The Arctic is possibly unique in being the northern hemisphere's ultimate peripheral habitat; for it is here that continental convergence brings together the northern limits of the flora of three continents.
The observation that in many polar areas some populations of plants are not fully self-sustaining has prompted the assertion that the Arctic as a whole should be considered as a marginal area (Svoboda & Henry, 1987). Such a concept implies that polar deserts and semi-deserts are marginal for the establishment of vascular plants (sensu strictu) as frequently they do not produce viable seed and therefore depend on propa-gules dispersed from more favourable habitats. It is also argued that these areas are marginal as the vegetation succession rarely progresses beyond the initial invasion stage of succession. The second phase rarely occurs, when it might be expected that under more favourable conditions stand formation and a build-up of a critical standing crop would lead to habitat improvement and finally to the replacement of the pioneer species.
Coastal sites being close to the sea frequently have their growing seasons limited by the proximity and duration of the sea ice. In some cases the growing season here can be so short that even though the plants may flower, there is not sufficient time for seed production and such sites are dependent on adjacent areas for an input of seeds. This is the case with the colonization of the lower shore by Saxifraga oppositifolia in western Spitsbergen, as the creeping ecotype of this species which survives on the cold wet shore has in the past rarely produced seed and regeneration has largely been dependent on seed produced by neighbouring populations on the beach ridge (see Fig. 2.24).
It is sometimes taken for granted that areas of extremely sparse vegetation which are often observed in polar deserts are due to the prevailing unfavourable climatic conditions inhibiting plant establishment, growth and survival. In a study of such areas in arctic Canada it has been claimed (Levesque & Svoboda, 1999) that the antagonistic factor is in fact the result of historical episodic adverse climatic anomalies. Such a case was considered to have taken place in Alexandra Fiord Lowland on Ellesmere Island (see map Fig. 6.20) in the Canadian High Arctic as a result of the recent Little Ice Age cooling which caused a dieback and even large-scale extinction of High Arctic plant communities that had taken centuries to develop. The Little Ice Age brought about new glacial advances, expansion of permanent snow cover and ice crusts over entire landscapes. It was argued that the newly formed ice (and snow) killed the underlying vegetation, thus creating what is referred to in the geological literature as 'lichen-kill zones'. In these zones the current plant diversity and abundance are exceedingly low and the plants are all relatively young and even-aged, as they are all part of a recolonization process that has been occurring only over the past 100-150 years. This vegetation, it is argued, has not yet reached equilibrium with the present prevailing climate and is still in an initial stage of succession (Levesque & Svoboda, 1999).
There are many boundary zones within the Arctic. Low and High Arctic and various subdivisions have already been mentioned (Fig. 6.3). There are many other lines that can be drawn in relation to the zonation of arctic plant communities. Despite the general recognition of these zones (Fig. 6.3), vegetation maps of the Arctic have proved very difficult to standardize on a circumpolar basis. The circumpolar map of Walker et al. (2005) in an attempt to reconcile North American, Scandinavian and Russian concepts of boundary zones includes 23 different floristic zones. The simplest circumpolar classification that was produced consisted of identifying bioclimatic types as defined by Elvebakk (1999), rather than mapping actual vegetation units. This simplified
approach reduces the number of circumpolar zones from 23 to five (Fig. 6.9 and Table 6.1).
The possibility of identifying at least 23 vegetation types highlights the extensive variation that is found between different regions of the Arctic. The process is made even more difficult because the actual geographical limits to the different zones are not abrupt boundaries but are instead a series of merging zonations (limes divergens or ecoclines; see Section 1.1).
Exceptions to the pattern of gradual change can be found in areas where there is an abrupt alteration in edaphic factors due to differences in substrate pH. Such a case is found at a sharp pH boundary along the northern front of the Arctic Foothills in Alaska which separates the non-acidic (pH>6.5) ecosystems to the north and the predominantly acidic (pH 5.5) ecosystems to the south. The edaphic boundary also marks abrupt changes to ecosystem processes. Comparison of two sites on either side of the boundary but only 7 km apart (sites 3 and 4, Fig. 6.10b) showed that the moist acidic vegetation had twice the gross photosynthetic activity and three times the respiration rate of the non-acidic site just to the north. Moist non-acidic tundra has greater heat flux, deeper summer thaw (active layer), is also less of a carbon sink, and is a smaller source of methane than the more southern moist acidic tundra (Walker et al., 1998).
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