at these latitudes do not appear to be damaged. It appears to be reseeding with grass-seed mixtures from more southern provenances that has made these once hardy northern pastures now sensitive to ice-encasement damage which is lethal and leaves a soil surface in spring that is entirely devoid of vegetation.
The relationship, or rather conflict between bogs and forests for dominance of the landscape, is complex. Trees survive on bogs in many northern areas and when they do so it is usually in regions where the soil is frozen to a considerable depth throughout the winter period and root metabolic activity will therefore be minimal during periods of prolonged oxygen deprivation. The black spruce (Picea mariana) and tamarack (Larix laricina) are examples of trees that are notable for their ability to grow on wet peatlands in the continental cold-winter regions of North America. When spring does arrive there is a rapid transition from frozen soils to a warm growing season. Consequently, during the summer growing season the active layer of the soil horizon will normally be aerated. However, when late spring flooding occurs on these forested peatlands it can be detrimental to growth for both P. mariana and L. laricina (Roy et al., 1999; Girardin et al., 2001).
The Siberian forests are particularly vulnerable due to the proximity of the Arctic Ocean coupled with the greater extent of the permafrost zone. Extensive boreal forest could exist under present temperature conditions within much of the southern zone of the subarctic tundra were it not for widespread bog development. In the Eurasian zone there is a region where the treeline is depressed southwards for over 200 km. In many of these locations colonization by shallow rooting native trees (Larix sibirica, L. gmelinii) would be expected on the basis of latitude, as these species are able to survive with a shallow active layer above the permafrost. Their absence may therefore be due to the counter-argument that climatic warming leads to a retreat of the forests due to bog and swamp formation.
In the West Siberian Lowlands over the past 6000 years there has been a 300-400 km retreat of forest, which in some areas used to extend almost to the shores of the Arctic Ocean (Kremenetski et al., 1998). Figure 5.4 shows the changes in the northern limits of the boreal forest proper and the approximate modern limits for the sporadic occurrence of individual tree species north of the limit of closed forest. Coloured zones to the north of the current treelines indicate the areas where Holocene macrofossils have been located and carbon dated. A pronounced southerly depression of the northern limit of the boreal forest in the region of the West Siberian Lowlands is due to the presence of enormous areas of bog which now cover terrain that was once covered with forest (MacDonald et al., 2000).
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